GJ-HAN-123

 

Vadose Zone Characterization Project at the Hanford Tank Farms

Tank Summary Data Report for Tank T-111
 

December 1998
 

Prepared for U.S. Department of Energy, Richland Operations Office, Richland, Washington.
Prepared by U.S. Department of Energy, Grand Junction Office, Grand Junction, Colorado.

Work Performed Under DOE Contract No. DE-AC13-96GJ87335 for the U.S. Department of Energy.


Table of Contents

Signature Page
Executive Summary
1.0  Introduction
2.0  Spectral Gamma-Ray Log Measurements
3.0  Review of Tank History
4.0  Boreholes in the Vicinity of Tank T-111
5.0  Discussion of Results
6.0  Conclusions
7.0  Recommendations
8.0  References
Appendix A. Spectral Gamma-Ray Logs for Boreholes in the Vicinity of Tank T-111


Executive Summary

The U.S. Department of Energy (DOE) Richland Operations Office tasked the DOE Grand Junction Office (GJO) with performing a baseline characterization of the gamma-ray-emitting radionuclides that are distributed in the vadose zone sediments surrounding the single-shell tanks (SSTs) at the Hanford Site.  Information regarding vadose zone contamination was acquired by logging the monitoring boreholes positioned around the SSTs using a spectral gamma logging system (SGLS).  This system uses a high-purity germanium detector and is designed to acquire laboratory-quality assays of the gamma-emitting radionuclides in the sediments.  This report documents the spectral gamma-ray logging results obtained from the monitoring boreholes that surround tank T-111.

Tank T-111 is currently classified as an assumed re-leaker and is estimated to contain 446,000 gallons (gal) of semi-solid sludge and 29,000 gal of pumpable liquid.  No supernatant liquid is reported.  The tank is currently on the Organics Watch List and is interim stabilized.

Nine of the ten boreholes surrounding tank T-111 are constructed with multiple casings and were finished with grout in the annular space.  This borehole configuration affects the calculated radionuclide concentrations detected by the SGLS.  The reported concentrations are only apparent concentrations and should be considered underestimated.

The man-made radionuclides cesium-137 (Cs-137) and cobalt-60 (Co-60) were detected by the SGLS in the boreholes surrounding this tank.  The measured Cs-137 concentrations were 2 picocuries per gram (pCi/g) or less and were mostly restricted to the upper few feet (ft) of backfill material.  The  Co-60 contamination was detected below the 69-ft depth in boreholes located northwest of tank T-111 at concentrations ranging from 0.06 to 4 pCi/g.

Some of the Co-60 contamination is probably the remnant or residual of a plume that consisted of additional short-lived radionuclides; the plume was first detected in borehole 50-08-07 in 1978.  Borehole 50-08-07 is farther from tank T-111 than borehole 50-11-11, but the contamination in borehole 50-08-07 occurs at a higher concentration and extends to a greater depth than in borehole 50-11-11.  The contamination probably originated closer to tank T-108 rather than tank T-111.  The contamination detected in these two boreholes is correlatable with data acquired in other monitoring boreholes north of tank T-111 that are associated with tank T- 108.

Very low levels of Co-60 contamination (0.08 pCi/g) detected in borehole 50-11-10 could represent the leading edge of a plume.  The depth of the Co-60 contamination correlates well with plumes identified in boreholes 50-08-07 and 50-09-05.

The absence of leak indications in the SGLS data and historical gross gamma-ray data is difficult to reconcile with the status of tank T-111 as a confirmed re-leaker, even though the estimated leak volume is only 2,000 gal or less on the basis of liquid-level measurements.  The leaker classification was determined on the basis of two separate liquid-level decrease incidents.  These measurements remain the only basis for declaring tank T-111 an assumed leaker.


1.0  Introduction

1.1  Background

The U.S. Department of Energy (DOE) Richland Operations Office tasked the DOE Grand Junction Office (GJO) with characterizing and establishing a baseline of man-made radionuclide concentrations in the vadose zone surrounding the single-shell tanks (SSTs) at the Hanford Site.  These tasks are being accomplished using spectral gamma-ray borehole geophysical logging measurements made in the boreholes surrounding the tanks.  The primary objective of this project is to provide data on the tanks for use by DOE organizations.  These data may also be used to develop an SST Closure Plan in compliance with the Resource Conservation and Recovery Act and to prepare an Environmental Impact Statement for the Tank Waste Remediation Systems program.

1.2  Scope of Project

The scope of this project is to locate and identify the gamma-ray-emitting radionuclides and determine their concentrations in the vadose zone sediment by logging the monitoring boreholes around the SSTs with a Spectral Gamma Logging System (SGLS).  Additional details regarding the scope and general approach to this characterization program are included in the project management plan (DOE 1997c) and baseline monitoring plan (DOE 1995b).  This project may help to identify possible sources of any subsurface contamination encountered during the logging and to determine the implications of the contamination for Tank Farm operations.  The acquired data will establish a contamination baseline that can be used for future data comparisons, for tank-leak verifications, and to help develop contaminant flow-and-transport models.

1.3  Purpose of Tank Summary Data Report

A Tank Summary Data Report (TSDR) will be prepared for each SST to document the results of the spectral gamma-ray logging in the boreholes around the tank.  Each TSDR provides a brief review and a summary of existing information about a specific tank and an assessment of the implications of the spectral gamma-ray log information, including recommendations on future data needs or immediate corrective action, where appropriate.  Appendix A of each TSDR presents logs of radionuclide concentrations versus depth for all boreholes around that specific tank.  A comprehensive Tank Farm Report will be prepared for each tank farm after completion of characterization logging of all boreholes in the subject farm.


2.0  Spectral Gamma-Ray Log Measurements

2.1  Data Acquisition and Processing

The concentrations of individual gamma-ray-emitting radionuclides in the sediments surrounding a borehole can be calculated from the activities in the gamma-ray energy spectra measured in the borehole using calibrated instrumentation.  Spectral gamma-ray logging is the process of collecting gamma-ray spectra at sequential depths in a borehole.  Figure 1 shows a gamma-ray spectrum with peaks at energies, from 0 to 2,700 kilo-electron-volts (keV), that are characteristic of specific radionuclides.  The spectrum includes peaks from naturally occurring radionuclides K-40, U-238, and Th-232 (KUT) and from man-made contaminants (e.g., Cs-137 and Co-60).  Gamma-ray source concentrations are cited in terms of picocuries per gram (pCi/g), even though this unit technically describes decay rate per unit mass of sample rather than concentration.  The use of decay rate per unit mass is widespread in environmental work, where health and safety issues relate to the radioactivity, not the chemical concentration.

Data are acquired in boreholes near the tanks according to methods described in the logging procedures (DOE 1997b).  Typical counting times used in the T Tank Farm at each measurement position are 200 seconds (s), with a spectrum being collected every 0.5 foot (ft) along the length of the borehole.

Longer data acquisition times can reduce the uncertainties in the calculated concentrations presented on the logs; however, economics and time constraints limit the amount of time available for data collection.  The statistical uncertainty for gamma rays emitted from low- activity radionuclides can be high for this counting time, and the logs will show high levels of statistical uncertainty, as evidenced by scatter in the plotted data and wide confidence intervals.

The 200-s counting time used at the T Tank Farm is twice the counting time used for baseline logging in other tank farms.  Because most T Tank Farm boreholes were double cased with cement grout in the annulus between casings, as discussed in Section 3.1.1, a longer counting time was needed to compensate for the attenuation associated with the second casing and to improve the low-level detection capability for the man-made contaminants.

The minimum detection level (MDL) of a radionuclide represents the lowest concentration at which the positive identification of a gamma-ray peak for that radionuclide is statistically defensible.  The spectrum analysis program calculates the MDL for a particular peak on the basis of a statistical analysis of the spectral background level in the vicinity of the peak.  The same equations that translate peak intensities into decay rates per unit-sample mass also translate the MDLs from counts per second (cps) to picocuries per gram.  A description of the MDL calculation is included in the data analysis manual (DOE 1997a).

The gamma-ray spectra measured in a borehole are processed using a variety of software programs to obtain the concentrations of individual gamma-ray-emitting radionuclides.  All of the algorithms used in the concentration calculations and their application are discussed in the data analysis manual (DOE 1997a).  These calculated data, which are usually presented as vertical profiles, are used to make an interpretation of vadose zone contamination associated with each borehole.  When data from all the boreholes associated with a specific tank have been processed and interpreted, a correlation interpretation is made of the vadose zone contamination surrounding each tank.

The initial SGLS calibration report (DOE 1995a) contains the results obtained from operating the logging tools in calibration models.  The calibration report presents the mathematical functions used to convert the measured peak area count rates to radioelement concentration in picocuries per gram.  The SGLS is routinely recalibrated (DOE 1998) to ensure the accuracy of the calculated radionuclide concentrations.

The calibration data from which conversion factors were derived were recorded with a logging tool in a borehole drilled through a uniform homogeneous isotropic gamma-ray-source material.  If the gamma-ray sources in the borehole being logged are not uniformly distributed in the sediments, the conversion factor produces apparent concentrations.  The concentrations calculated for the top and bottom of a borehole are also apparent concentrations, because the source-to-detector geometries at these locations differ from the source-to-detector geometries during calibration.

When gamma-ray spectra are measured in cased boreholes, a casing correction must be applied to the peak count rates to compensate for gamma-ray attenuation by the casing.  This correction function is described in the calibration report (DOE 1995a), and the data analysis manual (DOE 1997a) describes the application of the correction function in the data processing.

The radionuclide concentration values and the MDL values reported on the logs for the T Tank Farm boreholes contain an additional error that is not present in the logs from other tank farms.  This inaccuracy is caused by the fact that the boreholes contain dual casings and were grouted with an unknown thickness of grout placed between the borehole casings that was forced out into the formation through perforations in the casings.  A correction factor cannot be calculated or applied because the thickness of the grout is unknown and irregular, and the concentration values reflect that inaccuracy.  The maximum potential error caused by the grout is unknown at this time, but could be estimated using nuclear transport modeling codes or measured using a mock-up in a calibration model.

This inaccuracy must be considered when interpreting the log data.  Not only is the reported concentration inaccurate, but there may be vertical variations in the concentration log that do not represent actual formation variations because they are caused variations in grout behind the casing.  Concentrations shown on the log plots are reported as apparent concentrations to reflect the additional degree of uncertainty.  The MDL concentration shown on the logs shows the detection level of the radionuclide relative to the reported apparent concentration of the radionuclide.

The log data are useful for identifying areas of subsurface contamination in spite of this inaccuracy and they still represent a good baseline for future data comparisons.

2.2  Shape Factor Analysis

Insights into the distribution of the radionuclides identified by the SGLS can be provided by using an analytical method known as shape factor analysis (Wilson 1997, 1998).  Shape factor analysis uses the Compton downscattering caused by the interaction of gamma rays with matter between the gamma-ray source and the detector to help determine the location of the source relative to the detector.  Shape factor analysis is not generally applicable to spectral gamma measurements collected from boreholes in the T Tank Farm because of the presence of dual casings and grout in most of the boreholes; the additional thickness of steel and annular grout and the degree of grout penetration into the formation increase the attenuation of the gamma rays and distort the overall spectrum shape.  Annular grout thickness may be highly variable because the two casing strings are not necessarily concentric, and voids may exist.  The degree of grout penetration into the formation is also unknown.  Therefore, the effects of these elements on the various shape factors are not known, and interpretation of shape factors under these conditions is inconclusive.  There were a few boreholes, however, that did not have dual casings and grout or intervals of single casing.  Shape factor analysis performed for these few boreholes yielded interpretable results.

For a more extensive discussion of shape factor analysis as it is applied to log data from other tank farms, the reader is referred to the Tank Summary Data Reports for those farms.

2.3  Log Data and Plots

The results of the processing and analysis of the log data presented in Appendix A, "Spectral Gamma-Ray Logs for Boreholes in the Vicinity of Tank T-111," are grouped into a set of data for each borehole.  Each set includes a Log Data Report and log plots showing either radionuclide concentration versus depth, or apparent radionuclide concentration versus depth.

In the SGLS data, significant attenuation of the peaks associated with naturally occurring KUT was observed as a result of the double casing and grout in the boreholes.  Lower count rates lead to a higher degree of uncertainty, even with an increased counting time of 200 s.  For this reason, radionuclide concentrations are presented as apparent concentrations to reflect the higher degree of uncertainty associated with the effects of the dual casing and grout.

Log plots that show the spatial distribution of the detected man-made radionuclides are presented.  Plots of the natural gamma-ray-emitting radionuclides, at the same vertical scale as the man-made contamination plots, allow for interpretation of geologic information and the correlation of these data with the man-made contamination.  Rerun sections from selected boreholes are used to check the logging system for data acquisition repeatability.

The log plots show either concentrations or apparent concentrations of individual radionuclides or the total gamma count rate in counts per second in each borehole.  Where appropriate, log plots show the statistical uncertainties in the calculated concentrations at the 95-percent confidence level (±2 standard deviations).

A combination plot for each borehole shows the individual natural and man-made radionuclide concentrations or apparent concentrations, the total gamma log, and the Tank Farms gross gamma log.  The total gamma log is a plot of the total number of gamma rays detected during each spectrum measurement.  The combination plot provides information on the relative contributions of individual radionuclides to the total gamma-ray count.  The total gamma log also provides a means for comparing the spectral data with the historical Tank Farms gross gamma log data.

The Tank Farms gross gamma log data were collected with a non-spectral logging system previously used by DOE contractors for leak-detection monitoring at the Hanford Tank Farms.  This system does not identify specific radionuclides, but the logs provide an important historical record for the individual boreholes and offer a basis for temporal comparison.  The gross gamma logs shown on the plots in Appendix A are the latest data available.  When changes in the gross gamma log profiles are identified, plots of those changes over time are also provided.

Rerun sections in selected boreholes are used to check the logging system for data acquisition repeatability and are provided as separate plots.  Radionuclide concentrations or apparent concentrations shown on these plots are calculated independently from the separate gamma-ray spectra provided by the original and repeated logging runs.

The Log Data Report provides borehole construction information, casing information, logging system identification, and data acquisition parameters used for each log run.  A log run is a set of spatially sequential spectra that are recorded in the borehole with the same data acquisition parameters.  A single borehole may have several log runs, often occurring on different days because of the length of time required to log the deeper boreholes.  The Log Data Report also contains analysis information, including analysis notes and log plot notes.


3.0  Review of Tank History

3.1  T Tank Farm

3.1.1  Construction History

The 241-T Tank Farm was the first of the original four tank farms to receive waste.  This tank farm was constructed between 1943 and 1944 to store high-level radioactive waste generated by chemical processing of irradiated uranium fuel at the chemical separation plants.  The tank farm is located in the northern portion of the 200 West Area, north of 23rd Street and west of T Plant.  The T Tank Farm consists of twelve 100-series (75-ft diameter, 530,000-gal capacity) and four 200-series (20-ft diameter, 55,000-gal capacity) single-shell underground waste storage tanks constructed to the first-generation tank design.

The twelve 530,000-gal tanks are designated T-101 through T-112.  The four 55,000-gal tanks are designated T-201 through T-204.  Each tank is constructed of reinforced concrete with a mild steel liner covering the bottoms and sidewalls.  The original SST design specified a 0.25-in.-thick mild steel liner "whose sole function is to provide a liquid-tight container with a design life of 10 years" (Woodrich et al. 1992).

The 100-series tanks are about 75 ft in diameter and about 33 ft high from the bottom of the base to the top of the dome.  The reinforced concrete shells are about 1 ft thick in the sidewalls and dome.  The bases of the tanks are dished with the center about 1 ft lower than the perimeter.  The steel liner covers the bottom and sides to a height of about 18 ft.  The "knuckle," or transition between the sidewall and bottom, is constructed on a 4-ft radius.  The maximum operating depth is about 17 ft.  The concrete dome is unlined and is 13.25 ft above the top of the liner.  The tanks are covered with approximately 7 ft of overburden (Ewer et al. 1997).

The 200-series tanks are about 20 ft in diameter and 25 ft high.  These tanks also have a dished bottom with the center about 0.5 ft lower than the sides.  The 200-series tanks have a 0.25-in. steel liner on the bottom and sides.  The top of the tank is about 12 ft below the ground surface, and a concrete structure, which provides access to the tank, extends from the top of the tank to about 1 ft above grade (Ewer et al. 1997).

The T Tank Farm tanks were designed for non-boiling waste with a maximum fluid temperature of 220 °F (Brevick et al. 1995).  The larger tanks are connected in a cascade series of three tanks in a step configuration.  The tanks are in a three-by-four arrangement, with four cascade series: T-101 to -102 to -103, T-104 to -105 to -106, T-107 to -108 to -109, and T-110 to -111 to -112.  The bottom elevations of the successive tanks in each cascade are approximately 1 ft lower than the preceding tank.  There are also tie lines between the smaller 200-series tanks.  These tie lines are at the same elevation, which allowed the tanks to overflow and equalize tank volumes (Brevick et al. 1995).

In September and October 1944, before the T Tank Farm tanks received any waste, six monitoring boreholes were constructed around the periphery of the tank farm and a seventh monitoring borehole was installed within the interior of the tank farm.  These original boreholes were typically drilled to about 150 ft and completed with telescoped 12-in., 10-in., and 6-in. steel casings.

Additional boreholes were later installed around the 12 tanks for the purpose of externally detecting leaks.  Most of these boreholes were drilled to depths of 80 to 100 ft and completed with 6-in.-diameter casing in the early to mid-1970s.

There are a total of 67 monitoring boreholes associated with the T Tank Farm (Welty 1988).  A plan map showing the relative positions of the tanks in the T Tank Farm and the surrounding vadose zone monitoring boreholes is presented in Figure 2.  Tanks designated as "assumed leakers" are also indicated on Figure 2.

In 1977, a drilling campaign was initiated to remediate boreholes within the T Tank Farm.  The purpose of borehole remediation was to increase the monitoring depth and to eliminate the perceived problem of downward contaminant migration along the borehole casings.  Selected boreholes were deepened to depths of 120 to 125 ft to monitor deeper contaminant plumes originating from leaking tanks.  Borehole deepening was performed by extending the original 6-in. casing to total depth, perforating portions of the 6-in. casing (generally near the upper and lower regions of the borehole), installing a 4-in. casing inside the 6-in. casing, and grouting the annulus between the 4-in. and 6-in. casings.

3.1.2  Geologic Setting

Price and Fecht (1976) provide detailed descriptions of the geologic conditions beneath the T Tank Farm.  Caggiano and Goodwin (1991) provide geologic cross sections and further descriptions of geologic conditions based on a review of historical data and data from newly drilled groundwater monitoring wells.  Lindsey (1993) provides a summary of the geology and hydrogeology of the T Tank Farm based on information from Price and Fecht (1976) and Caggiano and Goodwin (1991) and data from newer RCRA groundwater monitoring wells, surface mapping, and older wells.  This section summarizes data from these documents using the stratigraphic nomenclature proposed in Lindsey (1993).

The three major stratigraphic units present within the vadose zone at the T Tank Farm from top to bottom are 1) the unconsolidated sand, silt, and gravel of the Hanford formation, 2) the early Palouse soil and the Plio-Pleistocene unit, and 3) the semiconsolidated sediments of the Ringold Formation.

The Hanford formation was deposited during the Pleistocene epoch by numerous cataclysmic flooding events caused by periodic ruptures of ice dams holding back large glacial lakes.  The Hanford formation sediments consist of coarse to very coarse gravel, fine- to coarse-grained sand, and silt.  Three distinct facies have been recognized by Lindsey (1993): gravel-dominated, sand-dominated, and silt-dominated (ordered from top to bottom of the Hanford formation).

In the vicinity of the T Tank Farm, the upper portion of the Hanford formation consists of gravelly deposits typical of the gravel-dominated facies of the Hanford formation.  The upper 38 ft of this material in the immediate vicinity of the tank farm has been excavated and redeposited as backfill material around the tanks.  Much material is poorly sorted and consists predominantly of cobbles, pebbles, and coarse to medium sand with silt.  The backfill extends from the ground surface to a depth of about 38 ft (i.e., from the surface elevation of about 671 ft to an elevation of about 633 ft above mean sea level) (Price and Fecht 1976).  A sand-dominated interval of the Hanford formation primarily composed of very coarse to medium sand with some pebbles occurs beneath the backfill material (Price and Fecht 1976).  Generally, this interval tends to become finer grained in the downward direction with the frequency of silt beds increasing.  The base of this unit lies about 75 to 100 ft below the ground surface (Lindsey 1993).  The contact appears to be irregular and generally slopes to the west and east.

Clastic injection dikes are commonly found in the Hanford formation.  These clastic injection dikes consist of thin, alternating vertical to subvertical layers of silt, sand, and granules that generally cross-cut bedding.  Clastic injection dikes are common in the vadose zone beneath the Hanford Site, although they are difficult to detect.  Where present, clastic injection dikes can act as barriers or pathways to fluid transport, depending on the content of the dike and the type of sediment through which it passes (Lindsey 1993).

Locally occurring strata separating the Pleistocene-aged Hanford formation and the Miocene- to Pliocene-aged Ringold Formation are informally defined as the early Palouse soil, the pre-Missoula gravels, and the Plio-Pleistocene unit.  The pre-Missoula gravels are not present in the vicinity of the T Tank Farm and will not be discussed further.

The Palouse soil and the Plio-Pleistocene unit contain similar sedimentary textures and are often difficult to differentiate on the basis of texture alone.  Available data from wells in the vicinity of the T Tank Farm are generally inadequate to distinguish between these units (Caggiano and Goodwin 1991).  However, a deep exploratory borehole (50-06-18) was drilled adjacent to tank T-106 in 1993.  On the basis of detailed lithologic information derived from the drilling log of this borehole, Freeman-Pollard et al. (1994) have identified and determined the local depth and thickness of each unit in this area.  This information is included in the description of each unit.

The early Palouse soil horizon is encountered at a depth of about 80 ft and is approximately 10 ft thick in the vicinity of tank T-106 (Freeman-Pollard et al. 1994).  This unit consists of massive, brown-yellow and compact, loess-like silt and minor fine-grained sand (Lindsey and Horton 1991).  The upper contact of the unit is poorly defined, and it may grade up-section into silty strata commonly found in the lower portion of the sand-dominated facies of the Hanford formation.  Magnetic polarity data indicate the unit to be early Pleistocene in age (Lindsey and Horton 1991).

The Plio-Pleistocene unit underlies the early "Palouse" soil and extends from about 91 to 105 ft in the vicinity of tank T-106 (Freeman-Pollard et al. 1994).  This laterally discontinuous unit is divided into the sidestream alluvium facies and the calcic paleosol facies (Lindsey and Horton 1991).  These facies are inferred to be late Pliocene to early Pleistocene in age on the basis of stratigraphic position and the magnetic polarity of interfingering loess units.  Weathered and unweathered basaltic gravels dominate the sidestream alluvium facies.  The calcic paleosol facies consist of massive calcium-carbonate-cemented silt, sand, and gravel (caliche) to interbedded caliche-rich and caliche-poor silts and sands.  The caliches are moderately to highly fractured.  Two prominent caliche layers are found within the Plio-Pleistocene unit below the T Tank Farm.  The upper layer occurs from about 91 to 93 ft and the lower layer occurs from about 100 to 103 ft.  The lower caliche layer is better developed than the upper caliche layer (Price and Fecht 1976).

The Plio-Pleistocene unit unconformably overlies the Ringold Formation.  Beneath the T Tank Farm, the Ringold Formation consists of both the upper unit and Unit E.  The upper unit consists of up to 25 ft of coarse- to fine-grained sand with lenses of interbedded silt.  The upper unit may pinch out near the southeast corner of the T Tank Farm.  The Ringold Unit E underlies the Ringold upper unit, where present, or the Plio-Pleistocene unit.  The Ringold Unit E consists of fluvial gravels with muddy zones and cemented zones above the water table that may form local perched water conditions (Lindsey 1993).  Beneath the T Tank Farm, the water table occurs in the gravels of the Ringold Unit E at a depth of about 218 ft (Hodges 1998).

Groundwater flow directions in the vicinity of the T Tank Farm have varied over time as a result of the influence of various effluent discharge sites in the 200 West Area.  Before the start of Hanford operations, groundwater flow was predominantly east or northeast, driven by recharge in the Cold Creek Valley and topographically high areas along the western boundary of the Pasco Basin.  During the late 1940s and early 1950s, groundwater flow was affected by discharges to the T Pond, located northwest of the T Tank Farm.  Groundwater mounding beneath T Pond resulted in a gradient to the south or southeast.  With the closure of T Plant processing operations in 1956, discharges to the T Pond decreased and other recharge sources (U Pond and Z Cribs) became more important, such that flow direction changed to the north or northeast (Hodges 1998).

Presently, groundwater flow is generally in a northeast direction and is controlled by the U Pond groundwater mound to the south.  The U Pond groundwater mound has been dissipating since the U Pond was deactivated in 1984.  A comparison of groundwater levels indicates that the groundwater table dropped more than 5 ft from 1985 to 1990.  The June 1990 water level in the vicinity of the T Tank Farm is reported as 464 ft above mean sea level (a depth of about 207 ft) (Caggiano and Goodwin 1991).  The June 1997 water table beneath the T Tank Farm was reported at an approximate elevation of 453 ft (a depth of about 218 ft) (Hodges 1998).  The groundwater flow direction under the T Tank Farm was primarily to the north when the groundwater mound developed beneath the U Pond.  As the mound dissipated following decommissioning of the U Pond in 1985, the groundwater flow direction shifted from the north to the northeast.  With further decline of the recharge mound, the groundwater flow direction is expected to shift toward the regional west to east direction (PNNL 1998).

3.1.3  Tank Contents

The T Tank Farm began receiving waste from T Plant processing operations in December 1944.  Three main waste types were produced by the bismuth-phosphate process used in T Plant until 1956.  The first waste type was metal waste and was produced when irradiated uranium fuel elements were dissolved in acid and plutonium was extracted.  The metal waste contained all of the uranium from the irradiated uranium fuel elements and 90 percent of the fission products (i.e., no effort was made to extract the uranium or fission products from the waste and they were disposed to the tanks) (Brevick et al. 1995).  Two decontamination cycles were performed to purify the plutonium product once the plutonium was extracted by the bismuth-phosphate process.  ("Decontamination" is used to describe the removal of contaminants from the plutonium product.)  The first decontamination cycle produced a waste stream (known as first- cycle decontamination waste) that contained almost 10 percent of the fission products from the original irradiated uranium fuel elements.  The second decontamination cycle produced a waste stream (known as second-cycle decontamination waste) that contained about 0.1 percent of the fission products from the original irradiated uranium fuel elements (DOE 1993).

The first cascade series (tanks T-101, -102, and -103) received metal waste and was full by February 1946.  The second cascade series (tanks T-104, -105, and -106) began receiving second-cycle decontamination waste in early 1946 and was full by August 1946.  The third cascade series (tanks T-107, -108, and -109) began receiving first-cycle decontamination waste in 1945 and was full by March 1946.  The remaining cascade series (tanks T-110, -111, and - 112) began receiving second-cycle decontamination waste in 1945 and was full by July 1946 (Brevick et al. 1995).

Waste was pumped from tanks T-107, -108, and -109 in 1951 and 1952 to create storage space for tributyl phosphate (TBP) waste.  TBP waste was produced at U Plant during the reprocessing of metal waste with TBP to recover uranium.  Tanks T-107, -108, and -109 began receiving TBP waste in 1952 and were full by January 1953 (Brevick et al. 1995).

Also during 1952, 224-Waste was added to tanks T-110, -111, -112, -201, -202, -203, and -204. 224-Waste was produced by the bismuth-phosphate process used to extract plutonium at the 224-Building of B Plant.  Because tanks T-110, -111, and -112 were already full, supernatant liquid cascaded to tank T-111 and then to tank T-112 as the 224-Waste was added to tank T-110.  The supernatant liquid was eventually pumped from tank T-112 to a crib (Brevick et al. 1995).

Additional first-cycle decontamination waste from the bismuth-phosphate process was added to the third cascade series (tank T-107, -108, and -109) in 1953.  Metal waste was added to the first cascade series (tanks T-101, -102, and -103) in 1955 (Brevick et al. 1995).

In 1956, tanks T-101, -102, and -103 were sluiced to make room for additional metal waste from T Plant and future coating waste from the Reduction and Oxidation Extraction Facility (REDOX).  REDOX was built in 1950 and 1951 and used methyl-isobutyl ketone (also known as hexone) to extract uranium and plutonium from irradiated fuel slugs.

In 1956, tanks T-105 and T-106 received coating waste from REDOX and tank T-108 received evaporator bottoms transferred from tank TX-117 (TX Tank Farm).  Coating waste was produced by the dissolution of aluminum and zircaloy cladding from the fuel elements before processing.  Evaporator bottoms were produced at the 242-T Evaporator, which processed tank wastes to reduce waste volumes and reclaim tank space for additional wastes (Brevick et al. 1995).

In 1957, evaporator bottoms waste from the 242-T Evaporator was transferred to tank T-109 from tank TX-117.  In 1965, a cross-site transfer of coating waste from tank C-102 (C Tank Farm) was received by tanks T-102 and T-103.  In 1967, coating waste was transferred to tank T-107 from tank C-102 (Brevick et al. 1995).  In 1967, first-cycle decontamination waste from tanks T-105, -107, -108, and -112 was transferred to tank TX-118 for processing at the 242-T Evaporator.

Tanks T-101 and T-105 received B Plant ion-exchange waste from tanks BX-101 and BX-104 (BX Tank Farm) in 1972 (Brevick et al. 1995).  B Plant ion-exchange waste was produced at the 221-B Building by passing supernatant and sluicing liquids through ion-exchange columns to remove cesium (DOE 1993).  Additional B Plant ion-exchange waste was transferred to tank T-107 from tank BX-104 in 1973 (Brevick et al. 1995).

In 1974, tanks T-101, -102, and -105 received coating waste and ion-exchange waste from tank S-110 (S Tank Farm).  The final recorded activity in the T Tank Farm was receipt of coating waste from tank SX-106 (SX Tank Farm) into tank T-101 in 1975 (Brevick et al. 1995).

Currently, the T Tank Farm is estimated to contain 1,888,000 gal of waste, which includes 1,860,000 gal of sludge and 28,000 gal of supernatant liquid.  The sludge contains an estimated 183,000 gal of drainable, interstitial liquid (Hanlon 1998).

The evaluation of leaks from tanks and transfer lines in the T Tank Farm is complicated by the presence of deliberate discharges of tank wastes to the vadose zone in cribs and trenches adjacent to the tank farm.  These discharges are documented in Waite (1991).  A total of about 38 million gal was intentionally discharged to cribs and trenches in the immediate vicinity of the T Tank Farm.  A few examples of these discharges include:

3.1.4  Tank Farm Status

All of the tanks in T Tank Farm are out of service.  Tanks T-102, -104, -105, -110, -112, -201, -202, -203, and -204 are categorized as sound.  Tanks T-101, -103, -106, -107, -108, -109, and -111 are categorized as "assumed leakers."  These assumed leaker tanks have leaked an estimated combined total of approximately 134,500 gal (Hanlon 1998).  This estimated leak volume includes an estimated 115,000-gal leak from tank T-106 in 1973.  The estimated leak volume for tank T-106 is based on measured liquid-level decreases and is considered fairly accurate; however, the leak estimates for the other tanks are considered questionable.

Tanks T-101, -102, -103, -105, -106, -107, -108, -109, -111, -112, -201, -202, -203, and -204 have been interim stabilized (Hanlon 1998).  Tank T-110 is on the watch list for hydrogen, and tank T-111 is on the watch list for organics (Hanlon 1998).  Tanks T-101 and T-107 were placed on the Ferrocyanide Watch List, but were removed from the list in 1993 and 1996, respectively (Hanlon 1998).

The T Tank Farm was scheduled to achieve controlled, clean, and stable (CCS) status by June 30, 1997; however, these activities have been deferred until funding is available (Hanlon 1998).

In November 1992, groundwater assessment monitoring was triggered in the T Tank Farm area because of high specific conductance in downgradient well 299-W10-15.  Well 299-W10-15 is located approximately 106 ft north of tank T-103.  In late 1995, specific conductance also increased rapidly in well 299-W11-27, exceeding the critical mean in the August 1996 sample.  Well 299-W11-27 is located approximately 150 ft northeast of tank T-101.  These increases in specific conductance were accompanied by concentration increases of technetium-99 (Tc-99) and other co-contaminants, including Co-60 (PNNL 1998).

An assessment of groundwater monitoring data for Waste Management Area (WMA) T concluded "with a high degree of certainty," that the source of the groundwater contamination detected in well 299-W11-27 is WMA-T (PNNL 1998).  WMA-T consists of the T Tank Farm and several other waste management facilities in the immediate vicinity of the tanks.  The assessment also concluded that the increase in specific conductance detected at well 299-W10-15, which triggered the assessment, was from the effects of a larger sodium-nitrate- tritium plume located in the area, not from "a present day tank farm source" (PNNL 1998).

The rapidly declining water levels in the area of the T Tank Farm complicate the assessment of groundwater contamination.  All of the wells in the RCRA monitoring network for WMA-T will probably be dry by the end of 1998 (PNNL 1998).

3.2  Tank T-111

Tank T-111 was constructed during 1943 and 1944 and was placed into service in 1945 (Welty 1988).  This tank is the second tank in a cascade series with tanks T-110 and T-112.  Tanks T-110, -111, and -112 were used extensively as settling tanks for suspended solids from processes that took place at the T Plant Processing Building.  In October 1945, tank T-111 began receiving second-cycle decontamination (2C) waste from the T Plant through the cascade overflow line running from tank T-110.  2C waste was part of T Plant's bismuth-phosphate process for separating plutonium from irradiated fuel rods.  The bismuth-phosphate process resulted in a waste stream that contained about 0.1 percent of the fission products from the original irradiated uranium fuel elements and about 1 percent of plutonium (Anderson 1990).  By early 1946, tanks T-110 and T-111 were full, and the 2C supernatant waste was cascading into tank T-112.  Cribbing of 2C waste started at the T Plant in September 1947, which resulted in significant waste-volume reductions.  After settling the 2C waste stored in the underground storage tanks, the supernatant was pumped directly into cribs.  The remaining solids, which contained nearly all of the initial fission activity but only a fraction (9 percent) of the original volume, were held in storage (Anderson 1990).  Between 1947 and 1952, all three tanks were cascading and discharging the 2C supernatant waste directly to the soil column through cribs and trenches for disposal (DOE 1992).

In late 1952, tank T-111 began receiving a second type of waste stream from the T-224 Bulk Reduction Building (224 waste).  This waste was part of the lanthanum-fluoride process used to concentrate and purify dilute plutonium solutions.  The fission activity of the 224 waste was low enough to permit ground disposal (less than 0.001 percent of that in the starting metal) (Anderson 1990).

Between 1945 and 1956, approximately 20,240,000 gal of 2C and 224-waste supernatant liquid cascaded between tanks T-110, -111, and -112, which left tank T-111 full of semi-solid sludge.  All of the supernatant that cascaded from tank T-111 to T-112 was discharged to the soil column through cribs, trenches, and other facilities for disposal.

T Plant was deactivated in 1956, concurrent with the phase-out of the bismuth-phosphate plutonium extraction process at the Hanford plants (DOE 1992).  This date coincides with a dramatic decrease in the primary waste stream volume cascading in and out of tank T-111.  Between 1956 and 1974, tank T-111 was full, but the tank continued to receive small quantities of waste that cascaded into tank T-112 for disposal.  An estimated historical total-traffic volume for tank T-111 is 21,963,000 gal, which is based on various T Plant processes and transfers from other tanks (Agnew et al. 1995).

Starting in mid-1973, an unexplained in-tank liquid-level decrease was noted that continued into early 1974.  Surface measurements slowly decreased by 0.30 in. over this time period.  Tank T-111 was declared as questionable integrity and removed from service on the basis of the observed in-tank liquid-level decline (Welty 1988); the total leak-volume estimate was less than 1,000 gal (Hanlon 1998).  Later that same year, approximately 42,000 gal of supernatant was pumped from tank T-111 to other tanks, thus stabilizing the liquid level at about 172 in. (Agnew et al. 1995).  Between 1974 and 1975, four new vadose monitoring wells were installed around tank T-111 (boreholes 50-11-05, 50-11-08, 50-11-07, and 50-11-10).  Between 1976 and 1978, salt-well pumping transferred approximately 13,000 gal of supernatant to tank T-101 (Agnew et al. 1995); this operation decreased the liquid level to about 162 in. (Welty 1988).

In 1984, several criteria designations, such as "questionable integrity," were merged into the category now reported as assumed leaker.  Tank T-111 was partially isolated in 1982 and declared an assumed leaker in 1984 (Brevick et al. 1995), although the suspected leak occurred in 1974.

In 1994, tank T-111 was declared an assumed re-leaker because of a decreasing trend in the liquid-level measurements (Tucker 1994).  From 1984 to January 1993, the in-tank liquid level (measured by an automatic FIC gauge) showed an increasing trend that leveled off at approximately 0.6 in. above the baseline (161 in.).  Between January 1993 and March 1994, the in-tank liquid level fell 1 in. below the baseline.  Additional measurement readings acquired in the liquid observation well (LOW) also seemed to decrease during this time period, although an exact value could not be determined (Tucker 1994).  On the basis of the observed in-tank liquid-level decrease, the volume of the tank leak was estimated to be less than 1,000 gal (Hanlon 1998).

On February 22, 1995, tank T-111 was declared interim stabilized after jet-pumping operations removed approximately 9,600 gal of supernatant liquid (Hanlon 1998).  Tank T-111 was placed on the Organic Watch List after a core sample identified a reactive component (exotherm) (Tucker 1994).

Tank T-111 presently contains 446,000 gal of non-complexed, semi-solid sludge with 29,000 gal of pumpable liquids (Hanlon 1998).  A LOW monitors the interstitial liquid level and is the primary method of leak detection (Hanlon 1998).  The FIC gauge was replaced with a manual ENRAF surface level measuring device in 1995.  Photographs from 1995 show the tank was nearly filled with sludge.  The sludge surface slopes slightly from the sidewall to the middle of the tank; the surface is flagstone-like, cracked, slightly bumpy, and has a liquid pool around a salt well.  The waste appears to be medium brown and has some craters that probably resulted from equipment removal.  Corrosion of the tank liner is indicated by the rust on the sludge around the tank perimeter (Brevick et al. 1995).


4.0  Boreholes in the Vicinity of Tank T-111

Ten vadose zone monitoring boreholes surround tank T-111.  These boreholes are 50-08-05, 50-10-10, 50-10-08, 50-00-06, 50-11-05, 50-11-07, 50-11-08, 50-11-10, 50-11-11, and 50-08-07.  The locations of these boreholes are shown in red on Figure 2.

The construction details vary from borehole to borehole and could not always be determined from the available records.  Many of the boreholes are double cased.  Shape factor analysis could not be applied to the spectral gamma data obtained from the boreholes surrounding tank T-111 because of the presence of multiple casings and the unknown effects of grout.  Construction specifics are provided, to the extent known, for the individual boreholes in the following sections and on the Log Data Reports included with Appendix A.

The T Tank Farm boreholes were modified in the early 1980s.  A knifing tool was lowered into the 6-in. casing, and the casing was perforated in the upper 20 ft and the lower 2 ft of each borehole.  A 4-in. casing was then placed inside the 6-in. casing and grout was pumped into the annulus.  The current configuration of the T Tank Farm boreholes is two steel casings with at least 1 in. of grout between the casings.  On the basis of the published thickness for schedule-40, carbon-steel casing, which was typically used for casing during the 1970s, the 4-in.-diameter casing thickness is assumed to be 0.237 in. and the 6-in.-diameter casing thickness is assumed to be 0.280 in.  A casing correction factor for a 0.5-in.-thick steel casing (the approximate thickness of the two casings) seemed to produce acceptable radioassay values.  Because this factor is slightly less than the combined thickness of the two casings (0.52 in.) and because of the unknown attenuation of grout between the two casings, the calculated radionuclide concentrations are probably only slightly underestimated.  It is unlikely that the double casings and grout have completely obscured the gamma rays from man-made or natural radionuclides in the vicinity of the boreholes.  Therefore, the intent of the logging operation for the T Tank Farm is to provide a relative radionuclide concentration baseline that can be used to compare future log data to detect and quantify changes.  Reported radionuclide concentrations shown on the logs in Appendix A are only apparent concentrations.

All of the boreholes around tank T-111 are completed above the water table, but were filled with various levels of terrestrial water (standing water) at the time they were logged.

Radionuclide concentrations were calculated from the SGLS logs using the radioassay techniques described in Section 2.1.

The following sections present results of the spectral gamma-ray logging of the boreholes surrounding tank T-111.  Appendix A contains the plots of the SGLS log data.  The most recent historical gross gamma data for each borehole are included on the combination plots in Appendix A.

4.1  Borehole 50-08-05

Borehole 50-08-05 is located approximately 12 ft northeast of tank T-111 and is also known by the Hanford Site designation 299-W10-143.  This borehole is associated with tank T-108, but is sufficiently close to tank T-111 to be useful in the characterization of the vadose zone in the vicinity of this tank.  Information concerning the construction of this borehole is provided in the drilling log and Chamness and Merz (1993).  Borehole 50-08-05 was drilled in March 1974 and completed to a depth of 94 ft using 6-in.-diameter casing.  In August 1980, the original 6-in. casing was perforated from 0 to 20 ft and 91 to 93 ft.  A 4-in. casing was placed inside the 6-in. casing and 109 gal of grout was pumped into the annulus.

It was not possible to calculate accurate radionuclide concentrations with this borehole configuration because correction factors for the double casing with annular grout are beyond the current system calibrations.  A 0.50-in. casing correction factor was used for data reduction, but a correction was not applied for the annular grout.  Therefore, the radionuclide concentrations identified are only apparent concentrations and should be considered underestimated.

Depth measurements were referenced to the top of the 4-in. casing, which is even with the ground surface.  A water level indicator identified standing water at 86.7 ft.  The total logging depth achieved by the SGLS was 90.0 ft.

The only man-made radionuclide detected in this borehole was Cs-137.  Cs-137 contamination was detected continuously from the ground surface to 2.5 ft at concentrations ranging from 0.2 to 1 pCi/g.  Cs-137 contamination occurs almost continuously from 11 to 18 ft at concentrations ranging from 0.2 to 0.4 pCi/g.  An isolated occurrence of Cs-137 contamination was detected at 79 ft at 0.4 pCi/g.  The maximum Cs-137 concentration for this borehole was 1 pCi/g at 0.5 ft.

The plot of the naturally occurring radionuclides shows the K-40 concentrations are about 11 pCi/g from the ground surface to a depth of 38 ft and increase to about 14 pCi/g between 38 and 53 ft.  Below the 53-ft depth, the K-40 concentrations decrease sharply to about 11 pCi/g and then steadily increase to 14 pCi/g at the bottom of the logged interval (90 ft).  K-40 and Th-232 concentrations increase at 71 ft.  The Th-232 and U-238 concentrations increase below the 83-ft depth.

The increase in the K-40 concentrations below 38 ft probably represents the boundary between the backfill material and the undisturbed sediments of the Hanford formation.  The increase in K- 40 concentrations between 38 and 53 ft may represent finer grained sediments.  The increase in the K-40 and Th-232 concentrations at 71 ft may represent a lithologic change in the Hanford formation.  This thin bed correlates with similar strata observed with Co-60 contamination in other nearby boreholes (50-11-11 and 50-08-07).  Below 84 ft, the increase in the U-238 and Th-232 concentrations probably represents the basal contact between the Hanford formation and the early Palouse soil.  This feature is observed in the same depth region in nearby boreholes, indicating that the feature is continuous and correlatable.  The drilling log reports mostly sand above 85 ft and mostly silt between 85 and 92 ft, confirming that the base of the Hanford formation and early Palouse soil occurs in this depth region.

The SGLS total gamma-ray plot reflects the influence of the Cs-137 contamination detected at the top of the logged interval and the variations in the naturally occurring radionuclides throughout the borehole.  The SGLS total count-rate profile clearly shows the contact between the backfill and the Hanford formation at 38 ft, the contact between the Hanford formation and the early Palouse soil at 84 ft, and a thin zone of elevated K-40 and Th-232 concentrations at a depth of 71 ft.

The historical gross gamma log data from 1974 to 1994 and the data summaries in Welty (1988) were reviewed.  The most recently acquired historical gross gamma data (February 17, 1994) are presented on the combination plot.  This profile generally corresponds to the SGLS total count- rate profile, but is much less sensitive.  Indications of anomalous gamma-ray activity in the upper 3 ft of the borehole are not apparent on this profile; however, gross gamma-ray data from the upper 2 to 5 ft of the boreholes were not always recorded on the historical gross gamma-ray logs.

The earlier gross gamma-ray data generally tend to follow the patterns evident in the historical gross gamma log included on the combination plot.  There is no indication of anomalous activity in the earlier historical gross gamma-ray logs.

The Cs-137 contamination detected between the ground surface and 3 ft probably resulted from a surface spill that migrated down into the shallow backfill surrounding the borehole.  The Cs-137 contamination detected between 11 and 18 ft and at the bottom of the borehole was probably carried down from the near-surface contaminated zone during the borehole construction or may be the result of statistical noise.

4.2  Borehole 50-10-10

Borehole 50-10-10 is located approximately 16 ft northeast of tank T-111 and is also known by the Hanford Site designation 299-W10-137.  This borehole is associated with tank T-110, but is sufficiently close to tank T-111 to be useful in the characterization of the vadose zone in the vicinity of this tank.  Information concerning the construction of this borehole is provided in the drilling log and Chamness and Merz (1993).  This borehole was originally drilled in February 1974 and completed to a depth of 94 ft using 6-in.-diameter casing.  In February 1981, the original 6-in. casing was perforated from 0 to 20 ft and 92 to 94 ft.  A 4-in. casing was placed inside the 6-in. casing and 109 gal of grout was pumped into the annulus.

It was not possible to calculate accurate radionuclide concentrations with this borehole configuration because correction factors for the double casing with annular grout are beyond the current system calibrations.  A 0.650-in. casing correction factor was used for data reduction, but a correction was not applied for the annular grout.  Therefore, the radionuclide concentrations identified are only apparent concentrations and should be considered underestimated.

Borehole 50-10-10 was logged in 1995 at the start of the Hanford Tank Farms Vadose Zone project.  Data acquisition parameters for this borehole differ from those of the other boreholes in the T Tank Farm.  This borehole was logged with the SGLS operating in continuous logging mode with a 200-s counting time while moving at 0.3 feet per minute (ft/min).  This method corresponds to a spatial data-acquisition interval of 1 ft.  Depth measurements were referenced to the top of the 4-in. casing, which is even with the ground surface.  A water level indicator showed standing water at 88.8 ft.  The total logging depth achieved by the SGLS was 90.0 ft.

Cs-137 was the only man-made radionuclide detected in this borehole.  Cs-137 contamination was detected continuously from 0.5 to 2.5 ft at concentrations that ranged from 0.2 to 2.5 pCi/g, and intermittently from 6.5 to 74.5 ft at concentrations ranging from 0.2 to 0.4 pCi/g.  The maximum  Cs-137 concentration of 2.5 pCi/g was measured at 1.5 ft.

The plot of the naturally occurring radionuclides shows K-40 concentrations varying between 9 and 15 pCi/g from the ground surface to 39 ft.  The K-40 concentrations increase to 21 pCi/g from 39 to 41 ft and decrease to about 19 pCi/g from 46 to 48 ft, varying between 15 and 19 pCi/g from 55 to 80 ft.  From 80 to 88 ft, K-40 concentrations range between 18 and 20 pCi/g, increasing to about 25 pCi/g below a depth of 88 ft.  U-238 and Th-232 concentrations generally vary between 0.5 and 1.0 pCi/g throughout most of the borehole.  Below 84 ft, concentrations increase perceptibly to between 1.0 and 1.5 pCi/g.

The variability of the naturally occurring radionuclides within the backfilled portion of the borehole (0 to 39 ft) is attributable to the presence of grout and normal variations in the backfill sediment.  The increase in the K-40 concentrations below 39 ft probably represents the transition between the backfill material and the undisturbed sediments of the Hanford formation.  The higher K-40 concentrations between 38 and 55 ft probably indicate finer grained sediments.  The increase in U-238 and Th-232 concentrations below 84 ft probably represents a lithology change and the contact between the basal units of the Hanford formation and the early Palouse soil.  The drilling log reports that mostly sand was detected above 85 ft and mostly silt was detected from 85 to 93 ft, confirming that the base of the Hanford formation and the early Palouse soil occur in this depth region.

A comparison of the KUT logs reveals that a significant portion of the lithologic detail for this borehole is not as clearly defined as it is for other boreholes in the vicinity of this tank.  This is the result of a shorter counting time and a lower spatial resolution.

The SGLS total gamma-ray plot reflects the influence of the Cs-137 contamination in the upper 2 ft of the borehole and the variations in the naturally occurring radionuclides.  The SGLS total count-rate profile closely follows the K-40 concentration profile throughout most of the borehole. The increase in the Th-232 and U-238 concentrations associated with the contact between the basal units of the Hanford formation and the early Palouse soil at 82 ft is clearly evident on the SGLS total count-rate profile.

The historical gross gamma log data from 1975 to 1994 and the log data summaries presented in Welty (1988) were reviewed.  The most recently acquired historical gross gamma log (January 1, 1993) is presented on the combination plot.  This profile generally corresponds to the SGLS total count-rate profile, but is much less sensitive.  A review of the historical gross gamma log data shows no evidence of anomalous activity from this borehole dating back to 1974.

Low levels of Cs-137 contamination were detected around this borehole.  The Cs-137 contamination detected between the ground surface and 3 ft probably resulted from a surface spill that migrated down into the shallow backfill surrounding the borehole.  The Cs-137 contamination detected discontinuously between 8 and 75 ft was probably carried downward from the near-surface contaminated zone during the borehole installation activity.  Below the 3-ft depth, the Cs-137 contamination shows a significant amount of statistical noise.

4.3  Borehole 50-10-08

Borehole 50-10-08 is located approximately 16 ft southeast of tank T-111 and is also known by the Hanford Site designation 299-W10-151.  This borehole is associated with tank T-110, but is sufficiently close to tank T-111 to be useful in the characterization of the vadose zone in the vicinity of this tank.  Information concerning the construction of this borehole is provided in the drilling log and in Chamness and Merz (1993).  This borehole was originally drilled in March 1975 and completed to a depth of 93 ft using 6-in.-diameter casing.  In February 1981, the original 6-in. casing was perforated from 0 to 20 ft and 91 to 93 ft.  A 4-in. casing was placed inside the 6-in. casing and 109 gal of grout was added to the annulus.

It was not possible to calculate accurate radionuclide concentrations with this borehole configuration because correction factors for the double casing with annular grout are beyond the current system calibrations.  A 0.650-in. casing correction factor was used for data reduction, but a correction was not applied for the annular grout.  Therefore, the radionuclide concentrations identified are only apparent concentrations and should be considered underestimated.

Borehole 50-10-08 was logged in 1995 at the start of the Hanford Tank Farms Vadose Zone program.  Data acquisition parameters for this borehole differ from those of other boreholes in the T Tank Farm.  This borehole was logged with the SGLS operating in continuous logging mode with a 200-s counting time while moving at 0.3 ft/min.  This method corresponds to a spatial data acquisition interval of 1 ft.  Depth measurements were referenced to the top of the 4-in. casing, which is even with the ground surface.  The bottom 1.5 ft of the borehole contains standing water.  The total logging depth achieved by the SGLS was 91.0 ft.

The only man-made radionuclide detected in this borehole was Cs-137.  Cs-137 contamination was detected continuously from 1.5 to 2.5 ft at concentrations ranging from 0.2 to 0.25 pCi/g.

The plot of the naturally occurring radionuclides shows irregular K-40 concentrations between the ground surface and 40 ft ranging from 10 to 15 pCi/g.  The K-40 concentrations increase to about 18 pCi/g between 40 and 55 ft and decrease to about 14 pCi/g below 55 ft.  The K-40 concentrations gradually increase to about 19 pCi/g at the bottom of the logged interval (91 ft).  U-238 and Th-232 concentrations increase perceptibly below 85 ft.

The increase in the K-40 concentrations below the 40-ft depth probably represents the transition between the backfill material and the undisturbed sediments of the Hanford formation.  The higher K-40 concentrations between 40 and 55 ft probably indicate finer grained sediments.  The increase in the U-238 and Th-232 concentrations below the 85-ft depth probably represents a change in lithology and the contact between the basal units of the Hanford formation and the early Palouse soil.  The drilling log reports that mostly sand is found above 84 ft and mostly silt is found between 84 and 93 ft, confirming that the Hanford formation and the early Palouse soil occur in this depth region.

A comparison of the KUT logs reveals that a significant portion of the lithologic detail for this borehole is not as clearly defined as it is for other boreholes in the vicinity of this tank.  This is the result of a shorter counting time and a lower spatial resolution.

The SGLS total gamma-ray plot reflects the influence of the Cs-137 contamination in the upper 2 ft of the borehole.  The variations in the naturally occurring radionuclides, particularly K-40 concentrations, throughout the borehole are evident on the total gamma-ray plot profile.  The increase in the Th-232 and U-238 concentrations associated with the contact between the basal units of the Hanford formation and the early Palouse soil at 84 ft is also clearly evident on the SGLS total count-rate profile.

The historical gross gamma log data from 1974 to 1994 and the log data summaries presented in Welty (1988) were reviewed.  The most recently acquired historical gross gamma log (November 1, 1993) is presented on the combination plot.  This profile generally corresponds to the SGLS total count-rate profile, but is much less sensitive.  A review of the historical gross gamma log data shows no evidence of anomalous activity from this borehole dating back to 1974.

Only low concentrations of Cs-137 contamination were detected near the ground surface in this borehole.  The Cs-137 contamination is probably from a surface spill that migrated down into the shallow backfill surrounding the borehole or may be the result of statistical noise.

4.4  Borehole 50-00-06

Borehole 50-00-06 is located approximately 43 ft southeast of tank T-111 and is also known by the Hanford Site designation 299-W10-55.  Information concerning the construction of this borehole is provided in the drilling log and Chamness and Merz (1993).  In addition, Welty (1988) mentions modifications to the borehole that may have been performed in 1973, but the information regarding these modifications was not available.

Records show that this borehole was drilled in September 1944, concurrent with tank farm construction, and completed at a depth of 150 ft.  The borehole was constructed with a 12-in.- diameter casing from the ground surface to about 100 ft and was extended to 150 ft with 6-in.-diameter casing.  Drilling records indicate that a cement (grout) plug was placed at the bottom of the 6-in.-diameter casing and the 6-in. casing was cut off at 90 ft.  The portion of the 6-in. casing between the ground surface and 90 ft was removed.  The 12-in. casing is perforated between 50 and 90 ft and the 6-in. casing is perforated between 90 and 150 ft.

Although the casing sizes referenced in the drilling records can be partially confirmed by field observations, the exact configuration for this borehole cannot be reconstructed.  The logging engineers report that a 5-in.-diameter casing is observed at the ground surface.  It is apparent that this borehole was modified, possibly in 1973.  Welty (1988) mentions that this borehole was a "new" borehole in 1973, when the 5-in.-diameter casing was placed inside the 12-in. and 6-in. casings.  There is no information regarding how deep the 5-in.-diameter casing extends, but for this report it is assumed to be the total depth of the borehole (150 ft).  Grout is also assumed to be present, because it was standard practice in the T Tank Farm to grout the annular space between the inner and outer casings after a borehole was modified.  Because of the uncertainties in the borehole construction, log data for this borehole were processed on the assumption that the borehole is double cased throughout its length, with 12-in. and 5-in. casings from the ground surface to 103 ft and 6-in. and 5-in. casings from 103 ft to the bottom of the logged interval (147.5 ft).

It was not possible to calculate accurate radionuclide concentrations with this borehole configuration because correction factors for the double casing with annular grout are beyond the current system calibrations.  Radioassays were calculated using a correction factor based on the known thicknesses of the 12-in., 6-in., and 5-in. casings.  A casing correction factor for a 0.650-in.-thick steel casing was used from 0 to 103 ft and a factor for a 0.50-in. steel casing was used from 103 to 147.5 ft.  A correction factor was not applied for the annular grout.  Use of these casing correction factors causes the calculated radionuclide concentrations to be greater than in other boreholes associated with tank T-111; therefore, the identified radionuclide concentrations are only apparent concentrations and should be underestimated.

The top of the 5-in. casing is the zero reference point for the SGLS, which is even with the ground surface.  There was no standing water in this borehole.  The total logging depth achieved by the SGLS was 147.5 ft.

The only man-made radionuclide detected in this borehole was Cs-137.  Cs-137 contamination was detected continuously from 0.5 to 2 ft at concentrations ranging from 0.2 (MDL) to 2 pCi/g.  Cs-137 contamination occurs intermittently from 110 ft to the bottom of the logged interval (147.5 ft) at concentrations ranging from the MDL to 0.6 pCi/g.  The maximum Cs-137 concentration for this borehole was 2 pCi/g detected at the 1-ft depth.

The K-40 concentrations increase slightly from a general background of about 7 pCi/g above 41 ft to about 11 pCi/g between 41 and 47 ft, and then decrease to less than 5 pCi/g at 49 ft.  From 50 to 57 ft, K-40 concentrations are about 12 pCi/g and then decrease to about 2 pCi/g at 60 ft. Between 65 and 82 ft, K-40 concentrations gradually increase from 8 to 11 pCi/g.  The Th-232 concentrations increase noticeably at 85 ft.  The KUT concentrations decrease dramatically below 92 ft and then increase at 109 ft.  The K-40 concentrations are about 14 pCi/g between 109 ft and the bottom of the logged interval (147.5 ft).

The increase in the K-40 concentrations below 41 ft probably represents the boundary between the backfill material and the undisturbed sediments of the Hanford formation.  The low K-40 concentrations identified at 49 and 60 ft may be related to borehole construction features, such as grout plugs, that are located at the top of the perforated casing.  The increase in the Th-232 concentrations at 85 ft probably represents a change in lithology and the contact between the Hanford formation and the early Palouse soil.  A similar contact was noted in borehole 50-08-07 at about the same depth.  The dramatic decrease in the KUT concentrations between 92 and 109 ft probably indicates a change in lithology representing the caliche beds that occur in the Plio-Pleistocene unit.  The increase in the K-40 and Th-232 concentrations below 109 ft probably represents a lithologic change and the contact between the caliche beds and the upper Ringold Formation.  Unfortunately, the sample description in the drilling log is not sufficiently detailed to provide insights into the lithologic section.

The SGLS total gamma-ray plot reflects the influence of the Cs-137 concentrations detected at the top of the logged interval and the variations in the concentrations of the naturally occurring radionuclides throughout the borehole.  The SGLS total count profile also reflects the contact between the backfill material and the Hanford formation sediments below 41 ft and the contact between the Hanford formation with the early Palouse soil below 85 ft.  The decrease in the KUT concentrations between 92 and 109 ft is very apparent on the SGLS total count-rate profile.

Historical gross gamma log data for borehole 50-00-06 were not available for review.  Welty (1988) notes that borehole monitoring was suspended in April 1975 because of misaligned casing, but no additional details were given.  Therefore, no historical gross gamma log is presented on the combination plot.  Log data summaries presented in Welty (1988) between 1973 and 1975 were reviewed and showed no anomalous activity.

Only low concentrations of Cs-137 were detected around this borehole.  The Cs-137 contamination detected between the ground surface and 3 ft probably resulted from a surface spill that migrated down into the shallow backfill surrounding the borehole.  The low concentrations of Cs-137 detected intermittently below 110 ft were probably carried downward from the near-surface contaminated zone during construction of the borehole.  The contamination at the bottom of the borehole is probably particulate matter that has fallen into the borehole from the ground surface.

4.5  Borehole 50-11-05

Borehole 50-11-05 is located approximately 6 ft from the southeast side of tank T-111 and is also known by the Hanford Site designation 299-W10-138.  The drilling log and Chamness and Merz (1993) provide borehole construction information.  This borehole was originally drilled in February 1974 and completed to a depth of 93 ft using 6-in.-diameter casing.  In 1980, the original 6-in. casing was perforated from 0 to 20 ft and 91 to 93 ft.  A 4-in. casing was placed inside the 6-in. casing and 74 gal of grout was added to the annulus.

It was not possible to calculate accurate radionuclide concentrations with this borehole configuration because correction factors for the double casing with annular grout are beyond the current system calibrations.  For data reduction, a 0.50-in. casing correction factor was used, but a correction was not applied for the annular grout.  Furthermore, this borehole was filled to the ground surface with standing water.  The appropriate water correction factor was not available, so no compensation was applied; therefore, the radionuclide concentrations identified in this section are only apparent concentrations and should be considered underestimated.

Depth measurements were referenced to the top of the 4-in. casing, which is even with the ground surface.  The water is terrestrial and probably ran into the borehole from rain and snow-melt runoff.  The water became trapped in the casing because the bottom of the borehole is plugged with grout.  The total logging depth achieved by the SGLS was 87.5 ft.

The only man-made radionuclide detected around this borehole was Cs-137.  Cs-137 contamination was detected at 0.35 pCi/g at the ground surface (0 ft).

The plot of the naturally occurring radionuclides shows the K-40 concentrations increase from a general background of about 8 pCi/g above 42 ft to about 12 pCi/g between 42 and 55 ft.  From 55 ft to the bottom of the logged interval (87.5 ft), the K-40 concentrations steadily increase from about 8 to 12 pCi/g.  The U-238 and Th-232 concentrations increase below the 82-ft depth.

The increase in the K-40 concentrations at 42 ft probably represents the transition between the base of the tank farm excavation and the undisturbed sediments of the Hanford formation.  The increase in the K-40 concentrations between depths of 42 and 55 ft may represent finer grained sediments, which have been identified in most of the boreholes discussed previously.  The increase in the KUT concentrations below 82 ft probably represents a change in lithology and the contact between the basal Hanford formation and the early Palouse soil.  The sample description in the drilling log reports mostly sand and silt were found below the 85-ft depth, indicating that the base of the Hanford formation occurs within this depth range.  The drilling log also reports caliche below 90 ft, but was not detected by the SGLS because this depth region was not accessible for logging.

The SGLS total gamma-ray plot reflects the Cs-137 contamination detected at the ground surface and the variations in concentrations of the naturally occurring radionuclides throughout the borehole.  This profile correlates well with the K-40 log concentrations, indicating K-40 contamination is the primary gamma-emitting radionuclide in the borehole.  The region of high K-40 concentrations between depths of 42 and 55 ft and the increase in KUT concentrations below 82 ft are clearly evident on the SGLS total count-rate profile.

The historical gross gamma log data from 1975 to 1994 and data summaries in Welty (1988) were reviewed.  The most recently acquired historical gross gamma data (February 17, 1994) are presented on the combination plot.  This plot corresponds to the SGLS total count-rate profile, but is much less sensitive.  Anomalous activity was not noted in any of the historical gross gamma-ray logs; however, the historical logs acquired after 1980 do show evidence of borehole modification.  The additional shielding provided by the second casing and annular grout decreased the observed gross gamma-ray count rate.

A low concentration of Cs-137 contamination was detected at the ground surface.  The Cs-137 contamination is probably related to direct radiation (shine) from contamination on the ground surface or from nearby contaminated equipment.

4.6  Borehole 50-11-07

Borehole 50-11-07 is located approximately 6 ft from the southwest side of tank T-111 and is also known by the Hanford Site designation 299-W10-152.  The drilling log and Chamness and Merz (1993) provide borehole construction information.  This borehole was originally drilled in February 1975 and completed to a depth of 94 ft using 6-in.-diameter casing.  In 1980, the original 6-in. casing was perforated from 0 to 20 ft and 92 to 94 ft.  A 4-in. casing was placed inside the 6-in. casing and 109 gal of grout was added to the annulus.

It was not possible to calculate accurate radionuclide concentrations with this borehole configuration because correction factors for the double casing with annular grout are beyond the current system calibrations.  For data reduction, a 0.50-in. casing correction factor was used, but a correction was not applied for the annular grout.  Furthermore, this borehole was filled to the ground surface with standing water.  The appropriate water correction factor was not available, so no compensation was applied; therefore, the identified radionuclide concentrations are only apparent concentrations and should be considered underestimated.

Depth measurements were referenced to the top of the casing, which is even with the ground surface.  The water is terrestrial and probably ran into the borehole from rain and snow-melt runoff that was trapped in the casing because the bottom of the borehole is plugged with grout.  The total logging depth achieved by the SGLS was 90.5 ft.

The only man-made radionuclide detected around this borehole was Cs-137.  The Cs-137 contamination was detected at the ground surface at a concentration of 0.15 pCi/g (just above the MDL) and continuously from 3 to 6 ft at concentrations ranging between 0.25 and 0.4 pCi/g.  Cs-137 contamination was also detected at the 84-ft depth at a concentration just above the MDL.  The maximum Cs-137 concentration for this borehole was 0.4 pCi/g at 5 ft.

The plot of the naturally occurring radionuclides shows the K-40 concentrations increase from a general background of about 9 pCi/g above 39 ft to about 12 pCi/g between 39 and 55 ft.  Between 55 ft and the bottom of the logged interval the K-40 concentrations vary from 8 to 12 pCi/g.  U-238 and Th-232 concentrations increase at 71 ft and below the 82-ft depth.

The increase in the K-40 concentrations at a depth of 39 ft probably represents the boundary between the backfill material and the undisturbed sediments of the Hanford formation.  Increased K-40 concentrations between 39 and 55 ft may represent finer grained sediments.  The increase in U-238 and Th-232 concentrations at 72 ft probably represents a change in lithology within the Hanford formation.  This thin bed appears to correlate with similar strata observed in other nearby boreholes.  The increase in the KUT concentrations below the 82-ft depth probably represents a change in lithology and the contact between the basal Hanford formation and the early Palouse soil.  A similar contact is shown in borehole 50-11-05 at about this same depth.  The drilling log reports mostly silt between the depths of 83 and 93 ft, indicating that the early Palouse soil is close to this depth region.

The SGLS total gamma-ray plot reflects the Cs-137 contamination detected at the ground surface and between depths of 2 and 5 ft.  Concentration variations in the naturally occurring radionuclides are also reflected throughout the borehole in the SGLS total count-rate profile.  The increase in the K-40 concentrations between depths of 39 and 55 ft and the increases in the U-238 and Th-232 concentrations at 71 ft and below 82 ft are clearly evident on the SGLS total count-rate log.

The historical gross gamma log data from 1974 to 1994 and the data summaries in Welty (1988) were reviewed.  The most recently acquired historical gross gamma data (February 17, 1994) are presented on the combination plot.  This plot generally corresponds to the SGLS total count-rate profile, but is much less sensitive.  Anomalous activity was present between the ground surface and about 6 ft on the earliest available log (1975), indicating contamination was present before that time.  Anomalous activity was not noted in the review of historical gross gamma data below the operating level of the tank.  After 1980, the borehole modification is evident in the historical gross gamma logs; the additional shielding provided by the second casing and annular grout decreases the observed gross gamma-ray count rate.

Only low concentrations of Cs-137 were detected in this borehole.  The Cs-137 contamination detected from 3 to 5 ft is probably related to surface spills that have migrated into the backfill material surrounding the borehole.  The Cs-137 contamination at the ground surface is probably attributable to direct radiation (shine) from surface contamination or from nearby equipment.  The Cs-137 contamination at 84 ft was probably carried downward from the near-surface contaminated zone during the borehole construction activities.  The locations and concentrations of the Cs-137 contamination are not indicative of a subsurface source.

4.7  Borehole 50-11-08

Borehole 50-11-08 is located approximately 12 ft from the southwest side of tank T-111 and is also known by the Hanford Site designation 299-W10-139.  The drilling log and Chamness and Merz (1993) provide borehole construction information.  This borehole was originally drilled in February 1974 and completed to a depth of 94 ft using 6-in.-diameter casing.  In 1980, the original 6-in. casing was perforated from 0 to 20 ft and 92 to 94 ft.  A 4-in. casing was placed inside the 6-in. casing and 109 gal of grout was added to the annulus.

It was not possible to calculate accurate radionuclide concentrations with this borehole configuration because correction factors for the double casing with annular grout are beyond the current system calibrations.  A 0.50-in. casing correction factor was used for data reduction, but a correction was not applied for the annular grout.  Furthermore, this borehole was filled to the ground surface with standing water.  The appropriate water correction factor was not available, so no compensation was applied; therefore, the identified radionuclide concentrations are only apparent concentrations and should be considered underestimated.

Depth measurements were referenced to the top of the 4-in. casing, which is even with the ground surface.  The water in this borehole is terrestrial and probably ran into the borehole from rain and snow-melt runoff that was trapped in the casing because the bottom of the borehole is plugged with grout.  The total logging depth achieved by the SGLS was 92.5 ft.

The only man-made radionuclide detected around this borehole was Cs-137.  The Cs-137 contamination was detected at the ground surface at a concentration of 0.5 pCi/g.  Cs-137 contamination was also detected at the MDL (0.2 pCi/g) at 0.5, 10.5, and 13 ft.  The maximum Cs-137 concentration for this borehole was 0.5 pCi/g at the ground surface.

The plot of the naturally occurring radionuclides shows the K-40 concentrations increase sharply from a general background of about 8 pCi/g above 39 ft to about 12 pCi/g between 39 and 55 ft.  The K-40 concentrations increase from a general background of about 9 to about 12 pCi/g between the depths of 71 and 73 ft; Th-232 concentrations also increase in this depth interval.  U- 238 and Th-232 concentrations increase noticeably below the 82-ft depth.  Below 85 ft, the K-40 concentrations increase to about 11 pCi/g.

The increase in the K-40 concentrations at 39 ft probably represents the boundary between the tank farm excavation surface and the undisturbed sediments of the Hanford formation.  The increase in the K-40 concentrations between 39 and 55 ft may indicate finer grained sediments that were identified in nearby boreholes.  The increase in K-40 concentrations between depths of 71 and 73 ft is coincident with an increase in the Th-232 concentrations and probably represents a change in lithology within the Hanford formation.  The thin bed appears to correlate with similar strata observed in boreholes 50-11-07 and 50-11-08.  Below 82 ft, the increase in the KUT concentrations probably represents a lithologic change and the contact between the basal Hanford formation and the early Palouse soil.  A similar contact was noted in boreholes 50-11-05 and 50-11-07 at about the same depth.  The drilling log (1975) reports mostly silt was observed in the interval between 80 and 90 ft, indicating that the early Palouse soil is close to this depth region.

The SGLS total gamma-ray plot reflects the Cs-137 contamination at the ground surface and the variations in the KUT concentrations.  The K-40 concentration increase between 39 and 55 ft and the KUT concentration increase at 72 ft and below 82 ft are clearly evident on the SGLS total count-rate log.

The historical gross gamma log data from 1974 to 1994 and the data summaries in Welty (1988) were reviewed.  The most recently acquired historical gross gamma data (February 17, 1994) are presented on the combination plot.  This plot generally corresponds to the SGLS total count-rate profile, but is much less sensitive.  Anomalous activity was not found in the review of the historical gross gamma-ray logs, nor is there any indication of anomalous activity below the operating level of the tank.  After 1980, the borehole modification is evident in the historical gross gamma logs.  The additional shielding provided by the second casing and annular grout decreases the observed gross gamma-ray count rate.

Only low concentrations of Cs-137 were detected in this borehole.  The Cs-137 contamination detected at the ground surface is probably attributable to direct radiation (shine) from contamination on the ground surface or nearby equipment.  The contamination at 0.5 ft is probably related to surface spills that have migrated into the backfill material surrounding the borehole.  The Cs-137 contamination detected below 10 ft could have been carried down from the ground surface during the borehole construction activities.  Regardless of how the contamination reached its present location, a subsurface source is not indicated.

4.8  Borehole 50-11-10

Borehole 50-11-10 is located approximately 16 ft from the northwest side of tank T-111 and is also known by the Hanford Site designation 299-W10-153.  The drilling log and Chamness and Merz (1993) provide borehole construction information.  This borehole was originally drilled in January 1975 and completed to a depth of 100 ft using 6-in.-diameter casing.  In 1980, the original 6-in. casing was perforated from 0 to 20 ft and 98 to 100 ft.  A 4-in. casing was placed inside the 6-in. casing and 109 gal of grout was pumped into the annulus.

It was not possible to calculate accurate radionuclide concentrations with this borehole configuration because correction factors for the double casing with annular grout are beyond the current system calibrations.  For data reduction, a 0.50-in. casing correction factor was used, but a correction was not applied for the annular grout.  Therefore, the radionuclide concentrations identified are only apparent concentrations and should be considered underestimated.

Depth measurements were referenced to the top of the 4-in. casing, which is even with the ground surface.  A water level indicator showed standing water at 90.4 ft.  The total logging depth achieved by the SGLS was 97.0 ft.

The man-made radionuclides Cs-137 and Co-60 were detected around this borehole.  Cs-137 contamination was detected continuously from the ground surface to 2.5 ft at concentrations of 0.2 pCi/g (just above the MDL).

Co-60 contamination was detected between 68 and 69 ft at very low concentrations ranging from the MDL (0.06 pCi/g) to 0.08 pCi/g.

The plot of the naturally occurring radionuclides shows the K-40 concentrations increase from a general background of about 9 pCi/g at 38 ft to about 12 pCi/g from 38 to 55 ft.  Between 55 and 90 ft, the K-40 concentrations steadily increase from 10 to 12 pCi/g.  Th-232 concentrations increase at the 72-ft depth.  U-238 and Th-232 concentrations increase noticeably below 82 ft.  Below 92 ft, the KUT concentrations decrease sharply.

The increase in the K-40 concentrations below 38 ft probably represents the boundary between the backfill material and the undisturbed sediments of the Hanford formation.  The increase in the K-40 concentrations between 39 and 55 ft may represent finer grained sediments that have also been detected in other nearby boreholes.  The increase in the Th-232 concentrations at 72 ft probably represents a change in lithology within the Hanford formation.  This thin bed correlates with similar strata observed with Co-60 contamination in other nearby boreholes (50-11-11 and 50-08-07).  The increase in the U-238 and Th-232 concentrations below the 82-ft depth probably indicates a change in lithology and the contact between the basal Hanford formation and the early Palouse soil.  Similar contacts were noted in boreholes 50-11-05 and 50-11-07 at about the same depth.  The sharp decrease in the KUT concentrations below 92 ft probably represents a change in lithology and indicates a caliche bed that occurs in the Plio-Pleistocene unit.  The drilling log reports mostly silt was detected in the interval between 80 and 92 ft, indicating that the early Palouse soil is close to this depth region.  The drilling log also reports that mostly caliche was found between 92 and 100 ft, confirming the interpreted caliche zone noted previously.

The SGLS total gamma-ray plot reflects the Cs-137 contamination at the ground surface and the variations in the KUT concentrations.  The increase in the K-40 concentrations between depths of 38 and 55 ft as well as the increases in the U-238 and Th-232 concentrations at 72 ft and below 82 ft are clearly evident on the SGLS total count-rate log.  Below 92 ft, the decrease in the measured KUT concentrations is apparent on the SGLS total count-rate profile.

The historical gross gamma log data from 1974 to 1994 and the data summaries presented in Welty (1988) were reviewed.  The most recently acquired historical gross gamma data (February 17, 1994) are presented on the combination plot.  This profile generally corresponds to the SGLS total count-rate profile.  Indications of anomalous gamma-ray activity in the upper 3 ft and at 69 ft are not apparent in the earlier historical gross gamma log data.  The lower count rate recorded on the historical gross gamma-ray logs acquired after 1980 reflects the additional shielding of the second casing and grout that was added when the borehole was modified.  There is no indication from the historical gross gamma data of anomalous activity in the region below the operating level of the tank or in deeper regions of the borehole.

Only low concentrations of Cs-137 contamination were detected near the ground surface in this borehole.  This contamination was just above the MDL and may be the result of a surface spill that migrated down into the shallow backfill surrounding the borehole or the result of statistical noise.

Traces of Co-60 contamination were detected at 68 and 69 ft at concentrations just above the MDL (0.06 and 0.08 pCi/g, respectively).  Only one Co-60 peak (1332 keV) was detected at both depths; no confirming peak was identified at the 1173-keV energy.  Therefore, the presence of Co-60 contamination at levels above the MDL is doubtful.  However, Co-60 contamination was detected at about this depth in nearby boreholes to the north (50-11-11, 50-08-07, and 50-09-05) at higher concentrations.  This may indicate that borehole 50-11-10 is near or at the southern margin of a Co-60 plume observed in the boreholes to the north.  Lateral spreading along the thin bed of strata identified at the 70-ft depth is present at the same depth region in boreholes 50-11- 11 and 50-08-07.  Consideration should be given to relogging the interval between 60 and 70 ft.  Any lateral movement of the plume to the south would then be evident as increased peaks relative to the baseline data, and it may be possible to estimate the breakthrough time at this point.  The source of the plume is not from tank T-111, but is probably associated with a plume that has migrated from the north where thicker intervals and higher concentrations of  Co-60 contamination are found.

4.9  Borehole 50-11-11

Borehole 50-11-11 is located approximately 5 ft from the northwest side of tank T-111 and is also known by the Hanford Site designation 299-W10-177.  The drilling log and Chamness and Merz (1993) provide borehole construction information.  This borehole was drilled in July 1979 and completed to a depth of 92 ft with 6-in.-diameter casing.  The casing was not perforated.  Grout is present from the ground surface to a depth of 20 ft, forming a grout collar around the outer borehole casing.  Eighteen gal of grout forms a grout plug between depths of 90 and 95 ft.

In those intervals where grout is known to be present, the calculation of accurate radionuclide concentrations was not possible because an appropriate correction factor is not available.  Therefore, the concentrations reported in the upper 20 ft of the borehole are only apparent concentrations; the radioassays in the ungrouted portion of the borehole (below 20 ft) are unaffected by borehole conditions.

Borehole 50-11-11 was logged with the SGLS in the move-stop-acquire mode, stopping every 6-in. and collecting spectra data for 200 s.  The top of the casing is the zero reference for the SGLS, which is even with the ground surface.  The total logging depth achieved by the SGLS was 85.0 ft.

The man-made radionuclides Cs-137 and Co-60 were detected around this borehole.  Cs-137 contamination was detected almost continuously from the ground surface to 14 ft at concentrations ranging from 0.15 to 0.9 pCi/g.  The maximum Cs-137 concentration was 0.9 pCi/g at 12 ft.

Co-60 contamination was detected continuously from depths of 68.5 to 71.5 ft at concentrations ranging from 0.1 to 0.3 pCi/g.  The maximum Co-60 concentration for this borehole was 0.3 pCi/g detected at 68.5 ft.

The KUT plot shows highly variable K-40 concentrations from the ground surface to about 38 ft.  The K-40 concentrations increase from a general level of 6 to 11 pCi/g above 38 ft to about 17 pCi/g from 38 to 53 ft.  At 53 ft the K-40 concentrations abruptly change to 11 pCi/g.  The K-40 concentrations steadily increase from about 11 to 16 pCi/g between 53 and 84 ft, and then decrease abruptly below the 84-ft depth.  U-238 and Th-232 concentrations generally vary between 0.4 and 0.8 pCi/g, with peaks of 0.9 to 1.1 pCi/g at 72 and 84 ft.

The variability of the concentrations of the naturally occurring radionuclides within the backfill portion of the borehole (0 to 38 ft) is attributable to the presence of grout and normal variations in the backfill sediment.  The increase in the K-40 concentrations below 38 ft probably represents the boundary between the backfill material and the undisturbed sediments of the Hanford formation.  The increased K-40 concentrations measured between 38 and 53 ft may represent finer grained sediments that have been encountered in all of the previously discussed boreholes.  The increase in the U-238 and Th-232 concentrations at 72 ft and below 82 ft probably represents a change in lithology.  This thin bed correlates with similar strata observed with Co-60 contamination in another nearby borehole (50-08-07).  Below the 82-ft depth, the increased U- 238 and Th-232 concentrations probably reflect a change in the lithology and the contact between the basal Hanford formation and the early Palouse soil.  A similar feature has been noted in the previously discussed boreholes at roughly comparable depths.  The decrease in the measured K-40 and U-238 concentrations below 84 ft may indicate a caliche bed encountered at about this same depth in borehole 50-11-10, although the original drilling log is not sufficiently detailed to confirm or refute this interpretation.

The SGLS total gamma-ray plot reflects the Cs-137 contamination detected in the upper portion of the borehole, the Co-60 contamination detected at the 70-ft depth, and the variations in the naturally occurring radionuclides.  The total gamma-ray profile clearly reflects the transition from the backfill to the undisturbed Hanford formation sediments below 38 ft and the transition to the early Palouse soil below 82 ft.

The interval between 60 and 75 ft was relogged as an additional quality check and to demonstrate the repeatability of the radionuclide concentration measurements made by the SGLS.  A comparison of the measured concentrations of the man-made and naturally occurring radionuclides using the data sets provided by the original and repeated logging runs is included with Appendix A.  The measurements repeat within two standard deviations (95-percent confidence level), indicating excellent repeatability of the measured gamma-ray spectral peak intensities used to calculate the radionuclide assays.

The historical gross gamma log data from 1974 to 1994 and the data summaries in Welty (1988) were reviewed.  The most recently acquired historical gross gamma data (February 17, 1994) are presented on the combination plot.  The gross gamma plot generally corresponds to the SGLS total count-rate profile, but anomalous gamma-ray activity is not apparent at the depths where the SGLS detected the Cs-137 and Co-60 contamination.  Welty (1988) notes anomalous gross gamma activity at about 67 ft (approximate depth of the Co-60 contamination) immediately after the borehole was installed and logged in July 1979.  When the borehole was being drilled, the drilling log noted contaminated soil at 69 ft measuring 2,500 counts per minute (cpm).

A plot of historical gross gamma logs between 1979 to 1994 is included with Appendix A.  Anomalous gamma-ray activity is evident on the earliest log plots between 65 and 70 ft, but mostly decayed to the detection limit of the gross gamma-ray system by about 1987.  Contamination appeared to increase between July 1979 and July 1980.  A slight downward migration of the gamma-ray-emitting radionuclides is also implied from the historical gross gamma-ray log plot.  This apparent downward migration is probably attributable to poor depth control during the collection of the historical gross gamma-ray data.

An additional plot showing historical gross gamma data between 62 and 75 ft is presented in Appendix A.  The data from 1979 to 1994 were reduced by calculating a grade thickness product value across the anomalous activity (62 to 75 ft) and plotting this value against time.  The decrease in gross gamma activity, as recorded on the historical logs, was compared to the decay curves for Ru-106 and Co-60.  Between early 1980 and mid-1982, the trend of the decreasing gross gamma activity closely matches the expected decay curve plotted for Ru-106, indicating that Ru-106 contamination was the predominant radionuclide within the contaminant plume during that time.  Most of the Ru-106 completely decayed away by mid-1982.  The subsequent trend of the decreasing gross gamma activity from mid-1982 to early 1987 matches portions of the Co-60 decay curve, indicating that residual Co-60 contamination was the primary constituent remaining in the contaminant plume.  By mid-1987, the trend of the gross gamma log and the Co-60 decay curve is separated, indicating that the residual Co-60 contamination was less than the background activity (about 20 cps) and could no longer be detected by the gross gamma logging system.

Shape factor analysis was not used to determine the distribution of the man-made radionuclides around this borehole.  The Cs-137 and Co-60 concentrations detected in this borehole were below 1 and 2 cps, respectively, which represents the minimum count rates required to quantify shape factor analysis.

The Cs-137 contamination detected from the ground surface to 14 ft may be the result of surface spills that have migrated through the backfill material surrounding the borehole.  The Cs-137 contamination detected in this interval could also have resulted from carrydown during borehole installation activities.

The Co-60 contamination detected at the 70-ft depth probably did not originate from tank T-111 because there are no other indications of contamination in the vicinity of the tank.  The contamination may have migrated from other sources, possibly from tank T-108, along the thin bed of strata identified in the Hanford formation at the 72-ft depth.  This zone of contamination correlates with Co-60 contamination identified in boreholes located a short distance to the north, especially boreholes 50-08-07 and 50-08-19.  The Co-60 contamination encountered in this borehole is probably the leading edge of a plume that migrated from a source to the north.  Most of the other radionuclides associated with the Ru-106 plume decayed at a much faster rate than the Co-60 contamination and are no longer detectable by the SGLS.

4.10  Borehole 50-08-07

Borehole 50-08-07 is located approximately 17 ft northwest of tank T-111 and is also known by the Hanford Site designation 299-W10-133.  This borehole is associated with tank T-108, but is sufficiently close to tank T-111 to be useful in characterizing the vadose zone in the vicinity of this tank.  Construction information for this borehole is provided by the drilling logs and Chamness and Merz (1993).  This borehole was originally drilled in February 1974 and completed at a depth of 94 ft using 6-in.-diameter casing.  In March 1981, the borehole was deepened to 120 ft and the 6-in. casing was perforated from 0 to 20 ft and 80 to 120 ft.  A 4-in. casing was placed inside the 6-in. casing and 209 gal of grout was pumped into the annular space between the casings.

It was not possible to calculate accurate radionuclide concentrations with this borehole configuration because correction factors for the double casing with annular grout are beyond the current system calibrations.  For data reduction, a 0.50-in. casing correction factor was used, but a correction for the annular grout was not applied.  Therefore, the identified radionuclide concentrations are only apparent concentrations and should be considered underestimated.

Depth measurements were referenced to the top of the 4-in. casing, which is even with the ground surface.  A water level indicator shows standing water at 118.3 ft.  The total logging depth achieved by the SGLS was 119.0 ft.

The man-made radionuclides Cs-137 and Co-60 were detected around this borehole.  Cs-137 contamination was detected almost continuously from the ground surface to 3 ft, at 101 ft, and from 117 to 119 ft.  The measured Cs-137 concentrations between the ground surface and 3 ft ranged from 0.15 to 2 pCi/g.  The Cs-137 concentration at 101 ft was just above the MDL at 0.15 pCi/g.  The Cs-137 concentrations ranged from just above the MDL to 0.4 pCi/g between 117 and 119 ft.  The maximum Cs-137 concentration was 1.9 pCi/g at a depth of 1 ft.

Co-60 contamination was detected continuously from 68.5 to 95.5 ft and intermittently from 103.5 to 110.5 ft.  The Co-60 concentrations range between 0.1 and 4 pCi/g from 68.5 to 71.5 ft, and from 0.07 pCi/g (just above the MDL) to 0.1 pCi/g between 103.5 and 110.5 ft.  The maximum Co-60 concentration was 3.9 pCi/g detected at 72 ft.

The plot of the naturally occurring radionuclides shows that the K-40 concentrations are variable from the ground surface to 38 ft.  The K-40 concentrations increase from a general background of about 11 pCi/g above 38 ft to about 17 pCi/g from 38 to 53 ft, and then steadily increase from about 11 to 13 pCi/g between 54 and 90 ft.  U-238 and Th-232 concentrations increase at 72 ft and between 82 and 93 ft.  The K-40 concentrations decrease to about 4 pCi/g at 95 and 104 ft, concurrent with a decrease in the Th-232 concentrations.  U-238 concentrations increase noticeably at the 104-ft depth.  Measured K-40 and Th-232 concentrations increase noticeably at 112 ft.

The increase in the K-40 concentrations below 38 ft probably represents the transition between the backfill material and the undisturbed sediments of the Hanford formation.  The increase in the K-40 concentrations between 39 and 53 ft probably indicates the presence of finer grained sediments observed in other nearby boreholes.  The increase in the U-238 and Th-232 concentrations at 72 ft and below 82 ft probably represents a change in lithology.  This thin bed correlates with similar strata observed in another nearby borehole (50-11-11) in which Co-60 contamination was detected.  Below the 82-ft depth, the increase in the U-238 and Th-232 concentrations probably indicates the contact between the basal Hanford formation and the early Palouse soil.  The drilling log reports mostly sand above the 85-ft depth and mostly silt from 85 to 93 ft, indicating that the base of the Hanford formation and early Palouse soil occurs within this depth range.

The noticeable decrease in the K-40 and Th-232 concentrations between 91 and 108 ft indicates a change in lithology that represents the caliche beds that occur in the Plio-Pleistocene unit.  The drilling log reports caliche between 95 and 112 ft, confirming the interpreted caliche zone in this depth range.  The increase in the U-238 concentrations at 104 ft is also a noticeable feature identified in many other nearby boreholes, but the significance of this concentration increase is unknown.  The increase in the K-40 and Th-232 concentrations below a depth of 110 ft represents the upper Ringold Formation.  Below 112 ft, the elevated K-40 and Th-232 concentrations and the silt reported in the drilling log indicate the upper units of the Ringold Formation occur within this depth range.

The SGLS total gamma-ray plot reflects the Cs-137 contamination detected from the ground surface to 3 ft and the Co-60 contamination below 69 ft.  The SGLS total count-rate profile also reflects the variations in concentrations of the naturally occurring radionuclides.  The increase in the K-40 concentrations between depths of 38 and 53 ft and the increases in the U-238 and Th-232 concentrations between 82 and 93 ft are clearly evident on the SGLS total count-rate log.  The decrease in the measured KUT concentrations from 92 to 108 ft is also readily apparent in the SGLS total count-rate profile.

The interval between 67 and 80 ft was relogged as an additional quality check and to demonstrate the repeatability of the radionuclide concentration measurements made by the SGLS.  A comparison of the measured concentrations of the man-made and naturally occurring radionuclides using the data sets provided by the original and repeated logging runs is included with Appendix A.  The measurements repeat within two standard deviations (95-percent confidence level), indicating excellent repeatability of the measured gamma-ray spectral peak intensities used to calculate the radionuclide assays.

The historical gross gamma log data from 1974 to 1994 and the data summaries in Welty (1988) were reviewed.  The most recently acquired historical gross gamma data (February 24, 1994) are presented on the combination plot.  The profile of the most recently acquired historical gross gamma-ray data generally corresponds to the SGLS total count-rate profile.  Indications of anomalous gamma-ray activity in the upper 3 ft of the borehole (in the vicinity of the Cs-137 contamination) are not apparent on this profile.  Anomalous activity is indicated between depths of 70 and 90 ft, with a small peak at about 73 ft.  This zone of anomalous gross gamma-ray activity corresponds with the zone of Co-60 contamination detected by the SGLS.

A plot representing a historical gross gamma time sequence, selected from the data acquired between 1975 to 1994, is included with Appendix A.  The region of anomalous gross gamma-ray activity between depths of 69 and 95 ft was not present in the earliest available log data (1975).  The first indication of anomalous activity occurs at 67 ft from log data acquired in January 1978; a second zone of anomalous activity appears at about 80 ft from log data acquired in May 1978.  The two zones remained distinctly separate until about December 1980, after which all of the activity between depths of 69 and 84 ft consistently exceeded 50 cps and the two intervals appear as one.  A slight downward migration of the gamma-ray-emitting radionuclides is also implied from the gross gamma time-sequence plot.  This apparent downward migration is probably attributable to poor depth control during the collection of the historical gross gamma-ray data.  There are no indications of anomalous activity below 90 ft in the historical gross gamma data.

A plot that graphically illustrates the average gross gamma activity recorded between 1975 and 1994 is included also with Appendix A.  The data were reduced by calculating a grade thickness product value across the anomalous activity (68 to 75 ft and 78 to 83 ft) and plotting the resulting value against time.  This average gross gamma activity plot shows the upper interval reaching a maximum activity in about 1981 and the lower interval reaching a maximum activity in late 1982.  The lower interval reached its maximum activity after the upper interval had decayed for some time.  This is probably because the contamination in the lower interval arrived at the borehole later than the upper interval, or the contamination from the upper interval migrated downward into the lower interval.  By 1988, most of the anomalous activity in those regions had decayed.  The plot also clearly shows the drop in the gross gamma-ray activity after the borehole was modified in 1981.

The Cs-137 contamination detected from the ground surface to 3 ft could be the result of surface spills migrating through the backfill material surrounding the borehole or might have resulted from carrydown during borehole construction activities.  The Cs-137 contamination detected at 101 ft was probably carried down from the near-surface contaminated zone during borehole construction activities.  The Cs-137 contamination at the bottom of the borehole is most likely particulate matter that has fallen into the borehole from the ground surface.

The Co-60 contamination detected below 69 ft probably did not originate from tank T-111, but migrated from tank T-108.  This plume reached borehole 50-08-07 in early 1978 after the Co-60 contamination began spreading laterally along the sediment boundary identified at 72 ft.  By mid-1978, a second plume reached this monitoring borehole by spreading laterally along the boundary separating the basal Hanford formation and the early Palouse soil at 81 ft.  Although the two plumes can be identified by different developments over time, they are probably from the same source.  The Co-60 contamination correlates with a plume occurring at about this same depth in several boreholes located immediately to the north, especially in boreholes 50-08-08 and 50-08-19.  The Co-60 contamination detected below the 85-ft depth was probably carried downward from the higher contaminated zone when the borehole was deepened.


5.0  Discussion of Results

Figures 3a and 3b present a correlation plot of the man-made radionuclide concentration profiles for the 10 boreholes surrounding tank T-111.

Cs-137 contamination was detected near the ground surface and the shallow subsurface in the vicinity of all the boreholes.  This contamination could have resulted from surface spills, airborne contamination releases, or a combination of these.  The contamination has migrated down into the sediments by precipitation infiltration, and, in some cases, was driven down along the borehole during the drilling.

Co-60 contamination was detected on the north side of the tank in boreholes 50-11-10, 50-11-11, and 50-08-07 at depths that indicate the contamination originated from a subsurface source.  The Co-60 contamination associated with boreholes 50-10-11 and 50-08-07 appears to be the residual of a plume containing additional short-lived contaminants.  This plume was first recognized in borehole 50-08-07 in 1978.  These short half-life contaminants have all decayed, and only the Co-60 contamination remains.  However, it is unlikely that the contamination in this plume originated from tank T-111.  The contamination in borehole 50-11-11, which is closest to tank T-111, is limited in both concentration and extent compared to the Co-60 contamination in borehole 50-08-07, which is farther away from tank T-111.  The contamination in boreholes 50-11-11 and 50-08-07 most likely originated from tank T-108.  The origin of the contamination identified in borehole 50-11-10 cannot be correlated with a specific tank, although the Co-60 contamination is coincident with a thin-bed boundary identified in boreholes 50-11-11 and 50-08-07.

The absence of indications of contamination in the monitoring boreholes is remarkable considering tank T-111 is classified both as a leaker and a re-leaker.  However, because the tank leak volume estimate is only 2,000 gal, the leak designation is probably based only on very small decreases in liquid level that may not be the result of a tank leak.

No contamination plumes were found that could be indicative of a leak.  Therefore, either the tank did not leak, or the leak plumes are limited in extent and have not been intercepted by any of the monitoring boreholes.

The KUT log plots derived from the SGLS data were very useful for determining lithologic contacts considering that the boreholes were grouted.  The K-40 log plots showed slight to moderate concentration increases at about 40 ft that most likely represent where the Hanford formation is in contact with the excavation surface.  Increases in the Th-232 log plots at about 82 ft most likely represent the contact with the early Palouse soil.

Below the 92-ft depth, the KUT plots showed noticeable concentration decreases that most likely represent the contact with a caliche bed in the Plio-Pleistocene unit.  The increase in K-40 and Th-232 concentrations below about 109 ft probably indicates a change in lithology and the top of the upper Ringold Formation.


6.0  Conclusions

The characterization of the gamma-ray-emitting contamination in the vadose zone surrounding tank T-111 was completed using the SGLS.  Data obtained from the SGLS and geologic and historical information indicate that surface spills have occurred, but do not indicate leakage from tank T-111.  The information obtained from in-tank liquid-level observations remains as the only evidence that tank T-111 ever leaked.  The contamination plumes that were identified in the vicinity of T-111 in boreholes 50-11-11 and 50-08-07 can be directly correlated to contamination detected in monitoring boreholes surrounding tank T-108.

The characterization of the KUT concentrations proved useful for identifying major lithologic boundaries and changes occurring in the stratigraphic column, including thin bedded strata and caliche beds.


7.0  Recommendations

Tank T-111 is currently classified as an assumed leaker and is interim stabilized.  However, as recently as 1994, tank T-111 was classified as an assumed re-leaker after additional unexplained liquid-level decreases were observed.  Continued monitoring of the boreholes surrounding this tank is recommended to help identify future leaks or changes in the distribution of the vadose zone contaminants on the north side of tank T-111.


8.0  References

Agnew, S.F., R.A. Corbin, T.B. Duran, K.A. Jurgensen, T.P. Oritz, and B.L. Young, 1995.  Waste Status and Transaction Record Summary for the Northwest Quadrant of the Hanford 200 West Area, WHC-SD-WM-TI-669, Rev. 1, Los Alamos National Laboratory, Los Alamos, New Mexico.

Anderson, J.D., 1990.  A History of the 200 Area Tanks Farms, WHC-MR-0132, Westinghouse Hanford Company, Richland, Washington.

Brevick, C.H., L.A. Gaddis, and W.W. Pickett, 1995.  Supporting Document for the Historical Tank Content Estimate for T Tank Farm, WHC-SD-WM-ER-320, Westinghouse Hanford Company, Richland, Washington.

Caggiano, J.A., and S.M. Goodwin, 1991.  Interim Status Groundwater Monitoring Plan for the Single-Shell Tanks, WHC-SD-EN-AP-012, Westinghouse Hanford Company, Richland, Washington.

Chamness, M.A., and J.K. Merz, 1993.  Hanford Wells, PNL-8800, prepared by Pacific Northwest Laboratory for the U.S. Department of Energy, Richland, Washington.

Ewer, K.L., J.W. Funk, R.G. Hale, G.A. Lisle, C.V. Salois, and M.R. Umphrey, 1997.  Historical Tank Content Estimate for the Northwest Quadrant of the Hanford 200 West Area, HNF-SD-WM-ER-351, Rev. 1, Fluor Daniel Northwest, Inc., Richland, Washington.

Freeman-Pollard, J.R., J.A. Caggiano, S.J. Trent, and EBASCO/Hart Crowser, 1994.  Engineering Evaluation of the GAO/RCED-89-157, Tank 241-T-106 Vadose Zone Investigation, BHI-00061, Rev. 0, Bechtel Hanford, Inc., Richland, Washington.

Hanlon, B.M., 1998.  Waste Tank Summary Report for Month Ending August 31, 1998, HNF-EP-0182-125, Westinghouse Hanford Company, Richland, Washington.

Hodges, F.N., 1998.  Results of Phase I Groundwater Quality Assessment for Single-Shell Tank Waste Management Areas T and TX-TY at the Hanford Site, PNNL-11809, prepared by Pacific Northwest National Laboratory for the U.S. Department of Energy, Richland, Washington.

Lindsey, K.A., 1993.  Memorandum to G.D. Bazinet with attached letter report Geohydrologic Setting, Flow, and Transport Parameters for the Single Shell Tank Farms, written by K.A. Lindsey and A. Law, 81231-93-060, Westinghouse Hanford Company, Richland, Washington.

Lindsey, K.A., and D.G. Horton, 1991.  Geologic Setting of the 200 West Area: An Update, WHC-SD-EN-TI-008, Westinghouse Hanford Company, Richland, Washington.

Pacific Northwest National Laboratory (PNNL), 1998.  Hanford Site Groundwater Monitoring for Fiscal Year 1997, PNNL-11793, Richland, Washington.

Price, W.H., and K.R. Fecht, 1976.  Geology of the 241-T Tank Farm, ARH-LD-135, Atlantic Richfield Hanford Company, Richland, Washington.

Tucker, R.P., 1994.  Occurrence Report, Subject: "Apparent Liquid Level Decrease in Single Shell Underground Storage Tank 241-T-111; Declared an Assumed Re-Leaker," WHC-TANKFARM-1994-0009, Westinghouse Hanford Company, Richland, Washington.

U.S. Department of Energy (DOE), 1992.  T Plant Source Aggregate Area Management Study Report, DOE/RL-91-61, Richland Operations Office, Richland, Washington.

__________, 1993.  200 West Groundwater Aggregate Area Management Study Report, DOE/RL-92-16, Richland Operations Office, Richland, Washington.

__________, 1995a.  Vadose Zone Characterization Project at the Hanford Tank Farms, Calibration of Two Spectral Gamma-Ray Logging Systems for Baseline Characterization Measurements in the Hanford Tank Farms, GJPO-HAN-1, prepared by Rust Geotech for the Grand Junction Projects Office, Grand Junction, Colorado.

__________, 1995b.  Vadose Zone Characterization Project at the Hanford Tank Farms, Spectral Gamma-Ray Logging Characterization and Baseline Monitoring Plan for the Hanford Single-Shell Tanks, P-GJPO-1786, prepared by Rust Geotech for the Grand Junction Projects Office, Grand Junction, Colorado.

__________, 1997a.  Hanford Tank Farms Vadose Zone, Data Analysis Manual, MAC-VZCP 1.7.9, Rev. 1, prepared by MACTEC-ERS for the Grand Junction Office, Grand Junction, Colorado.

__________, 1997b.  Hanford Tank Farms Vadose Zone, High-Resolution Passive Spectral Gamma-Ray Logging Procedures, MAC-VZCP 1.7.10-1, Rev. 2, prepared by MACTEC-ERS for the Grand Junction Office, Grand Junction, Colorado.

__________, 1997c.  Hanford Tank Farms Vadose Zone, Project Management Plan,MAC-VZCP 1.7.2, prepared by MACTEC-ERS for the Grand Junction Office, Grand Junction, Colorado.

__________, 1998.  Hanford Tank Farms Vadose Zone, Fourth Biannual Recalibration of Spectral Gamma-Ray Logging Systems Used for Baseline Characterization Measurements in the Hanford Tank Farms, GJO-HAN-14, prepared by MACTEC-ERS for the Grand Junction Office, Grand Junction, Colorado.

Waite, J.L., 1991.  Tank Waste Discharged Directly to the Soil at the Hanford Site, WHC-MR-0227, Westinghouse Hanford Company, Richland, Washington.

Welty, R.K., 1988.  Waste Storage Tank Status and Leak Detection Criteria, SD-WM-TI-356, Westinghouse Hanford Company, Richland, Washington.

Wilson, R.D., 1997.  Hanford Tank Farms Vadose Zone, Spectrum Shape-Analysis Techniques Applied to the Hanford Tank Farms Spectral Gamma Logs, GJO-HAN-7, prepared by MACTEC-ERS for the Grand Junction Office, Grand Junction, Colorado.

__________, 1998.  Hanford Tank Farms Vadose Zone, Enhancements, Validations, and Applications of Spectrum Shape-Analysis Techniques Applied to Hanford Tank Farms Spectral Gamma Logs, GJO-HAN-15, prepared by MACTEC-ERS for the Grand Junction Office, Grand Junction, Colorado.

Woodrich, D.D., G.S. Barney, G.L. Borsheim, D.L. Becker, W.C. Carlos, M.J. Klem, R.E. Van der Cook, and J.L. Ryan, 1992.  Summary of Single-Shell Waste Tank Stability, WHC-EP-0347 Supplement, Westinghouse Hanford Company, Richland, Washington.