Vadose Zone Characterization Project at the Hanford Tank Farms, T Tank Farm Report
DOE Grand Junction Office, Grand Junction, Colorado

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Executive Summary

The U.S. Department of Energy Grand Junction Office (DOE-GJO) was tasked by the DOE Richland Operations Office (DOE-RL) to perform a baseline characterization of the gamma-ray- emitting radionuclides that are distributed in the vadose zone sediments beneath and around the single-shell tanks (SSTs) at the Hanford Site.  The intent of this characterization is to determine the nature and extent of the contamination, to identify contamination sources when possible, and to develop a baseline of the contamination distribution that will permit future data comparisons.

The results of this initial baseline provide the information necessary to plan and prioritize more comprehensive characterization projects.  This characterization work also allows an initial assessment of the impacts of the vadose zone contamination as required by the Resource Conservation and Recovery Act (RCRA).  This characterization is limited to the depths and areal distribution of existing boreholes; no new boreholes were constructed for this project.

The scope of this project involves acquiring information regarding vadose zone contamination utilizing borehole geophysical logging methods, assessing and interpreting that information and documenting it in a series of reports for each SST (Tank Summary Data Reports) and for each tank farm (Tank Farm Reports).  The methods utilized are presently limited to detection of gamma-emitting radionuclides from both man-made and natural sources.

Logging operations utilized high-purity germanium detection systems to assess the distribution of the gamma-emitting radionuclides in the sediments surrounding and below the T Tank Farm tanks.  Log data were acquired in 67 existing vadose zone monitoring boreholes with two spectral gamma logging systems.  Logging of all the boreholes was completed by December 1998, and the last Tank Summary Data Report was issued in July 1999.  Twelve Tank Summary Data Reports were prepared for the tanks in the T Tank Farm (DOE 1995d, 1995e, 1998c, 1998d, 1998e, 1998f, 1999b, 1999c, 1999d, 1999e, 1999f, and 1999g).

Cesium-137 (Cs-137), cobalt-60 (Co-60), europium-154 (Eu-154), and, to a lesser degree, europium-152 (Eu-152), were the major gamma-emitting contaminants detected in the T Tank Farm vadose zone.  Limited and relatively isolated occurrences of niobium-94 (Nb-94), antimony-125 (Sb-125), tin-126 (Sn-126), uranium-235 (U-235), and uranium-238 (U-238) were also detected around several boreholes.

The spectral gamma log data were utilized to prepare visualizations showing the apparent distributions of Cs-137, Co-60, and Eu-154 in the vadose zone at the T Tank Farm.  The visualizations depict the distributions of these radionuclides as three-dimensional plumes of contamination.  Unfortunately, the precision of portions of the visualizations is limited due to inaccuracies in the assays resulting from double-cased and grouted boreholes and due to a lack of data with which to perform good geostatistical structural analyses.

Near-surface and shallow subsurface Cs-137 contamination was detected primarily in the central and eastern portions of the T Tank Farm.  This contamination most likely resulted from surface spills or leaks from piping systems related to routine tank farm operations.  The highest Cs-137 concentrations were detected within the near-surface backfill material in the east-central portion of the tank farm between tanks T-104 and T-107.  The thickest distributions of Cs-137 (27 feet [ft]) contamination were also detected in this region, suggesting a larger spill or several spills or leaks may have occurred in this area.

A vertically continuous, elongated Cs-137 plume was detected around one borehole near the southeast side of tank T-101, which is an assumed leaker.  The plume extends to a depth of more than 100 ft and appears to be the result of a large volume leak from spare fill lines on the southeast side of the tank that probably resulted from overfilling the tank.  A large Co-60 and Eu-154 plume identified near the south side of tank T-101 may also have originated from the same leak source.  Visualization data show that the Co-60 component of the plume trends in a southwesterly direction, passing under the southern portion of tank T-101 and portions of tanks T-104 and T-105.

A Cs-137 and Co-60 plume was identified in one borehole near the south side of tank T-102.   Although tank T-102 is classified as sound, the data indicate that the plume probably originated from a leak from the spare fill lines connected to this tank in a manner similar to spare fill-line leaks originating from adjacent tanks within the cascade series.  The Cs-137 component of the plume appears to have spread laterally from the suspected leak source to the south and west along the base of the tank farm excavation.

A distinct plume of Co-60, Eu-154, and Eu-152 contamination was intercepted by boreholes located near the southeast and south sides of tank T-103.  The contamination is believed to have originated from a leak in the seal of a spare fill line on the southeast side of the tank.  Some of the Co-60 and Eu-154 contamination within the plume has migrated laterally to the south and has apparently intermingled with contamination resulting from a large leak from tank T-106.

The very extensive plume of Cs-137, Co-60, Eu-154, and Eu-152 contamination identified around and below the base of the tank T-106 originated from large leak from the tank in 1973.  The leak source is located on the southeast side of the tank, and the resulting plume extends laterally as much as 100 ft and vertically beyond the depth of most of the boreholes.

Man-made radionuclide contamination was detected at the bottoms of several of the deepest T Tank Farm monitoring boreholes, indicating that the contamination has penetrated to a depth of at least 120 ft.  Because the boreholes were not deep enough to intercept the bottommost portions of the contaminant plumes, the vertical extent of these contaminants into the deeper regions of the vadose zone could not be determined.

In 1995 and 1996, groundwater samples collected from monitoring well 299-W11-27, which is located downgradient from the T Tank Farm, showed large increases in several parameters, including specific conductance, chromium, nitrate, tritium, technetium-99 (Tc-99), and Co-60.  A Phase I Groundwater Quality Assessment concluded that these contaminants are most likely the result of sources within T Tank Farm (Hodges 1998).  Brief summaries of groundwater data and previously published interpretations of groundwater contaminants are included in this report to allow the reader to understand the relation between vadose zone contamination and groundwater contamination.

Several waste disposal facilities adjacent to the T Tank Farm were investigated during the preparation of this report to evaluate the potential influence that wastes disposed of at these facilities may have had on the distribution of contamination at the T Tank Farm.  There is no indication that wastes discharged to these facilities are related to contamination detected in the vadose zone beneath the tanks.

The initial characterization of the T Tank Farm vadose zone is limited to the areal distribution and depths of the existing monitoring boreholes within the tank farm.  Because most of the boreholes are 120 ft deep or less and the local groundwater level is approximately 220 ft deep, only the upper portion of the vadose zone beneath the T Tank Farm has been characterized.  Although this characterization project is limited to detecting only gamma-emitting radionuclides, several plumes have been identified that define targets for additional, more comprehensive chemical and radiological characterization.