A GROUNDWATER QUALITY ASSESSMENT AT SINGLE-SHELL TANK FARMS AT THE DOE HANFORD SITE

Scientists from the Pacific Northwest National Laboratory have conducted an assessment of the groundwater quality under three buried past-practice waste storage tank farms at the DOE Hanford Site in eastern Washington. The case history presented in this paper covers current findings of the investigation, discussing problems unique to a past-practice radioactive storage facility. Innovative technologies have been deployed to unravel a complicated picture of groundwater contamination.

The dangerous waste management unit, comprised of the three tank farms, is regulated under the Resource Conservation and Recovery Act interim-status regulations (40 CFR 265, Subpart F) and Washington’s Hazardous Waste Management Act (HWMA, RCW 70.105). The unit was placed in assessment groundwater monitoring in 1996 when elevated conductivity was observed in a downgradient monitoring well. The purpose of the assessment was to determine if hazardous waste associated with the buried storage facilities had impacted groundwater quality.

The subsurface situation at this site is complicated by several factors. For example, the tank farms are surrounded by disposal facilities where waste of a similar composition was disposed directly to the soil column during the same time period the tanks were in active use. Thus it is difficult to track groundwater contamination back to a specific tank source. In addition the water table is essentially flat, caused by a highly transmissive, unconfined aquifer. Consequently conventional methods to determine flow direction based on water levels are not useful at this site. Furthermore the local flow direction has been artificially changed over time due to large discharges of liquid waste causing water table to rise and diverted the groundwater from its natural direction.

However, with the use of innovative technologies along with standard methods, it has been possible to determine the source for the groundwater contamination observed since the waste management unit was placed in assessment in the mid-1990s. Recent data evaluation studies center on pattern matching of trend plots and chemical fingerprinting using the ratio of nitrate to technetium-99 found in the groundwater. Results of both studies are presented and discussed.