Single-Shell Tank Waste Management Area U (WMA U) is in the 200 West Area on the Hanford Site. The area includes the U Tank Farm that contains 16 underground, single-shell tanks and their ancillary equipment and waste systems. WMA U is regulated under the Resource Conservation and Recovery Act of 1976 (RCRA) as codified in 40 CFR Part 265, Subpart F and Washington's Hazardous Waste Management Act (HWMA, RCW 70.105) and its implementing requirements in the Washington State dangerous waste regulations (WAC 173-303-400). Releases of hazardous wastes from WMA U have contaminated groundwater beneath the area. Therefore, the WMA U is being assessed to determine the rate of movement and extent of the contamination released and to determine the concentrations in groundwater. The original finding of groundwater impact was determined from elevated specific conductance in downgradient well 299-W19-41. The elevated specific conductance was attributed to the nonhazardous constituents calcium, magnesium, sulfate, and chloride. Tank waste constituents nitrate and technetium-99 are also present as co-contaminants and have increased over the past several years; however, at concentrations well below the respective drinking water standards. Chromium concentrations in downgradient wells have generally exceeded background levels, but similar levels were also observed in upgradient well 299-W18-25 in early 2000 before it went dry. The objective of this report is to present the current conceptual model for how and where contaminant releases have reached the water table and how that contamination has dispersed in the groundwater system. These efforts will achieve the requirements of a groundwater quality assessment under RCRA (40 CFR 265.93 (d)(4)). On that basis, a monitoring schedule with appropriate analyses and proposals for new wells and tests are presented in this document.
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