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SummaryPacific Northwest National Laboratory (PNNL) conducted a Phase I. Resource Conservation and Recovery Act of 1976 (RCRA) groundwater quality assessment for the Richland Field Office of the U.S. Department of Energy (DOE-RL), in accordance with the Federal Facility Compliance Agreement. The purpose of the investigation was to determine if the Single-Shell Tank Waste Management Area (WMA) S-SX has impacted groundwater quality.The WMA is located in the southern portion of the 200 West Area of the Hanford Site and consists of the 241-S and 241-SX tank farms and ancillary waste systems. The unit is regulated under RCRA interimstatus regulations ( 40 CFR 265, Subpart F) and was placed in assessment groundwater monitoring ( 40 CFR 265.93 [d]) in August 1996 because of elevated specific conductance and technetium-99, a non-RCRA co-contaminant, in downgradient monitoring wells. Phase I assessment, allowed under 40 CFR 265, provides the owner-operator of a facility with the opportunity to demonstrate that the regulated unit is not the source of groundwater contamination.Major findings of the assessment are summarized below:• Distribution patterns for radionuclides and RCRA/dangerous waste constituents indicate WMA S-SX has contributed to groundwater contamination observed in downgradient monitoring wells. Multiple source locations in the WMA are needed to explain spatial and temporal groundwater contamination patterns.• Drinking water standards for nitrate and technetium-99 are currently exceeded in one RCRA-compliant well (299-W22-46) located at the southeastern comer of the SX tank farm .. Technetium-99, the constituent with the highest concentration relative to a standard, is currently four to five times the U.S. Environmental Protection Agency (EPA) interim drinking water standard of 900 pCi/L. Technetium-99 also recently increased to just above the drinking water standard in an older well (299-W23-1) inside the Stank farm.• Technetium-99, nitrate, and chromium concentrations in downgradient well299-W22-46 (the well with the highest current concentrations) appear to be declining after reaching maximum concentrations in May 1997. Observations during the next four quarters are needed to confirm the apparent declining trend in this well.• Cesium-137 and strontium-90, major constituents of concern in single-shell tank waste, were not detected in any of the RCRA-compliant wells in the WMA network, including the well with the highest current technetium-99 concentrations (299-W22-46). This observation is consistent with the low expected mobilities of these constituents under Hanford Site conditions.• Low but detectable strontium-90 and cesium-137 were found in one old well (2-W23-7), located inside and between the S and SX tank farms. Additional investigation is needed to determine if the low level contamination is borehole related or is more broadly distributed in the aquifer. • Infiltration of snow melt runoff and/or artificial sources of water near vadose zone contamination sites within the WMA are possible causes ...

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I 20. I A BD-7400-172-2 (04/94) GEF097 21. DOE APPROVAL iir required) U Approved Approved u/ccmnerits tl Disapproved w/connnts Ctrl. No.

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