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Remedial Assessment <br />Cleanup Criterion for Alluvial Fill Source Term Materials <br />The goal of remedial measures to address solid source term materials residing in the alluvial fill is to <br />permanently isolate this material from contact with alluvial groundwater, greatly reducing or eliminating <br />the potential for groundwater leaching at concentrations in excess of the in- stream uranium standard for <br />Ralston Creek. Remedial measures will include excavation and removal of any source term material <br />having the potential to leach uranium in concentrations in excess of 0.03 mg/L. <br />The first approach for assessing a cleanup level that might achieve the remedial objective was to plot <br />results of Synthetic Precipitation Leaching Procedure (SPLP) testing (EPA Method 1312) for evaluation <br />of the relationship between U -nat concentrations in solid source term material and leachate generated <br />from the SPLP testing (Figure 8). A non - linear logarithmic function provided the best statistical fit to <br />these data, though the statistical significance of this curve is weak. Despite the statistically weak <br />relationship (R = 0.50), a correlative trend is nevertheless apparent and the fitted curve suggests that on <br />average, solid source term concentrations in excess of about 39 mg/kg could result in alluvial <br />groundwater concentrations in excess of 0.03 mg/L. <br />0.20 <br />0.18 <br />0.16 <br />0.14 <br />0.12 <br />0.10 <br />0.08 <br />0.06 <br />0.04 <br />0.02 <br />0.00 <br />• <br />y= 0.03Ln(x) - 0.06 <br />R -0.50 <br />0 500 1000 1500 2000 2500 3000 3500 <br />Solid Source Term U -nat ( mglkg) <br />11 <br />0.12 <br />0.09 <br />0.06 <br />0.03 <br />0.00 <br />Solid Source Term U-nat (mglkg) <br />Figure 8: Left: "best fit" non - linear curve fitted to SPLP data. Right: curve with axes truncated <br />to highlight the lowest values and a solid source term concentration that on average can be <br />expected to produce a leachate concentration of about 0.03 mg /L when subject to SPLP <br />testing. <br />Potential reasons for high variability in SPLP testing results include significant heterogeneity in rock/soil <br />particle size distributions and associated variability in the amount of oxidation and soluble salts present <br />on the surfaces of these source term materials. These samples were collected at different locations and <br />depths within the alluvial fill pad and variable degrees of prior oxidation and leaching are thus expected. <br />Because these data lacked a strong statistical correlation, further analysis was necessary to develop a <br />cleanup criterion that provides high confidence that remedial objectives will be achieved. <br />The second approach for assessing a potential cleanup level involved using SPLP data to evaluate ratios <br />of U -nat concentrations in solid source term materials to those in the leachate. This ratio is known as a <br />