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solid/liquid partition coefficient or Kd. If an average K value is sufficiently representative of the site, it <br />can be used to determine a site - specific Soil Screening Level (SSL) for releases to groundwater based on <br />guidance from the U.S. Environmental Protection Agency (EPA, 1996). The applicable equation from <br />this guidance (shown below) was used to calculate potential cleanup levels for solid source term materials <br />within the alluvial fill based on the target uranium water quality standard of 0.03 mg/L. Parameters not <br />applicable to U -nat were omitted, and site - specific estimates of soil porosity (0.15) and dry bulk soil <br />density (1.8 kg/L) were used (versus EPA default values of 0.3 and 1.5 kg/L respectively). <br />e <br />SSLu-nat _ — Cw K d + w <br />Pb <br />Where: <br />SSLu_nat = Soil Screening Level for natural uranium concentration (mg /kg) <br />C = Target soil leachate U -nat concentration (in this case, 0.03 mg/L) <br />Kd = Soil -water partition coefficient (L /kg) <br />O = Soil porosity (L /T <br />Pb = Dry bulk soil density (kg /L) <br />An equation for estimating Kd values based on SPLP leach testing data is available in the regulatory <br />guidance literature (NJDEP, 2008). Individual estimates of Kd values for the 17 samples subject to SPLP <br />testing were calculated with results ranging from 647 to 24,786 L /kg. The average Kd was nearly 5,000 <br />L /kg, indicating generally low uranium leaching potential under SPLP testing procedures. The calculated <br />SSL for U -nat based on this average Kd was thus high (148 mg/kg), nearly four times greater than the <br />potential cleanup level suggested in Figure 8. <br />The preceding results indicate that the SPLP leach test methodology is not sufficiently representative of <br />actual leaching conditions within the alluvial fill to be of practical value with respect to determining a <br />cleanup level. Fortunately, available field data associated with a particular set of site circumstances <br />provided an opportunity for a fundamentally different approach to this assessment. <br />The third and most reliable approach for determining a cleanup level involved using site - specific field <br />measurements to estimate a K value that is representative of actual leaching conditions within the <br />alluvial fill. Measured concentrations in the solid material were evaluated against measured <br />concentrations in alluvial groundwater at an area of the site that is upgradient of most mine facilities and <br />potential sources other than the solid material under investigation. <br />Groundwater collection at Sump 9 occurs in an area of the alluvial fill pad where Ralston Creek loses in- <br />stream flow and recharges groundwater in the alluvial fill. Much of this groundwater migrates towards <br />Sump 9 (Figure 9). This flow pattern is consistent with the original location of the stream channel <br />(Whetstone, 2011) and is supported by water quality data and field observations of seasonally daylighting <br />groundwater in this area. Groundwater quality at Sump 9 is clearly impacted, but the only potential <br />source of impact is the alluvial fill materials because upgradient locations are hydrologically upgradient <br />of any other possible sources. These circumstances are favorable for estimation of a truly representative <br />12 <br />