2012-10-01_REVISION - M1977310 (10)

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JD -6 Mine Environmental Protection Plan 17 would be presented in the field; (b) the tumbling process provides the maximum exposure of all potentially reactive sites to the leachant; and (c) the tumbling process exposes even more surface area through semi - autogenous grinding." DRMS (2006) The SPLP test uses a water:rock ratio of 20:1. If the results are applied to the site, the analysis inherently assumes that water is available in the environment to leach constituents from the rock. As discussed in the site hydrologic summary, potential evaporation (PE) exceeds precipitation (P) in the region. Although PE >P does not ensure that no water would ever migrate through the waste rock or temporary ore stockpile, the amount of water available to leach and transport elements is limited. 5.4.2.2 Transport The transport of constituents from the JD -6 Mine waste rock pile will be limited by low infiltration rates and therefore low transport velocities in the unsaturated zone (a.k.a., vadose zone). Infiltration modeling performed for the nearby JD -7 Mine reclaimed waste rock pile indicated that 0.21 in /yr (0.53 cm /yr) of meteoric water might infiltrate through the waste rock pile under normal site conditions. The model results indicated that approximately 98.1% of the precipitation would be consumed by evapotranspiration (ET) and runoff would be negligible. These results correspond well with reported average recharge rates for the arid- semiarid southwestern United States. Scanlon, et al., (2006) reported an average recharge rate of 0.44 in/yr (1.12 cm /yr), corresponding to annual precipitation of 11.9 inches (30.1 cm). If vegetation is fully established and sage brush succeeds grasses on the reclaimed waste rock pile, the model results indicated that infiltration would decrease to 0.10 in /yr (0.25 cm /yr). Given the low infiltration rates, the rate of vertical transport of soluble constituents from the waste rock pile to underlying groundwater would be low. Hydrologic modeling performed by GeoScience Services (2005) for the JD -6 Mine and the SM -18 Mine suggested that any downward vertical migration of fluid or constituents from the waste rock pile would be limited by the sorption coefficients (K and low transport velocities. For example, using a source term of 2.7 mg /L U and a Kd of 0.036 m /Kg in sand and 0.60 m /Kg in clay, uranium would migrate approximately 200 ft vertically at a concentration of 1/1000 of the original source concentration. Selenium would be transported 80 feet vertically at a concentration of 1 /1000` of the original source concentration. The PORFLOW model simulation is considered to be environmentally conservative for several reasons, including the constant flux rate of 1.2 in/yr which is nearly three times the infiltration rate predicted by the HYDRUS modeling of the reclaimed waste rock pile at the nearby JD -7 Mine. The PORFLOW model results showed that no contamination would reach an underlying groundwater system. For all the analytes modeled, none would migrate through the Salt Wash Member of the Morrison Formation. The Summerville Formation provides an additional barrier preventing the constituents of concern from impacting any groundwater in the underlying Entrada Sandstone. It is noted that higher infiltration rates through the unlined stormwater catchment, located adjacent to the JD -6 Mine waste rock pile, could have produced higher vertical transport velocities than what occurs elsewhere on the JD -6 Mine site. This stormwater catchment will be lined as described in the Stormwater Management Plan (Attachment 3). 5.4.2.3 Uptake The third component of an environmental exposure pathway is uptake by human or biological receptors. The inventory of surface water features (Section 8.1) indicates that no perennial streams have been mapped within two miles downstream of the JD -6 Mine. The surface water exposure pathway is limited to intermittent or ephemeral drainages that could contain water for very short time periods immediately after 4148A.120927 Whetstone Associates