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Prior to construction of the various drainage tunnels, which lowered the groundwater level within the <br /> diatreme, Squaw Gulch was the point at which groundwater discharged from the diatreme because the <br /> Precambrian-diatreme contact is at its lowest elevation in Squaw Gulch. Therefore, over geologic time, <br /> groundwater became enriched in dissolved constituents as it passed through mineralized zones within the <br /> diatreme before it discharged into Squaw Gulch. The solute-rich water would have enriched the soils and <br /> alluvium that host the shallow aquifer in Squaw Gulch, and current groundwater monitoring data reflect <br /> this natural enrichment. This explanation seems more reasonable to explain the elevated constituents <br /> than historic mining, because the bulk of historic mining took place within the diatreme, and since almost <br /> all of the precipitation that falls within the diatreme infiltrates into the diatreme and now discharges <br /> through the Carlton Tunnel, it is unlikely that after the construction of the drainage tunnels and prior to <br /> construction of VLF2 impacted water exited the diatreme to flow down Squaw Gulch. <br /> The construction of VLF2 has effectively eliminated groundwater recharge within the footprint of the <br /> facility. Therefore, VLF2 is not a new or increased source of sulfate, manganese, or zinc to Squaw <br /> Gulch. <br /> 6.2 Arequa Gulch <br /> The Arequa Gulch drainage contained numerous mines as well as two railroads and at least two mills <br /> (Arequa and Economic, see Figure 22). The Arequa mill recovered gold using cyanidation for oxide ore, <br /> and roasting followed by chlorination for sulfide ores (Lakes, 1899). No information was found regarding <br /> when the Arequa mill was constructed or its throughput, but it appears to have been destroyed by fire in <br /> 1903 (Mining and Scientific Press, 1903). Figure 26 is a photograph of the Arequa mill (Grimstad and <br /> Drake, 1983). <br /> s <br /> }f <br /> Figure 26: Photograph of the Arequa mill(Grimstad and Drake,1983). <br /> The Economic mill was built in 1899 (Levine, 1982) and utilized roasting and chlorination with hydrogen <br /> sulfide precipitation for gold recovery (Lakes, 1901). The Economic mill was a modestly-sized mill (300 <br /> tons per day), and operated until it was destroyed by fire in 1907 (Lindgren and Ransome, 1906; <br /> Henderson, 1926). Figure 27 is a photograph of the Economic Mill. <br />