Laserfiche WebLink
16 <br /> 8 14 <br /> 12 <br /> 7.5 <br /> I 10 <br /> YL e <br /> IVA <br /> a <br /> 6.5 <br /> 6 <br /> 6 <br /> 4 <br /> ` 2 <br /> 1 <br /> 5 _... _.._ _ _.._. ,_..---- <br /> 8 <br /> —WCMW 3-134 —.—WCMW 6234 -• Precip —Poly.(WCMW 3-134) —Poly.(WCMW&234) —Poly.(Predp) <br /> Figure 18: Field pH and precipitation data for Wilson Creek monitoring wells. Third order polynomial trends <br /> have been fit to the data to aid in recognition of correlations. <br /> 5.6 Grassy Valley <br /> Grassy Valley lies north of the primary area of mining in the Cripple Creek district. Minor historic mining <br /> and milling occurred in the upper half of the valley, but the level of activity did not approach that in the <br /> Squaw Gulch, Arequa Gulch, Poverty Gulch and Vindicator Valley drainages. <br /> Four monitoring wells are used for groundwater compliance monitoring in Grassy Valley (GVMW 8A-250, <br /> GVMW 86-50, GVMW 22A-70, and GVMW 226-30). The central portion of Grassy Valley is underlain by <br /> diatreme rocks, whereas the eastern and western portions are underlain by Precambrian rocks. As a <br /> result, the stream has both losing and gaining reaches, with the losing reach being above diatreme rocks, <br /> which permit a certain amount of vertical infiltration of surface water and groundwater. <br /> No apparent trends with precipitation were observed with respect to any of the constituents of interest. <br /> Sulfate concentration in the Grassy Valley monitoring wells is shown in Figure 19. Sulfate concentration <br /> in the Grassy Valley monitoring wells is substantially below the domestic well standard of 250 mg/L. <br /> In general the trend in sulfate concentration has been declining in all of the wells over time. The data for <br /> GVMW 86-50 tend to have more sample-to-sample variability than is observed in the other three wells. In <br /> addition, sulfate concentration spiked in this well in 2016. Previous work by Newmont showed that this <br /> spike resulted from a leak in the freshwater supply pipeline for the CC&V plant. The pipeline was buried <br /> beneath the East Cresson Overburden Storage Area (ECOSA). The water that leaked from the pipeline <br /> flushed sulfate from the ECOSA foundation, resulting in a temporary elevation in sulfate in groundwater <br /> sampled in GVMW 813-50. The other wells were not impacted by the broken pipeline. <br /> The occasional spikes in sulfate concentration measured in samples from GVMW 813-50 are believed to <br /> result from periods of heavy runoff, because the runoff carries elevated chemical loads from areas in <br /> which historic mining and milling activity have occurred. The runoff rate exceeds the rate at which water <br /> infiltrates into the diatreme, resulting in temporary elevations in dissolved constituents. <br />