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April2021 MNGW-I pH Memo♦9 <br /> 5.0 DISCUSSION <br /> A thin 20- to 25-foot-thick section of colluvium occurs northeast of the Henderson Mine <br /> surface facilities and overlies granitic bedrock.The colluvium comprises a thin surficial aquifer <br /> with water levels varying seasonally from less than 1 to 15 ft bgs (2.5 to 18 ft btoc). Water <br /> levels rise in the late-spring and early-summer in response to snowmelt and decline in the fall <br /> and winter months. Groundwater flow direction varies seasonally with a northeasterly <br /> component during the wetter spring and summer months, shifting to a more easterly flow <br /> direction during the drier months of the year. Groundwater velocities in the colluvial aquifer <br /> are estimated to range from 0.4 to 1.2 ft/day based on the range of estimated hydraulic <br /> conductivities of 1 to 3 ft/day and an average hydraulic gradient of 0.08 ft/ft. <br /> The elevation of water in the No Name Gulch,in the diversion ditch near the monitor wells, <br /> is above the static water levels when it is flowing. Therefore, it is a recharge source to the <br /> shallow colluvial aquifer when it is flowing. <br /> MNGW-1 water chemistry is similar to the naturally impacted surface water in No Name <br /> Gulch(at NNG-40 and NNG-50)with low sulfate concentrations during the summer months <br /> (Figure 13), and low specific conductance field measurements (Table 3). However, unlike <br /> NNG-40 and NNG-50, MNGW-1 samples do not show elevated concentrations of <br /> manganese or zinc(Figures 11 and 12).Analytical results at MNGW-1 for indicator parameters <br /> established in the GWMP are below respective NPLs; this includes samples collected from <br /> MNGW-1 during periods of low pH. <br /> The lowest pH values in MNGW-1,as well as MNGW-3 and MNGW-4, often occur during <br /> the spring runoff and summer periods, suggesting a potential relationship between aquifer <br /> recharge and groundwater chemistry. Historical low pH values at MNGW-1 have also been <br /> recorded during low flow/winter conditions,when No Name Gulch is dry,which may reflect <br /> residual effects of summer recharge or that groundwater may also be slightly acidic. <br /> Measurements of pH from No Name Gulch,including from the diversion ditch portion (i.e., <br /> NNG-50) and from sampling stations in the natural stream south (upstream) of the Mine <br /> surface facilities (i.e.,NNG-40),have generally been below 5 s.u.The pH of No Name Gulch <br />