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April2021 MNGW-1 pH Memo♦10 <br /> generally increases in a downstream direction. There are three channels that flow into No <br /> Name Gulch from the southwest (from Harrison Mountain). These channels could be <br /> contributing alkalinity to No Name Gulch as it flows through the diversion ditch. It is also <br /> likely that construction material used for the diversion channel provides some alkalinity to <br /> neutralize a portion of the natural acidity in the surface water conveyed off Red Mountain. It <br /> is evident from Figure 9 that this alkalinity contribution is not sufficient to increase the pH <br /> into the neutral range. There also was little observed temporal change in surface water pH <br /> along No Name Gulch. <br /> Based on the data collected to date,the naturally low pH water conveyed by No Name Gulch <br /> off of Red Mountain is contributing to the low pH conditions observed in MNGW-1 and is <br /> likely contributing or directly causing pH values at the POC to go below 6.5 s.u..Based on the <br /> vast extent of the current water level and water chemistry dataset, which now includes over <br /> five years of monthly monitoring data, and given the consistency and repeatability of <br /> monitoring results, monthly monitoring activities are no longer recommended. Henderson <br /> should continue to review the results of the triannual monitoring at MNGW-1 and make <br /> adjustments to monitoring frequency should unexpected conditions or trends develop. <br />