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April2021 MNGW-1 pHMemo♦7 <br /> • MNGW-1 pH (since 2012) has ranged from 5.4 to 8.0 standard units (s.u.). The <br /> median MNGW-1 pH over this time period is 6.4 s.u. <br /> • MNGW-3 pH (since installation in 2014) has ranged from 5.7 to 6.6 s.u. The median <br /> MNGW-3 pH value over this time period is 6.3 s.u. <br /> • MNGW-4 pH (since installation in 2014) has ranged from 6.0 to 7.0 s.u. The median <br /> MNGW-4 pH value over this time period is 6.3 s.u. <br /> NNG-40 was sampled six times between 2018 and 2020, and pH has ranged from 3.5 to 4.5 <br /> s.u. The pH of samples collected from NNG-50, located adjacent to MNGW-1 (Figure 2), <br /> since initiation of sampling in July 2013 has ranged from 3.9 to 7.4 s.u.. The median pH of <br /> samples collected from NNG-50 is 4.6 s.u..Figure 8 shows that the pH in NNG-40 and NNG- <br /> 50 samples is consistently lower than groundwater samples collected from the three <br /> monitoring wells. <br /> To further the understanding of low pH measurements in No Name Gulch and MNGW-1,a <br /> field sampling program was performed in 2017 starting at the headwaters of No Name Gulch <br /> on Red Mountain and continuing to NNG-50. Field measurements along the gulch between <br /> the headwaters and NNG-50 support low pH values (below 5.0 s.u.) as shown on Figure 9. <br /> It can be seen from Figure 9 that there is an increasing trend in the pH along No Name Gulch <br /> in the direction of flow (from sampling points 1 to 18). During this study, pH was observed <br /> to be lowest at the upper reaches of No Name Gulch (-3.4 at sampling location 1). It then <br /> generally increases as No Name Gulch flows towards the West Fork of Clear Creek (4.2 at <br /> sampling location 18 near NNG-50). <br /> Other GWMP Analytes <br /> The following is a summary of the occurrence and concentrations of other GWMP analytes <br /> based on sampling results from MNGW-1, MNGW-3, MNGW-4, NNG-40, and NNG-50 <br /> since 2014. Note there have been no other exceedances of a GWMP Numeric Protection <br /> Limit (NPL) in POC monitor well MNGW-1 with the exception of the previously discussed <br /> pH results. <br />