2017-05-11_PERMIT FILE - C1981012 (7)

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ARCADIS United States Geological Survey (USGS) Gw Chart tool (http://water.usgs.gov/nrp/gwsoftware/GW—Chart/GW_Chart.html) was used to create Piper diagrams. Because the Piper diagram method is based on the calculation of ternary ratios, exclusion of any ionic species from the calculation based on a lack of data during a single sampling event may result in a skewed result that is not representative of the water type. Therefore, at each monitoring location, data was selected from dates where analytical results were reported all relevant ionic species. Average concentrations of each ionic species over the full period of monitoring were used to determine the average water composition for each sampling location. In addition to comparing the ionic compositions of waters, historical average values of: pH, total dissolved solids (TDS), EC, total iron, total recoverable iron, dissolved iron, total manganese and dissolved manganese were compared between sampling stations. The characteristics of surface water samples collected at upstream (PRS -1) and downstream (PRS -4) stations are similar to one another and are distinct from the composition of mine waters (NEW -2 and NEW -4) (Figure 2, Table 1). The concentration of TDS and EC values measured in surface water are lower than those measured in mine water (Table 1). Surface waters are characterized by a high average proportion of calcium (72.4 to 73.5 percent) and similar average proportions of magnesium (13.1 to 13.3 percent) and sodium plus potassium (13.3 to 14.5 percent). In contrast, mine waters are characterized by a high average proportion of sodium plus potassium (95.6 to 98.2 percent) with a dominant contribution from sodium — 279 to 619 mg/L sodium, compared to 6 to 6.4 mg/L potassium. Surface waters are also characterized by lower carbonate plus bicarbonate concentrations (140.6 to 140 mg/L) than mine waters (544 to 1,417 mg/L). As illustrated by clustering of data points in Figures 3 and 4 (surface waters) and Figures 5 and 6 (mine waters) the compositions of these waters show relatively little variability over time. At mine water station NEW -2 (Figure 9) two data points appear to be anomalous. These samples were collected in 1996 and 2008 and show a high proportion of calcium, a low proportion of sodium, a low concentration of TDS, and a low EC relative to other samples collected at this location. One data point at surface water station PRS -1 (Figure 3) appears to be anomalous, with a relatively low bicarbonate concentration. As illustrated in Figures 7 through 10, the ionic composition of alluvial groundwater shows a greater degree of temporal variability than that of surface waters and mine waters. The EC values and average ionic compositions of alluvial groundwater samples are between those observed in surface waters and mine waters (Table 1). Alluvial groundwater has a lower average proportion of calcium than surface water (43.3 to 49.3 percent) and a lower average proportion of sodium plus potassium than mine water (33.0 to 48.1 percent). With the exception of upgradient alluvial well PAW -1, alluvial groundwater samples also have TDS and carbonate plus bicarbonate concentrations between those measured in surface water and mine water. The ionic composition of the upgradient alluvial well (PAW -1) is somewhat distinct from that of downgradient alluvial wells (PAW -2, PAW -8 and PAW -9), with a signature more similar to that of surface water with a higher proportion of calcium relative to sodium, lower concentrations of carbonate plus bicarbonate and TDS, and a lower EC. These results suggest that either mining activities or a change in lithology in the downgradient direction influence groundwater ionic compositions. Page cWserstwtm psm�appdaW*)cahnuDsDtaMmsW*mporaymftmetfiles%cmientoudD"6ui#llVwmkcoah0acmaWnfkjenwassessm trrwp.dock 4/8