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ARCADIS <br />In addition to ionic compositions, pH and metals concentrations have the potential to elucidate potential <br />influence of mining activities on surface water quality. Maximum, minimum and average pH and <br />concentrations of total iron, total recoverable iron, dissolved iron, total manganese, and dissolved <br />manganese are presented in Table 2. As shown, there is little variability in pH between surface water, <br />alluvial groundwater, and mine water, with average values ranging between 7.6 and 8.1. Average <br />concentrations of total iron, total recoverable iron, and total manganese are generally higher at alluvial <br />wells, including upgradient well PAW -1, relative to surface water and mine water sampling stations. <br />Because metals concentrations are elevated in the upgradient alluvial well, and are only slightly elevated <br />in mine water relative to surface water, based on the available data presence of these constituents in <br />groundwater and surface water cannot be directly attributed to mining activities. <br />4. Comparison of Upgradient and Downgradient Surface Water Quality <br />To evaluate the potential effect of mine dewatering activities on surface water quality in the Purgatoire <br />River, three key water quality parameters were compared over time at upstream station PRS -1 and <br />downstream station PRS -4: total recoverable iron, EC, and SAR (Figures 11 through 13). <br />Iron may serve as an important indicator parameter for the influence of mining activities, although as <br />mentioned in the previous section, the presence of elevated iron concentrations in the upgradient alluvial <br />well suggests that elevated iron concentrations cannot be fully attributed to mining activities. The chronic <br />aquatic life standard for total recoverable iron is 1 mg/L. As shown in Figure 11, this concentration was <br />exceeded during one sampling event at PRS -4 in 2008. With the exception of the period between 2006 <br />and 2008, concentrations of total recoverable iron at the upstream and downstream stations show a <br />similar temporal pattern and are similar in magnitude, and therefore do not indicate an influence of mine <br />water discharge on surface water quality. <br />Waters of the Purgatoire River are designated for beneficial use in agriculture, and the salinity of irrigation <br />water is an important control on rates of water infiltration. Waters with low values of EC (less than 200 to <br />500 pmhos/cm), and waters with waters with high EC values and with high sodium to calcium ratios, tend <br />to reduce the infiltration capacity of soils (Colorado Water Quality Control Division [WQCD] 2008; Ayers <br />and Westcot 1985). Therefore, both the EC and sodium content of waters that may be designated for <br />beneficial use in agriculture must be considered. The SAR is a measure of the sodicity of water and is an <br />expression of the relative concentration of sodium ions to calcium and magnesium ions (WQCD 2008). In <br />cases where high concentrations of bicarbonate are also present (at concentrations exceeding <br />approximately 150 mg/L), the adverse effect of a sodium imbalance may be worsened by the loss of <br />calcium due to carbonate mineral precipitation. In these cases, an index known as the adjusted SAR is <br />most appropriate (WQCD 2008; Lesch and Suarez 2009). The adjusted SAR was calculated for each <br />monitoring event conducted at the upstream and downstream Purgatoire River stations based on the <br />guidance outlined in Lesch and Suarez (2009). <br />Page <br />c WsersVmftmpsm4ipoab bcaknuDsoftmdows%temporary infixnetfileskwtBntm&ok*BufMiNewekcoalsufacewatennimeeassessmentnemo dou 5/8 <br />