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t SECTIONSEVEN <br />J <br /> <br /> <br /> <br /> <br /> <br />u <br />Groundwater Chemistry <br />Groundwater samples were collected from the monitoring wells at the end of deve]ol~ment, and a <br />sample was collected from the extraction well neaz the end of the pump test. These samples <br />were analyzed for major ions and metals, including chloride, sulfate, carbonate alkalinity, <br />bicarbonate alkalinity, total dissolved solids, calcium, iron, magnesium, manganese, ;md sodium. <br />Groundwater analytical results are compiled in Table 1 and presented in Figures 9 through 11. <br />The concentrations of major ions are highest at the extraction well site and steadily dE;crease <br />toward monitoring well HMMW-03, which is located 100 feet to the west of HMEX-Ol. For <br />example, total dissolved solids (TDS) concentration decreases by 50 percent between HMEX-01 <br />and HMMW-03. Iron, manganese, and calcium concentrations aze highest at HMMV~/-02, and <br />alkalinity is highest in HMMW-03. <br />The analytical results suggest a preferential flowpath for groundwater through the portion of the <br />alluvial channel between HMEX-Ol and HMMW-02. The concentrations of tailing-related <br />constituents are highest in these wells, which suggests that a relatively large proportion of the <br />seepage from the tailing impoundment flows through this portion of the alluvium. The alluvial <br />sediments reach their greatest thickness in this azea and the historical aerial photos reveal that t}te <br />main channel of this stream tributary existed in this area prior to construction of the tailing <br />impoundment. <br />The presence of elevated concentrations of iron and manganese combined with pH less than 7.0 <br />and available dissolved oxygen (shallow groundwater) provide conditions suitable for <br />precipitation of iron and/or manganese oxides. When in contact with oxygen, ferrous ions <br />become unstable and change to ferric ions. Ferric ions are almost completely insoluble in <br />weakly acidic waters like those found at this site. Manganese is present as manganous. <br />bicarbonate, which can precipitate out of solution when carbon dioxide is liberated from the <br />water as a result of groundwater level drawdown near a pumping well (Driscoll, 1995). <br /> <br /> <br /> <br /> <br /> <br />1 <br /> <br />~~` wWiWECTSSe1~Sw_NpgERSOM_NY~fEIAAP_opeo PpW_D6MAEVISED TEd wEYPPt.DDgtrm, l~tM /-1 <br /> <br />