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Moro and Assoelafes, LC <br />• Groundwater acquires most of its CO,tg~ in the soil zone where the partial pressure of CO, <br />greatly exceeds atmospheric levels. This CO, combines with water to form carbonic acid <br />according to <br /> <br />Carbonic acid dissociates into H' and HCO; as <br /> <br />The H' ions temporarily decrease the pH of [he water but are quickly consumed by the <br />dissolution of carbonate minerals that are abundant in the soil zone and in most western U.S. <br />sedimentary rocks. Carbonate mineral dissolution is represented as <br />2H' + CaMg(CO,): = Ca''' + Mg" + 2HC0;, and (3) <br />(dolomite) <br />• H' + CaCO, = Ca" + HCO;. (4) <br />(calcite) <br />The net effect of reactions 2 through 4 is to increase the pH and the Ca", Mg'', and HCO; <br />contents of waters. Dissolution of small amounts of gypsum, which is present in many <br />formations in the region, can elevate the Ca" and SO;- contents in the absence of additional <br />CO„gl and H' according to <br />CaSO~•2H_O = Ca" + SO;" + 2H,0. <br />(gypsum) <br />(5) <br />Elevated Nay concentrations may result from either the dissolution of small amounts of very <br />soluble halite or from ion exchange on clay particles or on sodium zeolites. Halite <br />Characterization of Groundwater Systems in the Vicinity of the West Elk Mine, Somerset, Colorado <br />29 January 1999 <br />Page 2~ <br />