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Moro and /Issodafea, l~ <br />' TU indicates that the water is of mixed origin with modem recharge water (i.e. < 50 years). <br />What this means is that a component of the water discharging from the portal is more than <br />20,000 years old and a component of the water is very young. <br />Geochemical Modeling <br />Chemical reactions <br />Solute compositions of groundwaters are the result of interactions between groundwaters and <br />bedrock lithologies and between groundwaters and atmospheric and soil gases. The general <br />reactions responsible for the chemical evolution of groundwaters in the vicinity of the West <br />Elk lease area and inside the coal mine are described below. <br />' Groundwater typically acquires most of its COZ~R~ in the soil zone where the partial pressures <br />of COQ greatly exceeds atmospheric levels. Groundwater may also acquire appreciable <br />' quantities ofCO2 during maturation oforganic material, by the oxidation and bacterial action <br />on CH„ and by other mechanisms. The S"C content of [he fault waters indicate that these <br />waters have acquired considerable CO, by bacterial action on CH,. Regardless of the source, <br />~ CO2 combines with water forming carbonic acid according to <br />CO,~~~ + H,O = H2C0, <br />Carbonic acid dissociates into H' and HCO; as <br />The H' ions temporarily decrease the pH of the water but are quickly consumed by the <br />dissolution of carbonate minerals that aze abundant in the soil zone and in most aquifers. <br />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 Ca2+ and SO; contents in the absence of additional <br />I CO,~g~ and H' according to <br />' CaSO,•2H,O = Ca" + SO,Z- + 2H,O. (5) <br />(gypsum) <br />edwspr.doc 4 20 January 1998 <br />