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" Mato and Assoelafes, Lf <br />TU indicates that the water is of mixed origin with modem recharge water (i.e. < 50 yeazs). <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 />Geochemicat Modeling <br />C{:emical reactions <br />' Solute compositions of groundwaters are the result of interactions between groundwaters and <br />l bedrock lithologies and between groundwaters and atmospheric and soil gases. The general <br />11 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 C0,1g1 in the soil zone where [he partial pressures <br />of COQ greatly exceeds atmospheric levels. Groundwater may also acquire appreciable <br />quantities of CO, during maturation of organic material, by the oxidation and bacterial action <br />on CH„ and by other mechanisms. The S"C content of the 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 /> <br />Carbonic acid dissociates into H' and HCO; as <br /> <br />The H' ions temporarily decrease the pH of the water but are quickly consumed by the <br />dissolution of carbonate minerals that are abundant in the soil zone and in most aquifers. <br />Cazbonate mineral dissolution is represented as <br />2H' + CaMg(CO,), = Ca" + Mg~* + 2HC0;, and <br />(dolomite) <br />(3) <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'-', Mgz+, 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,Ig~ and H' according to <br />CaSO,•2HzO = Ca" + SO,'- + 2H2O. (5) <br />(gypsum) <br />edwspr.doc 4 20 January 1998 <br />