GROUNDWATER CHEMICAL EVOLUTION IN A SANDY SILICATE AQUIFER IN NORTHERN WISCONSIN .2. REACTION MODELING

Major chemical trends along flow paths in a sandy silicate aquifer (Kenoyer and Bowser, this issue) are interpreted using a mass balance approach and reaction simulation modeling. Because of the diverse mineral assemblage present and uncertainties in the thermodynamic constants, it is not possible to derive a unique solution. However, considering relative reaction kinetics and mineral abundance, a model involving the dissolution of plagioclase and small amounts of biotite and pyroxene to kaolinite in an open-CO2 system is a simple, thermodynamically valid and reasonable explanation for the major chemical changes. The favored reaction model would result in apparent rates of dissolution of 2 x 10(-18) mol cm-2 s-1 plagioclase (An41), 3 x 10(-20) mol cm-2 s-1 biotite, and 3 x 10(-17) mol cm-2 s-1 diopside. These rates are lower than published rates from laboratory studies, that range from 10(-11) to 10(-17) mol cm-2 s-1 for plagioclase, but are in the same range as rates derived from field studies. In predictive simulations where current silicate dissolution rates are assumed to hold constant but the pH of the recharge water is lowered from 6.4 to 3.99 (simulating increasing acidity of precipitation), the pH and alkalinity of the groundwater are reduced significantly.