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‐CO 2 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 × 10 −18 mol cm −2 s −1 plagioclase (An 41 ), 3 × 10 −20 mol cm −2 s −1 biotite, and 3 × 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 p H of the recharge water is lowered from 6.4 to 3.99 (simulating increasing acidity of precipitation), the p H and alkalinity of the groundwater are reduced significantly.