GROUNDWATER CHEMICAL EVOLUTION IN A SANDY SILICATE AQUIFER IN NORTHERN WISCONSIN .1. PATTERNS AND RATES OF CHANGE

Patterns and rates of chemical evolution of groundwater in a sandy silicate aquifer were determined by detailed analysis of groundwater velocity and chemistry along short (120 m) flow paths. Groundwater enters the aquifer mainly as seepage from a dilute lake and evolves chemically under open-CO2 conditions for at least 8 years before discharging to another lake. Spatial trends in the aquifer are evident for calcium, magnesium, silicon, sodium, CO2, alkalinity, pH, temperature, and dissolved oxygen. The kinetics of silicate mineral dissolution regulates additions of calcium, magnesium, and alkalinity to the groundwater, causing them to increase steadily along flow paths at about 12-mu-mol L-1 yr-1 (Ca), 6-mu-mol L-1 yr-1 (Mg), and 39-mu-eq L-1 yr-1 (alkalinity) respectively, for at least 3-5 years. Silicon and sodium increase by 41-mu-mol L-1 yr-1 and 13-mu-mol L-1 yr-1 respectively for about 3 years, then level off as the groundwater approaches saturation with respect to kaolinite and smectite.