KINETIC AND MINERALOGIC CONTROLS ON THE EVOLUTION OF GROUNDWATER CHEMISTRY AND SR-87 SR-86 IN A SANDY SILICATE AQUIFER, NORTHERN WISCONSIN,USA/

Substantial flowpath-related variability of Sr-87/Sr-86 is observed in groundwaters collected from the Trout Lake watershed of northern Wisc onsin. In the extensive shallow aquifer composed of sandy glacial outw ash, groundwater is recharged either by seepage from lakes or by preci pitation that infiltrates the inter-lake uplands. Sr-87/Sr-86 Of groun dwater derived mainly as seepage from a precipitation-dominated lake n ear the head of the watershed decreases with progressive water chemica l evolution along its flowpath due primarily to enhanced dissolution o f relatively unradiogenic plagioclase. In contrast, Sr-87/Sr-86 Of gro undwater derived mainly from precipitation that infiltrates upland are as is substantially greater than that of precipitation collected from the watershed, due to suppression of plagioclase dissolution together with preferential leaching of Sr from radiogenic phases such as K-feld spar and biotite. The results of a column experiment that simulated th e effects of changing residence time of water in the aquifer sand indi cate that mobile waters obtain relatively unradiogenic Sr, whereas sta gnant waters obtain relatively radiogenic Sr. Nearly the entire range of strontium-isotope composition observed in groundwaters from the wat ershed was measured in the experimental product waters. The constant m obility of water along groundwater recharge flowpaths emanating from t he lakes promotes the dissolution of relatively unradiogenic plagiocla se, perhaps due to effective dispersal of clay mineral nuclei resultin g from dissolution reactions. In contrast, episodic stagnation in the unsaturated zone along the upland recharge flowpaths suppresses plagio clase dissolution, perhaps due to accumulation of clay mineral nuclei on its reactive surfaces. Differences in redox conditions along these contrasting flowpaths probably enhance the observed differences in str ontium isotope behavior. This study demonstrates that factors other th an the calculated state of mineral saturation must be considered when attempting to simulate chemical evolution along flowpaths, and that re action models must be able to incorporate changing contributions from reacting minerals in the calculations.