9725295 Martin The extent of chemical exchange and mixing between surface water, ground water, and aquifer rocks has been poorly constrained in karst aquifers, although they provide (25% of the world's potable water. Although these aquifers are important sources of water, they appear to be more susceptible to contamination than other types of aquifers because of large amounts of mixing and rapid subsurface flow. Mixing, as well as fluid-solid reactions in the subsurface, should be reflected in natural chemical variations of water discharged from karst aquifers. This hypothesis will be tested along a 10 km stretch of the Santa Fe river where it flows through the subsurface at the boundary of confined and unconfined portions of the Floridan aquifer in north-central Florida. Samples will be collected from the sink and rise at high frequency (one sample every two hours) for two 4 day periods: one at low discharge and one at high discharge. Variations in compositions of non-reactive solutes (Cl and (18O) with river stage should provide information on residence time of the water in the subsurface. Additional sets of samples will be collected at high and low discharge from the sink , rise, and two water supply wells, one at the sink and the other at the rise. Sampling of water at the rise will be delayed by the length of time necessary for water to flow from the sink to the rise. The sink, rise, and well samples will be measured for a suite of solutes (Ca, Mg, Na, K, DIC, NO3, SO4, Cl, F, Si, Mn, Fe concentrations, (D and (18O values pH, alkalinity, and DO) that will serve as constraints for NETPATH models of mixing and natural fluid-solid reactions. Measurement of residence time, calculation of the extent of mixing, and models of fluid-solid reactions should provide information about the susceptibility of karst aquifers to contamination including the length of time available for natural remediation of polluted water, the quantity of polluted water that might recharge the aquifer, and t he background natural chemical variability caused by reactions. These results should be important for management of water supplies in the Floridan aquifer, the primary water source for all of Florida.
