Hydrochemical evidence for mixing of river water and groundwater during high-flow conditions, lower Suwannee River basin, Florida, USA

Karstic aquifers are highly susceptible to rapid infiltration of river wate r, particularly during periods of high flow. Following a period of sustaine d rainfall in the Suwannee River basin, Florida, USA, the stage of the Suwa nnee River rose from 3.0 to 5.88 m above mean sea level in April 1996 and d ischarge peaked at 360 m(3)/s. During these high-now conditions, water from the Suwannee River migrated directly into the karstic Upper Floridan aquif er, the main source of water supply for the area. Changes in the chemical c omposition of groundwater were quantified using naturally occurring geochem ical tracers and mass-balance modeling techniques. Mixing of river water wi th groundwater was indicated by a decrease in the concentrations of calcium , silica, and Rn-222; and by an increase in dissolved organic carbon (DOC), tannic acid, and chloride, compared to low-flow conditions in water from a nearby monitoring well, Wingate Sink, and Little River Springs. The propor tion (fraction) of river water in groundwater ranged from 0.13 to 0.65 at W ingate Sink and from 0.5 to 0.99 at well W-17258, based on binary mixing mo dels using various tracers. The effectiveness of a natural tracer in quanti fying mixing of river water and groundwater was related to differences in t racer concentration of the two end members and how conservatively the trace r reacted in the mixed water. Solutes with similar concentrations in the tw o end-member waters (Na, Mg, K, Cl, SO4, SiO2) were not as effective tracer s for quantifying mixing of river water and groundwater as those with large r differences in end-member concentrations (Ca, tannic acid, DOC, Rn-222, H CO3).