Changes in the isotopic and chemical composition of ground water resultingfrom a recharge pulse from a sinking stream

The Little River, an ephemeral stream that drains a watershed of approximat ely ss km(2) in northern Florida, disappears into a series of sinkholes alo ng the Cody Scarp and flows directly into the carbonate Upper Floridan aqui fer, the source of water supply in northern Florida. The changes in the geo chemistry of ground water caused by a major recharge pulse from the sinking stream were investigated using chemical and isotopic tracers and mass-bala nce modeling techniques, Nine monitoring wells were installed open to the u ppermost part of the aquifer in areas near the sinks where numerous subterr anean karst solution features were identified using ground penetrating rada r. During high-flow conditions in the Little River, the chemistry of water in some of the monitoring wells changed, reflecting the mixing of river wat er with ground water. Rapid recharge of river water into some parts of the aquifer during high-flow conditions was indicated by enriched values of del ta O-18 and delta deuterium (-1.67 to -3.17 per mil and -9.2 to -15.6 per m il, respectively), elevated concentrations of tannic acid, higher (more rad iogenic) Sr-87/Sr-86 ratios, and lower concentrations of Rn-222, silica, an d alkalinity compared to low-how conditions. The proportion of river water that mixed with ground water ranged from 0.10 to 0.67 based on binary mixin g models using the tracers O-18, deuterium, tannic acid, silica, Rn-222, an d Sr-87/Sr-86. On the basis of mass-balance modeling during steady-state ho w conditions, the dominant processes controlling carbon cycling in ground w ater are the dissolution of calcite and dolomite in aquifer material, and a erobic degradation of organic matter. (C) 1998 Elsevier Science B.V. All ri ghts reserved.