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

Abstract

The Little River, an ephemeral stream that drains a watershed of approximately 88 km² in northern Florida, disappears into a series of sinkholes along the Cody Scarp and flows directly into the carbonate Upper Floridan aquifer, the source of water supply in northern Florida. The changes in the geochemistry of ground water caused by a major recharge pulse from the sinking stream were investigated using chemical and isotopic tracers and mass-balance modeling techniques. Nine monitoring wells were installed open to the uppermost part of the aquifer in areas near the sinks where numerous subterranean karst solution features were identified using ground penetrating radar. During high-flow conditions in the Little River, the chemistry of water in some of the monitoring wells changed, reflecting the mixing of river water with ground water. Rapid recharge of river water into some parts of the aquifer during high-flow conditions was indicated by enriched values of delta ¹⁸O and delta deuterium (−1.67 to −3.17 per mil and −9.2 to −15.6 per mil, respectively), elevated concentrations of tannic acid, higher (more radiogenic) ⁸⁷Sr/⁸⁶Sr ratios, and lower concentrations of ²²²Rn, silica, and alkalinity compared to low-flow conditions. The proportion of river water that mixed with ground water ranged from 0.10 to 0.67 based on binary mixing models using the tracers ¹⁸O, deuterium, tannic acid, silica, ²²²Rn, and ⁸⁷Sr/⁸⁶Sr. On the basis of mass-balance modeling during steady-state flow conditions, the dominant processes controlling carbon cycling in ground water are the dissolution of calcite and dolomite in aquifer material, and aerobic degradation of organic matter.

Introduction

Along the transition zone between the Northern Highlands and the karstic Gulf Coastal Lowlands in northern Florida, referred to as the Cody Scarp (White, 1970), all streams with the exception of the Suwannee River disappear underground near the toe of the scarp. Other karst features, such as sinkholes, abandoned spring heads, and dry stream courses are also common along the scarp transition zone (Crane, 1986). Direct localized recharge of river water through sinkholes along the Cody scarp typically receives little filtration as it enters the Upper Floridan aquifer, the principal source of water supply in northern Florida. Natural recharge of water from sinking streams to this aquifer can result in water-quality problems, such as high concentrations of iron and hydrogen sulfide, high concentrations of organic material, and undesirable bacteria, protozoa, and fungi (Krause, 1979; McConnell and Hacke, 1993). The susceptibility of the Upper Floridan aquifer to contamination is compounded by the direct hydraulic connections between the aquifer and numerous sinkholes which typically contain highly conductive material (Katz et al., 1995b). This material can be more hydraulically conductive than the surrounding hydrologeologic unit (Lee et al., 1991; Sacks et al., 1992) and may provide little opportunity for attenuation of contaminants prior to entering the aquifer system.

This article presents the results of a study designed to evaluate the processes controlling changes in the chemical composition of ground water resulting from a major recharge pulse from a sinking stream. The Little River is an ephemeral stream that disappears into a series of sinkholes along the Cody Scarp and flows directly into the Upper Floridan aquifer. As part of this study, several chemical and isotopic tracers [major ions, tannic acid, dissolved organic carbon, ¹⁸O/¹⁶O (δ¹⁸O), ²H/¹H (δD), ¹³C/¹²C (δ¹³C), tritium (³H), radon (²²²Rn), and strontium-87/strontium-86 (⁸⁷Sr/⁸⁶Sr)] are evaluated, regarding their effectiveness in identifying and quantifying hydrochemical interactions between river water and the Upper Floridan aquifer. Geochemical models, which incorporate these chemical and isotopic data along with analyses of aquifer mineralogy, are used to calculate mass transfer for dominant biogeochemical processes and to estimate the amount of mixing of river water in ground water. Results from this study provide a framework for a better understanding of the hydrochemical interactions between ground water and surface water in karst areas and the processes controlling the evolution of ground-water chemistry during low- and high-flow conditions. Knowledge of the hydrochemical interaction between river water and ground water is vital for evaluating the susceptibility of ground water to contamination.