USE OF CHEMICAL AND ISOTOPIC TRACERS TO CHARACTERIZE THE INTERACTIONSBETWEEN GROUND-WATER AND SURFACE-WATER IN MANTLED KARST

In the mantled karst terrane of northern Florida, the water quality of the Upper Floridan aquifer is influenced by the degree of connectivit y between the aquifer and the surface. Chemical and isotopic analyses [O-18/O-16(delta(18)O),H-2/H-1(delta D),C-13/C-12(delta(13)C), tritium (H-3), and strontium-87/strontium-86(Sr-87/Sr-86)]along with geochemic al mass-balance modeling were used to identify the dominant hydrochemi cal processes that control the composition of ground water as it evolv es downgradient in two systems. In one system, surface water enters th e Upper Floridan aquifer through a sinkhole located in the Northern Hi ghlands physiographic unit. In the other system, surface water enters the aquifer through a sinkhole lake (Lake Bradford) in the Woodville K arst Plain. Differences in the composition of water isotopes (delta(18 )O and delta D) in rainfall, ground water, and surface water were used to develop mixing models of surface water (leakage of water to the Up per Floridan aquifer from a sinkhole lake and a sinkhole) and ground w ater. Using mass-balance calculations, based on differences in delta(1 8)O and delta D, the proportion of lake water that mixed with meteoric water ranged from 7 to 86% in water from wells located in close proxi mity to Lake Bradford, In deeper parts of the Upper Floridan aquifer, water enriched in O-18 and D from five of 12 sampled municipal wells i ndicated that recharge from a sinkhole (1 to 24%) and surface water wi th an evaporated isotopic signature (2 to 32%) was mixing with ground water. The solute isotopes, delta(13)C and Sr-87/Sr-86, were used to t est the sensitivity of binary and ternary mixing models, and to estima te the amount of mass transfer of carbon and other dissolved species i n geochemical reactions. In ground water downgradient from Lake Bradfo rd, the dominant processes controlling carbon cycling in ground water were dissolution of carbonate minerals, aerobic degradation of organic matter, and hydrolysis of silicate minerals. In the deeper parts of t he Upper Floridan aquifer, the major processes controlling the concent rations of major dissolved species included dissolution of calcite and dolomite, and degradation of organic matter under oxic conditions. Th e Upper Floridan aquifer is highly susceptible to contamination from a ctivities at the land surface in the Tallahassee area. The presence of post-1950s concentrations of H-3 in ground water from depths greater than 100 m below land surface indicates that water throughout much of the Upper Floridan aquifer has been recharged during the last 40 years . Even though mixing is likely between ground water and surface water in many parts of the study area, the Upper Floridan aquifer produces g ood quality water, which due to dilution effects shows little if any i mpact from trace elements or nutrients that are present in surface wat ers.