Bg. Katz et al., CHEMICAL EVOLUTION OF GROUNDWATER NEAR A SINKHOLE LAKE, NORTHERN FLORIDA .1. FLOW PATTERNS, AGE OF GROUNDWATER, AND INFLUENCE OF LAKE WATERLEAKAGE, Water resources research, 31(6), 1995, pp. 1549-1564
Leakage from sinkhole lakes significantly influences recharge to the U
pper Floridan aquifer in poorly confined sediments in northern Florida
. Environmental isotopes (oxygen 18, deuterium, and tritium), chlorofl
uorocarbons (CFCs: CFC-11, CCl3F; CFC-12, CCl2F2; and CFC-113, C2Cl3F3
), and solute tracers were used to investigate groundwater flow patter
ns near Lake Barco, a seepage lake in a mantled karst setting in north
ern Florida. Stable isotope data indicated that the groundwater downgr
adient from the lake contained 11-67% lake water leakage, with a limit
of detection of lake water in groundwater of 4.3%. The mixing fractio
ns of lake water leakage, which passed through organic-rich sediments
in the lake bottom, were directly proportional to the observed methane
concentrations and increased with depth in the groundwater flow syste
m. In aerobic groundwater upgradient from Lake Barco, CFC-modeled rech
arge dates ranged from 1987 near the water table to the mid 1970s for
water collected at a depth of 30 m below the water table. CFC-modeled
recharge dates (based on CFC-12) for anaerobic groundwater downgradien
t from the lake ranged from the late 1950s to the mid 1970s and were c
onsistent with tritium data. CFC-modeled recharge dates based on CFC-1
1 indicated preferential microbial degradation in anoxic waters. Verti
cal hydraulic conductivities, calculated using CFC-12 modeled recharge
dates and Darcy's law, were 0.17, 0.033, and 0.019 mid for the surfic
ial aquifer, intermediate confining unit, and lake sediments, respecti
vely. These conductivities agreed closely with those used in the calib
ration of a three-dimensional groundwater flow model for transient and
steady state flow conditions.