THE ORIGIN AND AGE OF SPRING WATERS IN THE ST. JOHNS RIVER WATER MANAGEMENT DISTRICT, CENTRAL FLORIDA

Eleven first and second magnitude springs in the St. Johns River Water Management District, Florida, were sampled in February and March 2001 for water chemistry, CFC’s, tritium-helium-3, SF6, and nitrogen isotopes. Six of the springs contained calcium-bicarbonate type water; four contained sodium-chloride type water; and one contained sodium-chloride, calcium-bicarbonate type water. The evolution of water chemistry for calcium-bicarbonate springs was modeled by reactions of rain water with soil organic matter, calcite, and dolomite under oxic conditions. The water chemistry for sodium-chloride and sodium-chloride, calcium-bicarbonate type springs was modeled by reactions of either rain water or Upper Floridan aquifer water with soil organic matter, calcite, and dolomite under oxic conditions and mixing with varying proportions of Lower Floridan aquifer water. For the sodium-chloride and sodium-chloride, calcium-bicarbonate springs, the proportions of Lower Floridan aquifer water ranged from 4 to 53%.

Generally concordant CFC-113 and 3H/3He average residence times of ground water discharging from these springs can best be explained with a binary mixing model, in which varying proportions of young water (recharged during early 1990s) have mixed with older water containing little or no tritium or CFCs (approximately early 1950s and older). Mixing fractions of young water ranged from about 0.1 to 0.7. Adjusted carbon-14 ages for all spring waters calculated by using NETPATH are modern and are consistent with other chemical and isotopic data.

Nitrate-N concentrations ranged from 0.03 to 1.28 mg/L and exceed background levels (0.20) of nitrate-N in ground water in seven of the springs. Delta 15N values in these springs ranged from 5.8 to 12.9 per mil and suggest a range of sources from mixtures of inorganic (fertilizers) and organic (animal wastes or sewage) to primarily organic. The chemical and isotopic data, along with the geochemical modeling results suggest that ground water discharging from these springs is locally derived and contains variable amounts of recently recharged water.