GROUNDWATER-SURFACE WATER EXCHANGE IN TAMPA BAY: RESULTS FROM A PRELIMINARY GEOCHEMICAL AND GEOPHYSICAL SURVEY

The reversible exchange of ground water from the Floridan aquifer system to Tampa Bay bottom waters is controlled foremost by hydraulic head gradients between these two water bodies, as well as by the hydraulic conductivity and thickness of the overburden deposits. Superimposed on such physical hydrogeologic characteristics are the effects of climate, i.e., precipitation/evaporation and our own increasing demand on ground water reserves. Saltwater intrusion can consequently occur wherever the potentiometric surface of a coastal aquifer lies below sea level. Similarly, ground water has the potential to be discharged as submarine ground water discharge when the potentiometric surface lies above sea level. Past estimates indicate as much as 50 million gallons of ground water can be discharged into Tampa Bay per day (~189,270 cubic meters per day) – roughly 20 % of the combined surface water runoff.

To begin to quantify this exchange of water across the sediment/water interface in Tampa Bay, we developed two field techniques – a geochemical survey of select isotopic tracers and trace elements measured across the sediment-water interface, as well as a novel geophysical technique using streaming resistivities. Multi-port piezometers and Lee-type seep meters were used to collect interstitial waters from discrete sediment depth intervals and were analyzed for such constituents as 87Sr/86Sr, Cl-, nutrients, and trace elements (U, V, Mo, Ba, Sr, Fe, Mn). Pore water results indicate that exchange across the sediment-water interface involves mostly recycled seawater at the four sites. Similarly, an interpretation of select streaming resistivity profiles does not appear to indicate significant freshened water masses close to the sediment-water interface. These findings, in light of the prolonged drought and lowered potentiometric surfaces surrounding Tampa Bay, suggest that the upward migration of Floridan ground water may be considerably less than what has been previously estimated.