Tracing groundwater flow on a barrier island in the north-east Gulf of Mexico

Groundwater flow on St. George Island, a barrier island in the north-east G ulf of Mexico, was monitored downfield from wastewater systems using artifi cial tracer techniques. Sulphur hexafluoride and fluorescein dye were used to determine groundwater flow velocity, hydraulic conductivity, and dispers ivity at selected sites on the island. Monthly hydraulic head measurements illustrate the aquifer's dependence on recharge associated with rainfall. H owever, during periods of little rain, tidal stage also influences the dire ction and magnitude of groundwater flow within approximately 30 m of the wa ters edge. Estimated hydraulic conductivities ranged from 3 to 180 m day(-1 ), with an overall estimate of 36 m day(-1). Groundwater tracers showed ver y little dilution and calculated longitudinal dispersivities were approxima tely 0.1 to 0.5 m, which is in the same range as previous studies of sandy aquifers. The total groundwater flux into adjacent Apalachicola Bay was also evaluate d using two independent techniques. Darcy's law was applied using an estima ted cross sectional area and the experimental horizontal transport rates to estimate the volumetric flow. In addition, a simple water balance calculat ion was used, which accounted for all known sources and sinks of water to t he aquifer. The two independent approaches agreed very well, with an estima ted groundwater flux from the surficial aquifer to Apalachicola Bay between 1-9 x 10(6) m(3) year(-1). (C) 2000 Academic Press.