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 Gulf of Mexico, was monitored downfield from wastewater systems using artificial tracer techniques. Sulphur hexafluoride and fluorescein dye were used to determine groundwater flow velocity, hydraulic conductivity, and dispersivity at selected sites on the island. Monthly hydraulic head measurements illustrate the aquiferʹs dependence on recharge associated with rainfall. However, during periods of little rain, tidal stage also influences the direction and magnitude of groundwater flow within approximately 30 m of the waters edge. Estimated hydraulic conductivities ranged from 3 to 180 m day-1, with an overall estimate of 36 m day-1. Groundwater tracers showed very little dilution and calculated longitudinal dispersivities were approximately 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 evaluated using two independent techniques. Darcyʹs law was applied using an estimated cross sectional area and the experimental horizontal transport rates to estimate the volumetric flow. In addition, a simple water balance calculation was used, which accounted for all known sources and sinks of water to the aquifer. The two independent approaches agreed very well, with an estimated groundwater flux from the surficial aquifer to Apalachicola Bay between 1–9×106 m3 year-1.