Characterization of freshwater lenses for construction of groundwater flow models on two sandy barrier islands, Florida, USA

Groundwater models are being constructed to evaluate the impact of increased development on two adjacent sandy barrier islands on the northern Gulf coast of Florida, USA. To characterize the hydrostratigraphy and seasonal variability we are conducting resistivity and electromagnetic profiles across the freshwater lens and seepage meter and well sampling of freshwater fluxes and heads. The islands, Dog Island and St. George Island, are a “drumstick” and a strip barrier island, ~100-2000 m x 10 km and ~250-1000 m x 40 km, respectively. Dog Island relies exclusively on shallow, mostly nearshore, wells for its water supply. St. George Island has a much higher population density and meets most of its water demands via an aqueduct from the mainland. Potential effects on the hydrostratigraphy of St. George Island from the artificial recharge include increased freshwater lens volume, increased submarine groundwater discharge (SGD) rates, and a degradation of groundwater quality. Both islands use septic systems as the primary means of waste disposal. The maximum lens thickness on both islands is shifted seaward of the island’s center and ranges from ~2 to 25 m on Dog Island, and from ~5 to 30 m on St. George Island. The lenses extend down through unconsolidated sands into underlying limestone. Density-dependent groundwater flow models show that a spatially variable recharge, correlated with vegetation, can account for the asymmetry observed in the freshwater lenses. Groundwater seepage into 1-2 m deep water between St. George Island and the mainland varies systematically with island width, ranging from 0.01 to 0.025 m/day. Seepage measurements closer to shore show more small scale variability. Ongoing monitoring will assess seasonal variations.