Submarine groundwater discharge (SGD) is an often overlooked yet possibly significant process in the geochemical and nutrient budgets of marine nearshore waters. In this research, subsurface water movement was evaluated with natural and artificial tracers in the karst limestone of the Florida Keys and a sandy barrier island in the Northeast Gulf of Mexico. One of the main outcomes of this research was the development of an approach which integrates geochemical and hydrogeological techniques for assessing directions and rates of subsurface flow and, specifically, how to quantify this flow into surface waters.

In the Florida Keys, natural tracers (222Rn and CH4) were used to locate areas of increased groundwater/surface water interactions and artificial tracers (SF6, 131I, 32P) were used to establish a direct link of contaminated groundwaters to surface waters. Collectively, these results indicate a greater flow of groundwater along the inside of the Keys. Nutrient flux estimates, based on interstitial nutrient concentrations and groundwater flux measurements, suggest that groundwater in the eastern area of Florida Bay may provide as much nitrogen and phosphate as surface freshwater sources from the Everglades.

Artificial tracers injected directly into sewage injection wells indicate rapid flow of groundwater beneath the Keys. Results suggest that nutrients injected in the subsurface are removed rapidly from solution and thus may not have a significant impact on surface waters. Approximately 95% of the phosphate injected into the subsurface could be removed in 20 to 50 hours. Approximately 65% of the nitrate was removed over several days, suggesting a denitrification rate of 2700 micro-moles m^-3 groundwater hr^-1, comparable to estimates of denitrification in other groundwater systems.

On St. George Island, natural (SiO4) and artificial tracers (SF6) were used to establish groundwater flow paths and rates downgradient from septic and aerobic wastewater systems, and evaluate losses of wastewater-derived nutrients in groundwater along these flow paths. Evidence indicates that developing an aerobic system that is raised by as little as 1 m from the natural ground surface on St. George Island would provide the most efficient means for removing contaminants from the wastewater before it is introduced to the groundwater.