Tracing Submarine Groundwater Discharge into the Ocean

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Submarine springs and seeps discharge a substantial but not precisely defined quantity of water into the ocean. Recent estimates suggest that the magnitude of this discharge, driven by gravitational and convective forces, may be globally significant. Discharged fluids may consist of fresh water, connate waters, brines derived from evaporite deposits, or recirculated seawater. Because the composition of submarine hydrologic discharge may be markedly different than seawater, this process is capable of contributing significantly to the marine geochemical cycling of many elements. The proposed research will determine the potential of using water column standing crops of the tracers 222Rn, 226Ra and CH4, constituents greatly enriched in groundwater relative to seawater, for the purpose of locating and assessing submarine groundwater discharge to the ocean. A detailed geochemical mass balance will be constructed for the single best tracer, 222Rn, within a relatively small area on the continental shelf of northwest Florida, an area where we expect to find appreciable submarine discharge. Groundwater tracer concentrations and fluxes will be measured, both in the form of discrete springs and as disseminated seeps. Benthic fluxes and other processes, such as advection and air-sea exchange, which contribute to changes in the water column standing crop will be evaluated. These measurements will be modeled to evaluate the prospects of developing a simple tracer-based technique for determining the magnitude of groundwater discharge to the ocean applicable to other areas.