Estimating the groundwater contribution into Florida Bay via natural tracers, 222Rn and CH4

Groundwater may represent a significant pathway for nutrients and other dissolved solutes into Florida Bay, especially near the Keys where wastewater disposal practices add large amounts of nitrogen and phosphorus to the subsurface each year. Previously, we suggested that high water column inventories of the tracers Rn-222 and CH4 may be indicative of groundwater discharge. In this study, we employed mass balance calculations to determine that the total benthic fluxes required to maintain the measured water column tracer inventories were significantly larger than diffusive fluxes and varied between 4.2-5.6 dpm m(-2) min(-1) and 5.8-15.4 nmoles m(-2) min(-1) for Rn-222 and CH4, respectively. Independent estimates of the diffusive flux and porewater activities/concentrations allowed us to calculate an advective groundwater velocity, assuming that the difference between the total benthic flux (given above) and the diffusive flux is driven by seepage-driven porewater advection. These calculated velocities ranged from 0.2 to 4.3 cm d(-1) for all sites, tracers, and sampling periods, with a best estimate of approximately 1.7 cm d(-1). These estimates of groundwater velocities compare very well with previous measurements of groundwater flux (1-3 cm d(-1)) at the same sites via seepage meters.