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

Groundwater may represent a significant pathway for nutrients and other dis solved solutes into Florida Bay, especially near the Keys where wastewater disposal practices add large amounts of nitrogen and phosphorus to the subs urface each year. Previously, we suggested that high water column inventori es of the tracers Rn-222 and CH4 may be indicative of groundwater discharge . In this study, we employed mass balance calculations to determine that th e total benthic fluxes required to maintain the measured water column trace r inventories were significantly larger than diffusive fluxes and varied be tween 4.2-5.6 dpm m(-2) min(-1) and 5.8-15.4 nmoles m(-2) min(-1) for Rn-22 2 and CH4, respectively. Independent estimates of the diffusive flux and po rewater activities/concentrations allowed us to calculate an advective grou ndwater velocity, assuming that the difference between the total benthic fl ux (given above) and the diffusive flux is driven by seepage-driven porewat er advection. These calculated velocities ranged from 0.2 to 4.3 cm d(-1) f or all sites, tracers, and sampling periods, with a best estimate of approx imately 1.7 cm d(-1). These estimates of groundwater velocities compare ver y well with previous measurements of groundwater flux (1-3 cm d(-1)) at the same sites via seepage meters.