Ra and Rn isotopes as natural tracers of submarine groundwater discharge in Tampa Bay, Florida

摘要

A suite of naturally occurring radionuclides in the U/Th decay series (~(222)Rn, ~(223,224,226,228)Ra) were studied during wet and dry conditions in Tampa Bay, Florida, to evaluate their utility as groundwater discharge tracers, both within the bay proper and within the Alafia River/estuary — a prominent free-flowing river that empties into the bay. In Tampa Bay, almost 30% of the combined riverine inputs still remain ungauged. Consequently, groundwater/surface water (hyporheic) exchange in the discharging coastal rivers, as well as submarine groundwater discharge (SGD) within the bay, are still unresolved components of this system's water and material budgets. Based on known inputs and sinks, there exists an excess of ~(226)Ra in the water column of Tampa Bay, which can be evaluated in terms of a submarine groundwater contribution to the bay proper. Subrriarine groundwater discharge rates calculated using a mass balance of excess ~(226)Ra ranged from 2.2 to 14.5 L m~(-2) day~(-1), depending on whether the estuarine residence time was calculated using ~(224)Ra/xs~(228)Ra isotope ratios, or whether a long term, averaged model-derived estuarine residence time was used. When extrapolated to the total shoreline length of the bay, such SGD rates ranged from 1.6 to 10.3 m~3 m~(-1) day~(-1). Activities of ~(222)Rn were also elevated in surface water and shallow groundwater of the bay, as well as in the Alafia River estuary, where upstream activities as high as 250 dpm L~(-1) indicate enhanced groundwater/surface water exchange, facilitated by an active spring vent. From average nutrient concentrations of 39 shallow, brackish, groundwater samples, rates of nutrient loading into Tampa Bay by SGD rates were estimated, and these ranged from 0.2 to 1.4 x 10~5 mol day~(-1) (PO_4~(3-)), 0.9-6.2 x 10~5 mol day~(-1) (SiO_4~-), 0.7-5.0 x 10~5 mol day~(-1) (dissolved organic nitrogen, DON), and 0.2-1.4 x 10~6 mol day~(-1) (total dissolved nitrogen, TDN). Such nutrient loading estimates, when compared to average river discharge estimates (e.g., TDN = 6.9 x 10~5 mol day~(-1)), suggest that SGD-derived nutrient fluxes to Tampa Bay are indeed important components to the overall nutrient economy of these coastal waters.