Submarine groundwater discharge to Tampa Bay: Nutrient fluxes and biogeochemistry of the coastal aquifer

Abstract

To separately quantify the roles of fresh and saline submarine groundwater discharge (SGD), relative to that of rivers, in transporting nutrients to Tampa Bay, Florida, we used three approaches (Darcy's Law calculations, a watershed water budget, and a 222Rn mass-balance) to estimate rate of SGD from the Pinellas peninsula. Groundwater samples were collected in 69 locations in the coastal aquifer to examine biogeochemical conditions, nutrient concentrations and stoichiometry, and salinity structure. Salinity structure was also examined using stationary electrical resistivity measurements. The coastal aquifer along the Pinellas peninsula was chemically reducing in all locations sampled, and that condition influences nitrogen (N) form and mobility of N and PO 4 3−. Concentrations of NH 4 +, PO 4 3− and ratio of dissolved inorganic N (DIN) to PO 4 3− were all related to measured oxidation/reduction potential (pε) of the groundwater. Ratio of DIN: PO 4 3− was below Redfield ratio in both fresh and saline groundwater. Nitrogen occurred almost exclusively in reduced forms, NH 4 + and dissolved organic nitrogen (DON), suggesting that anthropogenic N is exported from the watershed in those forms. In comparison to other SGD studies, rate of PO 4 3− flux in the seepage zone (μM m − 2 d − 1 ) in Tampa Bay was higher than previous estimates, likely due to 1) high watershed population density, 2) chemically reducing conditions, and 3) high ion concentrations in fresh groundwater. Estimates of freshwater groundwater flux indicate that the ratio of groundwater discharge to stream flow is ∼ 20 to 50%, and that the magnitudes of both the total dissolved nitrogen and PO 4 3− loads due to fresh SGD are ∼ 40 to 100% of loads carried by streams. Estimates of SGD based on radon inventories in near-shore waters were 2 to 5 times greater than the estimates of freshwater groundwater discharge, suggesting that brackish and saline SGD is also an important process in Tampa Bay and results in flux of regenerated N and P from sediment to surface water.