TRACING NUTRIENT FLUXES ASSOCIATED WITH SALINE SUBMARINE GROUNDWATER DISCHARGE IN A SEMI-ARID ESTUARY USING STABLE AND RADIOACTIVE ISOTOPES

The role of saline submarine groundwater discharge (SGD) on nutrient cycling in semiarid estuaries has rarely been hypothesized and addressed. In this study we use SGD rates derived from radon [²²²Rn] and radium [²²⁶Ra, ²²⁴Ra, and ²²³Ra] mass balances to calculate fluxes of available nutrients to the hypersaline oxygenated surface water of Baffin Bay (Texas) across three seasons. The δ¹⁸O and δD signatures were also used to evaluate changes in source waters and mixing and indicated that porewaters were more depleted (δ¹⁸O: 0.91 to 1.89‰ and δD: 6.07 to 13.56‰) than surface water (δ¹⁸O: 2.21 to 4.29‰, δD: 13.87 to 21.01‰) in summer. SGD rates derived from ²²²Rn and ²²⁶Rn activities were slightly higher in summer than in winter and fall and ranged from 1.6 ± 0.2 to 41.3 ± 4.1 cm d^-1. The insignificant differences in ²²²Rn and ²²⁶Rn-derived SGD rates, defines the likely total SGD is dominated by recirculated, deoxygenated terrestrial saline inputs from underlying sediments, which also supported by the stable isotopes. SGD-derive NH₄⁺ flux rates averaged 4.2x10⁵ μmol∙d^-1 and were the highest in summer (7x10⁵ μmol∙d^-1). The persistent reducing conditions [July Oxidation Reduction Potential-ORP: -375 to -320 mv) in porewaters are believed to be the cause of elevated NH₄⁺ levels (1.18x10³ µM) and consequently high SGD-derived solute fluxes. In addition, the availability of dissolved organic carbon [(DOC): avg. 9 mg/L] in highly reducing environments, also supports reducing terrestrial inputs and favorable conditions for NH₄⁺ production through dissimilatory nitrogen reduction (DNRA). The reducing conditions of porewaters and the overall low pH (<7) and presence of elevated dissolve ions and inorganic nitrogen are favorable conditions for the enhanced release of Ra into the porewater alongside with production of NH₄⁺. Thus, biogeochemical conditions of porewaters in semiarid estuaries likely drive processes that could simultaneously impact the production and release of NH₄⁺ and Ra to the surface waters and they may vary by season.