Groundwater Contribution of Elevated Nutrients to a Hypersaline Estuary: Laguna Salada (Baffin Bay), Texas

There is limited information related to the extent of submarine groundwater discharge (SGD) and its significance on the solute budgets in shallow, well-mixed, and hypersaline estuaries in low-gradient watersheds and semi-arid climates. This study quantifies the monthly variability of SGD and solute fluxes in the hypersaline Laguna Salada (LS), a primary bay part of Baffin Bay system in the semi-arid area of south Texas, using radon [222Rn] and radium [226Ra, 224Ra, and 223Ra] isotopes. Given the area's low precipitation rates, flat topography, and occurrence of sandy soils, it is predicted that groundwater and sediment fluxes comprise the most significant source of nutrients (NUT) to the bay. Surface-, ground-, and porewater samples are collected monthly over the course of one year (February 2020 to January 2021) and analyzed for stable isotopes of δ18O, δD, and δ13C, NUT, dissolved organic carbon (DOC), DON, Chl-a, total alkalinity, metal, and major ions. Two locations along LS and one inland at Los Olmos Creek have been chosen as test sites. Two transect-monitoring wells at each bay test site (four total) where groundwater elevations are monitored continuously with in-situ data loggers to investigate groundwater-surface water interactions as driven by terrestrial hydraulic gradients and tidal pumping. Preliminary data indicates that NUT concentrations in groundwater are higher closer to shore. Porewater NUT indicates mixing with surface water as concentrations in porewater are lower than in groundwater and higher than in surface water. Tidal changes, which are wind-induced, are mimicked by groundwater elevation fluctuations with very little lag effect, thus, tidal pumping is likely driving the groundwater-bay interaction. Exchange with Los Olmos Creek is limited to tidal fluctuations with reverse-to-no-flow/discharge, depending on windspeed, direction, and precipitation. Thus, nutrient inputs are expected to be derived from exchange with groundwater and porewater through seawater recirculation.