Submarine groundwater discharge and nutrient input to a semiarid and hypersaline estuary: Baffin Bay, Texas

Abstract

This study evaluates the role of submarine groundwater discharge (SGD)-derived nutrients to Baffin Bay, a semi-arid, hypersaline bay in south Texas. SGD can be equivalent to riverine input in some places, making SGD’s role in nutrient input important. Dissolved inorganic nitrogen (DIN), and total alkalinity combined with SGD measurements, using two different geochemical tracers (radium-226 [226Ra], radium 224 [224Ra] and radon [222Rn]) and geophysical investigations were conducted. SGD rates were found to have slight spatial variation with higher rates near the shoreline around areas characterized by course-grained sediments and relic serpulid reefs. The222Rn and 226Ra-based SGD estimates produced agreeable results, within the range of uncertainties, and no significant changes in SGD from July to November, within the same year, were observed. However, 226Ra and 224Ra activities decreased from July to November and are associated with large decreases in porewater DIN concentrations. July and November 222Rn-derived SGD rates were 31.4±32.7 and 30.0±30.9 cm∙d-1, respectively while those derived from 226Ra were 16.6±1.7 and 13.2±1.3 cm∙d-1, respectively. Given the lack of change in SGD between the two seasons, organic matter (OM) decay may be the driving force for changes in radium activities as it can lead to reducing conditions that enhance radium solubility from sediments. In addition to OM remineralization from phytoplankton, a shift from a seawater to a terrestrial groundwater source in the subterranean estuary is also likely to be responsible for the larger porewater radium activities and nutrient concentrations in July. A comparison of bay-wide solute fluxes indicates that DIN inputs, mainly in the form of ammonium (NH4+), are almost five orders of magnitude higher in the SGD component than the surface runoff. Therefore, regardless of the magnitude of SGD and its nature (i.e. fresh or saline; groundwater or recirculated saline), the associated nutrient input is likely significant in this shallow bay system in warmer months. This study helps provide an understanding of the possible effects of OM decay on radium and DIN fluctuations and inputs in a hypersaline estuary. Studying these relationships is important as hypersalinity is a developing problem in freshwater-limited environments.