Examining the relationship between alkalinity and submarine groundwater discharge in a semi-arid estuary

Copano Bay, located along the semiarid south Texas coast, has been experiencing long-term acidification over several decades, including pH and total alkalinity decline. Copano Bay also exhibits a clear signature of an additional alkalinity sink that acerbates the alkalinity decline, particularly during periods of low freshwater inflow and extended residence times. It is important to understand this alkalinity sink because Copano Bay supports a commercial oyster fishery which may be vulnerable to the changing chemistry, and most of the alkalinity decline cannot be attributed to calcification. The goal of this study was to investigate submarine groundwater discharge (SGD) as a potential source of acidity driving the alkalinity decline in Copano Bay. For a two-year period, we conducted monthly water column sampling for carbonate system parameters and major ion analyses. Continuous radon surveys were conducted along the entire inland side of Copano Bay during both winter and summer seasons for SGD rate estimates. Porewater samples were collected directly from the sediment column as well as from intervals along sediment cores to estimate sediment-water fluxes of porewater constituents associated with SGD. We found notable spatial and temporal variability in SGD. While Copano Bay periodically experiences droughts resulting in hypersalinity, and the influence of SGD on the solute concentrations of a bay is most notable during drought periods, hypersaline conditions were not reached during this project. There are many drivers controlling the carbonate system in estuaries, and the role of SGD in carbonate system variability has received little attention compared to other drivers; therefore, the findings from this study may help inform other regions experiencing similar biogeochemical controls.