Proceedings of the 2019 Mississippi Water Resources Conference

Year: 2019 Authors: Moody A., Shiller A.M., Sanial V.

Submarine groundwater discharge (SGD) in the Mississippi Sound is an understudied component of nutrient and trace metal cycling. Submarine groundwater discharge is the combined flow of freshwater from aquifers and the recirculation of seawater through sediments that occurs along the coastline and across the continental shelf. In July 2017, a low oxygen (less than 2 mg/L) event occurred in the Mississippi Sound causing a "jubilee" event, where large masses of demersal organisms came towards shore. We collected water samples at five locations along Mississippi beaches and analyzed them for chemical species typically enriched in SGD (e.g., Ra isotopes, Ba, methane). During this period of low oxygen there were increased groundwater signatures, suggesting a correlation between the hypoxic conditions and groundwater release. Dissolved methane (CH₄) and ²²⁴Ra (half life = 3.66 d) were significantly higher than what we observed during more normal conditions. The high levels and short half lives of both indicators suggest that there was a recent release, and nearby source, of groundwater. Historically, low oxygen events have been observed in the Sound, so it is important to understand what causes them. Our results suggest that understanding the origins and forcing factors for local SGD may be an important aspect of predicting and managing hypoxia in the Sound. In order to determine if SGD may enhance or lead to hypoxia in the Mississippi Sound, an ongoing time series along the coastline has been collecting radium, nutrients, barium, and oxygen data. In order to understand if there are spatial components of SGD, radon surveys are also being conducted throughout the Sound. Preliminary results indicate that SGD is highest along the coastline. However, more work is needed to determine the sources and impacts of SGD within the Mississippi Sound.