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The influence of Submarine Groundwater Discharge on the quality of Mississippi coastal waters: example of hypoxic events in summer 2016 and 2017
Proceedings of the 2019 Mississippi Water Resources Conference
Year: 2019 Authors: Sanial V., Shiller A., Moore W.
The quality of the Mississippi Sound and Bight ecosystem, and as a consequence of economic activities such as tourism and fisheries, is directly affected by land-derived chemical elements. Rivers supply large amounts of allochthonous nitrogen that impact the ecosystem by stimulating primary production, which sometimes leads to coastal eutrophication as well as harmful algal blooms. The Mississippi Sound and Bight, located to the east of the Mississippi River Delta, experience hypoxia that is often attributed to nutrient-rich Mississippi River waters. However, oxygen isotopes show a limited influence of the Mississippi River waters in the Mississippi Bight in spring and summer 2016, but rather a dominant freshwater source originating from local rivers with much lower nutrient concentrations. Therefore, we hypothesize that there is likely an additional factor, namely submarine groundwater discharge (SGD), that significantly impacts the quality of Mississippi coastal waters by playing a role, in particular, in hypoxia. SGD is a hidden pathway for the transfer of chemical substances (such as nutrients, metals, and pollutants) from the land to the coastal ocean. Unlike rivers, SGD is difficult to monitor due to its diffuse nature, which limits the use of direct physical measurements. Tracing techniques, measuring geochemical species such as radium isotopes (Ra) that are naturally enriched in groundwater, constitute a powerful tool to assess the extent of SGD influence in the coastal zone. Hypoxic Mississippi Bight bottom waters in summer 2016 were enriched in Ra, but also in barium (Ba), and nutrients that cannot be accounted for sediment diffusion or river inputs, which suggests the presence of SGD. Spatial distribution in bottom water concentrations of certain dissolved trace elements (e.g., Mn, V, REEs) also suggests spatial differences in fluxes of species from the sediments are affected by bottom oxygen. Further evidence of SGD comes from the increase of Ra associated with a rise in nutrients and methane in coastal Mississippi Sound waters shortly after a Jubilee event in July 2017.