Investigating groundwater inputs to Mississippi River Deltaic wetlands using spatial and temporal responses of the geochemical tracer, 222Rn

Submarine groundwater discharge (SGD) has been recognized as a significant coastal process that transports terrestrial freshwater, nutrients, and anthropogenic contaminants to the ocean. Globally, total influxes of terrestrial SGD to the ocean are equal to 5 to 10% of the annual global river water discharge into the ocean. In particular, several recent SGD studies have reported significant SGD fluxes in global deltaic regions such as the Yellow River Delta and Ganges-Brahmaputra Delta. The Mississippi River Delta (MRD) is the seventh largest delta on Earth, and was formed by thick layers of sandy sediments that were transported and deposited by numerous ancient river channels. In particular, the point bar aquifer, characterized as having a high sediment permeability and porosity, has developed along the Mississippi River (MR) natural levee. Considering the increased difference of hydraulic head between the MR and nearby swamps at high flood stage of the MR, the point bar aquifer and buried paleo river channels may be a conduit for groundwater to the MRD. To understand the hydrologic interaction between the MR and nearby swamps, a natural radioisotope radon (222Rn) was utilized as a groundwater tracer. In addition, dissolved organic carbon, total nitrogen, stable isotopes, and 222Rn activities in surface waters were measured to understand the biogeochemical transports of SGD in the MRD. The average SGD seepage rate in MRD was found to be 2.1 cm/day, or 1.3 × 108 m3 day-1 to the MRD. The source of the SGD in the MRD was a mixture of MR and precipitation based on stable isotope results. In addition, the concentration of biogeochemical constituents in SGD was at maximum two orders of magnitude higher than in surface waters. The main control factor of SGD in the upper MRD was influenced by the seasonal MR water stage. Thus, SGD in the MRD is not only a significant biogeochemical source, but also contributes freshwater the wetland sustainability in the MRD.Master of Scienc