Semi-Permeable Paleochannels as Conduits for Submarine Groundwater Discharge in Mississippi River Delta Stratigraphy, Barataria Bay, Louisiana

Alexander Breaux Louisiana Universities Marine Consortium and Department of Earth and Environmental Sciences, Tulane University; Alexander S. Kolker Louisiana Universities Marine Consortium and Department of Earth and Environmental Sciences, Tulane University; Katherine Telfeyan Department of Earth and Environmental Sciences, Tulane University; Jihyuk Kim Department of Marine Sciences, University of North Carolina, Chapel Hill; Karen Johannesson Department of Earth and Environmental Sciences, Tulane University , Jaye E. Cable Department of Marine Sciences, University of North Carolina, Chapel Hill

Many studies have focused on hydrological and geochemical fluxes from land to the ocean via submarine groundwater discharge (SGD), however few have assessed these contributions of SGD in deltaic settings. The Mississippi River delta is the largest delta in North America, and the magnitude of groundwater that discharges from the River into its delta is relatively unknown. Hydrological budgets have indicated that there is a large magnitude of water lost in the Mississippi’s Delta. Recent evidence in our study indicates that paleochannels, or semi-permeable buried sandy channels that were former distributaries of the River, allow for groundwater to discharge out of the Mississippi’s main channel and into its Delta. Our study uses geophysical data, including CHIRP and continuous resistivity profiling, to detect the location of these paleochannels in Barataria Bay, a coastal bay located in the Mississippi Delta. CHIRP data show that these paleochannel features are ubiquitous in the Mississippi Delta, while resistivity profile data indicate lower salinity water is found to upwell during high river flow in bays proximal to the River. Analysis of bulk sedimentary properties of sediment cores that have been collected in the study area is used to characterize delta facies sediments. Down core variability in hydraulic conductivity values of sediments, based on calculations from bulk sediment properties, represent changes in delta facies. These calculated hydraulic conductivity values are further compared to experimental values measured using a permeameter in the lab, in order to ground truth our calculations. The geophysical and sediment core data will be used to contextualize geochemical data collected in the field, which includes an assessment of major cations and anions, as well as in situ Rn-222 activities, a method that has been proven to be useful as a tracer of groundwater movement. The results may be useful in understanding the potential global magnitude of hydrological and geochemical fluxes of other large rivers with abandoned distributaries and can have implications for urban planning and planning of coastal restoration projects, as many large global deltas sustain significant populations.