Biogeochemistry of surface waters and groundwaters at Myrtle Grove, LA

Telfeyan Department of Earth and Environmental Sciences, Tulane University, New Orleans, LA *correspondence: ktelfeya@tulane.edu; A. Breaux Louisiana Universities Marine Consortium, Cocoderie, LA, Department of Earth and Environmental Sciences, Tulane University, New Orleans, LA; J. Kim Department of Marine Sciences, University of North Carolina, Chapel Hill, NC; K.H. Johannesson, K.H. Department of Earth and Environmental Sciences, Tulane University, New Orleans, LA; A.S. Kolker, A.S. 2Louisiana Universities Marine Consortium, Cocoderie, LA, Department of Earth and Environmental Sciences, Tulane University, New Orleans, LA; J.E. Cable Department of Marine Sciences, University of North Carolina, Chapel Hill, NC

The chemistries of near-surface pore waters are commonly studied in coastal marshes, especially with respect to iron and sulfur redox changes. However, deeper groundwaters and more specifically, terrestrially-derived groundwater discharge as a source of fresh water to coastal marshes has been largely understudied. Owing to the hydraulic gradient established by elevated river stage relative to the interdistributary zone, Mississippi-derived water may flow to the surrounding wetlands via buried sand channels. This project investigates the influence of this fresh groundwater on the redox chemistry of marsh groundwaters. Groundwaters and surface waters were sampled at five stations over the length of Myrtle Grove Canal during three sampling campaigns in 2014. Samples were collected for the analysis of major cations and anions, redox-sensitive trace elements (i.e., Fe, Mn, As, V), and auxiliary parameters (e.g., dissolved organic carbon, pH, conductivity). Radon concentrations were also determined to understand the influence of Mississippi River-derived water on groundwater fluxes throughout Myrtle Grove. Concentrations of trace elements and major ions vary through both time and space and behave both conservatively and non-conservatively with respect to salinity. In surface waters, major ions generally increase with distance from the river, suggesting mixing of river-sourced water and saline water occurs. In groundwaters, sulfide, sulfate, and DOC covary suggesting that microbially-driven sulfate reduction occurs in the marsh. Additionally, 10-20 cm short cores were taken in April and September 2014 to sample pore water changes with depth and determine the effects of sulfate and iron reduction with seasonal changes and fluctuating water levels. Finally, samples of both surface and groundwaters were filtered through sequentially smaller pore sizes to estimate the fraction of trace elements in the colloidal and dissolved phases. Overall, concentrations of major and trace elements vary through time and suggest the importance of tides and seasons in providing advective fluxes as well as altering the redox state of marsh groundwaters.