SEDIMENTARY, SEASONAL, AND STORM INFLUENCES ON SHALLOW GROUNDWATER HYDROLOGY IN COASTAL MARSHES IN GRAND BAY NATIONAL ESTUARINE RESEARCH RESERVE, MISSISSIPPI

Climate change and relative sea level rise (SLR) is resulting in saltwater intrusion and inundation of vital coastal marshes, which affects healthy ecosystem function and causes severe deviations in marsh hydrology. This study investigates sediment variability and associated influences on marsh hydrology in Grand Bay National Estuarine Research Reserve (GNDNERR) near Pascagoula, Mississippi.

Emphasis is placed on establishing a baseline understanding of factors that affect shallow groundwater dynamics, including sediment composition, seasonal variability, and coastal storms. Analysis of sediment cores includes color, organic C, carbonate, magnetic susceptibility, and particle size. Shallow groundwater hydrologic trends between Summer 2015 and Fall 2016 are established by monitoring water levels, water temperature, and conductivity at the surface and in piezometers at depths of 0.75m, 1.5m, and 2.25m installed at 4 sites along a salinity gradient.

Munsell soil color analysis indicates that the marsh is dominated by reducing conditions and redoximorphic concentrations of iron and magnesium oxides. Sediment samples have an average of 4.6% organic C and 2.3% carbonate shell hash. Magnetic susceptibility increases seaward along the salinity gradient. The relative abundance of sand particles is associated with other soil characteristics and shallow groundwater conditions.

Shallow groundwater levels were observed to fluctuate corresponding to diurnal tidal cycles as well as longer seasonal cycles and short-term storm influences. Conductivity generally follows the expected salinity gradient across the marsh; it is typically slow to fluctuate and does not have a strong diurnal tidal signal. During drier periods, the salinity gradient gradually shifts to the middle of the marsh such that salinity is higher there than in outer marsh areas. This is likely the result of evapotranspirationally-increased surface salinity. Conductivity generally decreases with depth suggesting that a seaward flow of lower salinity water in the shallow subsurface coupled with low permeability fine-grained sediments resists the seepage of higher density saline surface water. Temperature fluctuations are rarely more than a degree in 24hrs.