Salt Marsh Groundwater Dynamics: North Inlet Salt Marsh, Georgetown, South Carolina

Salt marsh groundwater dynamics play a key role in coastal surface water chemistry and ecosystem health, and spatial variability in marsh groundwater dynamics may be important to the development of sharp boundaries indicative of acute marsh dieback. To determine the spatial and temporal variability of groundwater flow within a salt marsh, a field and modeling study was conducted at an acute marsh dieback site in North Inlet salt marsh, Georgetown, South Carolina. Site stratigraphy is variable with 1 – 2m of mud overlying fine sand. Seven piezometer nests in two transects oriented N-S and E-W were installed with 3 piezometers screened at 1, 2, and 4m at each nest. In situ measurements of groundwater temperature and hydraulic head were made from January 2007 through May 2008. Semidiurnal tides inundate the marsh twice a day. Hydraulic head data reveal lateral variations in flow with downward flow indicated near marsh creeks and stagnant conditions at the marsh center controlled primarily by evapotranspiration. Vertical flow variability corresponds to stratigraphic and tidal variability, with groundwater movement focused primarily within the underlying sand. The marsh was not inundated at high tide (< 3.5 ft above MLLW) for approximately 5% of the tidal record during the study period. These unusual tide conditions lasted anywhere from 12 hours to 4 days. Coincident head measurements suggest enhanced drainage near marsh creeks particularly during extraordinarily low tides (≤ MLLW), and potential lateral flow from the marsh center toward the creeks. The absence of tidal flushing would also increase the potential for development of hypersaline remnant pools and surface desiccation. A two-dimensional groundwater flow model (SUTRA) is currently being developed to calculate flow and solute distribution between nests and to hindcast to the time when the dieback episode occurred in 2001 – 2002.