NATURAL HYDROLOGIC VARIABILITY IN AN INTERTIDAL SALT MARSH AND ITS IMPACT ON GROUNDWATER TRANSPORT OF SELECT GEOCHEMICAL TRACERS AND PROXIES

Intertidal salt marsh groundwater dynamics are strongly influenced by variations in hydrologic forcing. This variability affects marsh redox chemistry, solute distributions, and solute and water exchanges with coastal surface water. It also provides a primary control on porewater radium isotope activity, which is an important but typically poorly constrained end-member when used to estimate submarine groundwater discharge (SGD). We installed piezometers (1-4 m) and passive diffusion samplers (< 1 m) in a salt marsh island at North Inlet, South Carolina, to assess the impact of hydrologic variability on marsh geochemistry. Significant temporal variations in tidal inundation, rainfall, and evapotranspiration were observed. The island was typically inundated twice daily, but occasional 19 – 21 hour periods occurred in winter and spring when the marsh was not inundated. Enhanced exposure resulted in seasonal redox chemistry changes, as indicated by changes in the ratio of ferrous iron [Fe(II)] to total iron [Fe(II) + Fe(III)]. Mean porewater salinity increased with depth and was more variable near the marsh surface. This pattern reflected the effects of precipitation and evapotranspiration influenced by the frequency of tidal inundation and the downward movement of porewater through the marsh mud. With the exception of seasonal variations in temperature and pH (ANOVA; P =0.000), significant variability in the concurrently measured factors (salinity, temperature, pH, and redox potential) known to control porewater radium activity was not found. Porewater radium activity varied by up to a factor of 8 in areas of the marsh near large creeks that received considerable tidal flushing and up to a factor of 60 in areas of the marsh near small creeks that were subject to less frequent high-energy tidal flushing. These results show that temporal variations in radium activity at a fixed point within the marsh are entirely due to changes in groundwater flow. Further interpretation of these results is underway using a 3D hydrogeologic model. In spite of its simplicity as a hydrologic system, natural variability in this marsh resulted in changes to shallow porewater salinity and redox conditions important to wetland ecosystem health and to porewater radium activity resulting in temporal differences up to an order of magnitude.