Hydrologic and Geophysical Constraints on Submarsh Groundwater Flow to an Estuary in the Georgia Bight

Previous research in the Georgia coastal zone suggests that low-permeability muds clog the sediment matrix at the edges of uplands, thereby impeding freshwater flow to the estuary across seepage faces. A significant portion of fresh groundwater flux from upland to estuary may instead occur along intricate submarsh flow pathways. In this study, we combine hydrologic and geophysical measurements to directly constrain submarsh flow pathways and various scales of aquifer heterogeneity beneath a 100-200 m wide Spartina marsh that separates forested upland from a tidal creek at a Long Term Ecological Research (LTER) site on North Sapelo Island. Our data provide both qualitative and quantitative constraints on hydraulic conductivity variations in the marsh sediments. Continuous vibracores (to depths of ~5 m) at the site of our monitoring well network reveal a complex distribution of sediments both vertically and horizontally. In general, the thin, near-surface layer of fine muds gives way to relatively clean sands and then a deeper layer of clay with sand stringers. Where these stringers are laterally continuous, they may serve as critical high permeability conduits for groundwater flow in submarsh sediments. Grain size analyses constrain hydraulic conductivity of the distinct lithologies within the cores, while time and spectral domain analyses of tidal pumping data bracket field-scale hydraulic conductivity near each marsh monitoring well. Our data also allow us to infer information about possible pathways for submarsh groundwater flow. Geophysical surveys (inductive EM and DC resistivity) and borehole conductivity logs suggest a complicated distribution of fresher and more saline pore waters in the submarsh. Preliminary analysis of the geophysical data implies the presence of fresher pore waters at depths of several meters beneath the marsh. A marsh monitoring well within ~10 m of the upland intersects this relatively fresh submarsh flow system. To first order, the density stratification of saline pore waters at shallow levels over fresher water deeper in the marsh should be unstable. We postulate that the persistence of saline over fresher pore waters at this site may imply active flow of the fresher waters along high permeability pathways beneath the marsh.