COUPLED PHYSICAL AND CHEMICAL CONSTRAINTS ON GROUNDWATER FLOW ACROSS THE UPLAND-ESTUARY INTERFACE IN PEAT AND CLASTIC SETTINGS

Noninvasive geophysical techniques (e.g., DC resistivity, electromagnetic induction) are an important, but underutilized, tool in constraining groundwater flow and transport from uplands to estuaries. Here we report on geophysical surveys conducted across the upland-estuary interface in peat-dominated systems in Massachusetts (Plum Island estuary) and clastic systems in South Carolina (Okatee estuary) and Georgia (Altamaha estuary). Most importantly, most surveys--whether in peat or clastic environments--have revealed freshening of pore waters with depth in estuarine marshes adjacent to uplands. Such results imply that a significant fraction of groundwater discharge into estuaries may occur via submarsh flow, not across seepage faces or tidal creek banks. Our results also provide provocative evidence for variable degrees of interaction between the upland freshwater lens and more saline surface and groundwaters in adjacent estuaries. In some settings, the geophysical data suggest relatively fresh pore waters near the marsh edge, implying some freshwater discharge from the upland into the marsh-tidal creek complex. At other sites with different morphology, vegetation, and aquifer parameters, saline water intrudes the upland, and we postulate that discharging groundwater consists mostly of estuary waters that earlier intruded the upland and were modified by chemical and biological processes and by mixing with fresher groundwaters. At Georgia Bight sites, direct measurements (geochemical analyses and electrical conductivity logs) in shallow (< 5 m) discrete monitoring wells confirm our geophysically-based inferences about the extent of saline water intrusion. Coincident geophysical and hydrologic measurements collected since 1998 at these sites also permit us to assess groundwater transport to estuaries over a range of time scales. Borehole electrical conductivity logging and EM measurements collected repeatedly during a tidal cycle yield the surprising result that groundwater transport to the estuary appears dominantly diffusive, not advective, at some locations. At a longer time scale (~1.5 years), seasonally repeated geophysical measurements reveal a strong correlation between recharge and head gradients, which ultimately control groundwater flux from upland to estuary.