SUBMARINE GROUNDWATER DISCHARGE IN THE CHESAPEAKE BAY: QUANTIFYING THE IMPORTANCE OF FIELD PARAMETERS

Terrestrial and oceanic forces drive fluid flow within the coastal zone to produce submarine groundwater discharge (SGD). Groundwater flowing from the seabed serves as a significant pathway for contaminants and nutrients, producing an active biogeochemical reaction zone. In order to quantify the importance of this SGD flow in geochemical and hydrologic budgets for the lower Chesapeake Bay, three coastal Virginia transects (southern Eastern Shore, Elizabeth River, and Ocean View beach) with different topographic gradients were modeled using similar boundary conditions and consistent treatment of hydrogeologic layers. A sensitivity study was performed on the variables of recharge rate, seawater density, and hydraulic permeability. Each two-dimensional transect is approximately 5 km in the shore-perpendicular direction with vertical elevations ranging from 10 m above sea level to 50 m below sea level. A pre-processing suite of code displays NOAA topography and bathymetry data, allows the user to identify a desired transect, and outputs a cross-sectional numerical domain for a series of steady-state calculations solved by a USGS program called SUTRA. SUTRA’s hybrid finite element and finite difference method captures buoyancy-driven and variable-density flow, computes fluid flow patterns, solves the coupled solute transport equation, and predicts areas of discharge and recharge across the nearshore coastal zone. Models suggested SGD in all transects, with common flow pattern characteristics including freshwater discharging below the elevation of sea level, seawater recirculating in steep bathymetry, and intervening zones of relatively low flow. Although fluid velocity at the low tide mark was significantly dependent upon the slope of the transect, response to recharge rate was small over a reasonable range of modeled values. Permeability had the greatest effect on SGD; varying hydraulic conductivity over an order of magnitude produced at least a 50% change in discharge. Overall, this series of models provides a framework for identifying zones of high groundwater flow, reveals the variability of SGD rates between locations, and suggests which field measurements would be most valuable to better constrain the groundwater contribution to the coastal zone.