SUBMARINE GROUNDWATER DISCHARGE IN THE CHESAPEAKE BAY: CONSTRAINTS FROM NUMERICAL MODELS

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 transects were modeled subject to similar boundary conditions but different hydrogeologic layers, recharge rates, seawater densities, and coastal Virginia physical environments (e.g., relatively high-relief southern Eastern Shore, riverine-dominated Elizabeth River, wave-dominated Ocean View beach). Each two-dimensional transect is approximately 5 km in the shore-perpendicular direction with vertical extent ranging from 10-100 m. 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 velocity. 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.