Use of Regional Groundwater Models to Estimate Potential Submarine Groundwater Discharge along the Chesapeake Bay

A regional groundwater model of the Delmarva Peninsula was used to predict potential quantities and locations of submarine groundwater discharge (SGD) and its nutrient load.  The model is a representation of the water-table aquifer system, in which the depth to water is typically small and the groundwater flow highly controlled and defined by the local stream networks.  The model was constructed using MODFLOW, has an areal grid-resolution of 500 ft, and has seven layers that extend to a depth of 300 ft below land surface.  Regional recharge applied to the model was estimated from stream base flows, and the model was calibrated using 48 water levels and 24 tritium-helium ages.  The median groundwater return time to the streams from the water table is estimated from the model to be about 30 years.  This substantial groundwater lag time has important implications for the response time of the water quality of the Chesapeake Bay following reductions in nitrogen loadings at the land surface.

An analysis was made using MODPATH to estimate the quantity of water and nutrients that potentially discharge to the Chesapeake Bay from the Delmarva Peninsula.  Overall, only a few percent of the total recharge is predicted to discharge directly to the Bay below the mean tide line.  Similarly less than 1 million lbs/yr of nitrogen is predicted to enter the Bay by SGD compared to 40 million lbs/yr predicted to discharge through base flow to streams.  Results also indicate potential locations where SGD is likely to be enhanced.  Areas with a high potential for SGD include estuaries, where groundwater discharge becomes focused, and areas with high transmissivity at the coastline.  Local prediction of SGD is not possible with this scale of model, however, without more detailed knowledge of the subsurface permeability at the coastline.