Submarine groundwater discharge (SGD) is a small portion of the global water budget,
but a potentially large contributor to coastal nutrient budgets due to high concentrations relative
to stream discharge. A numerical groundwater flow model of the Inland Bays Watershed,
Delaware, USA, was developed to identify the primary hydrogeologic factors that affect
groundwater discharge rates and transit times to streams and bays. The distribution of
groundwater discharge between streams and bays is sensitive to the depth of the water table
below land surface. Higher recharge and reduced hydraulic conductivity raised the water table
and increased discharge to streams relative to bays compared to the Reference case (in which
66% of recharge is discharged to streams). Increases to either factor decreased transit times for
discharge to both streams and bays compared to the Reference case (in which mean transit times
are 56.5 and 94.3 years, respectively), though sensitivity to recharge is greater. Groundwaterborne
nitrogen loads were calculated from nitrogen concentrations measured in discharging fresh
groundwater and modeled SGD rates. These loads combined with long SGD transit times suggest
groundwater-borne nitrogen reductions and estuarine water quality improvements will lag
decades behind implementation of efforts to manage nutrient sources. This work enhances
understanding of the hydrogeologic controls on and uncertainties in absolute and relative rates
and transit times of groundwater discharge to streams and bays in coastal watersheds.
