Construction of a Watershed-Scale Model to Assess Submarine Groundwater Discharge to Indian River Bay, Delaware

Three-dimensional numerical groundwater models were developed to improve understanding of controls on submarine groundwater discharge (SGD) into Indian River Bay, on Delaware’s Atlantic coast. The models quantify watershed-scale groundwater fluxes, quantify factors controlling watershed-scale groundwater flow and guide geophysical and geochemical field studies.  The models focus on the 490 km² Inland Bays Watershed, which contributes waters into Indian River Bay—a saline coastal estuary.  The modeled aquifer system consists of a 75-130m thick, stratified aquifer system that occurs within Neogene-age unconsolidated sediments ranging in size from clay to gravel.  Discontinuous muddy lenses from millimeters to tens-of-meters in thickness separate aquifers within this sequence.  Underlying this system is a thick muddy unit that acts as a regional confining unit.  Previous studies have found that Holocene surficial deposits function as confining units in some parts of the watershed, especially in the bay itself, causing freshened SGD far from the bayshore.  Single and variable-density steady-state groundwater flow models of the system were constructed with USGS MODFLOW and SEAWAT codes using the ArgusOne GUI.  The model includes 9 layers and about 300,000 active cells that are 152 m on a side.  Existing data from federal and state agencies were used to construct and calibrate the models.  Sensitivity analyses were conducted on recharge and aquifer hydraulic properties.  These models were used primarily to better understand the range of SGD flux and its distribution in Indian River Bay.  Model parameters and results have also interfaced with geophysical surveys and hydrologic study of the Indian River Bay, both gaining from their results and directing future efforts.  Additionally, simulations were used to explore theories about the hydrogeologic and seasonal factors controlling SGD.