USE OF A GIS MODEL TO ESTIMATE GROUND-WATER DISCHARGE TO REHOBOTH AND INDIAN RIVER BAYS, DELAWARE

GIS models for water budget and flownet analyses were developed to refine estimates of ground-water discharge to coastal bays in Delaware. Rehoboth and Indian River bays are part of a shallow coastal ecosystem with a total surface area of ~70 km². The bays serve as the discharge area of a watershed with an area of ~700 km² with the highest percentage of land use being agriculture (~40%). The results of previous discharge models indicate 21 to 43 million gallons per day of direct submarine ground-water discharge to the bays through the unconfined, clastic surficial aquifer. These second-generation models increase the spatial resolution of these discharge estimates.

In addition to easily facilitating calculations and offering flexibility for model updates, the use of GIS allows for easier comparison of results with other spatial datasets that may help constrain discharge locations. Datasets for model input include watershed boundaries, digital elevation models of the land surface, water table and base of the surficial aquifer, and hydrogeologic cross-sections. The models are being constrained/tested by comparison with thermal infrared imagery from aircraft and satellites and other field measurements (head, seepage, chemistry).

Discharge from the surficial aquifer system contains a significant amount of nitrate, the primary environmental stressor in the bays, and is a factor in the occurrence of nuisance algal blooms, fish kills, and low benthic diversity. Determining the locations and rates of ground-water discharge and associated nutrient levels will enable a better evaluation of the impact on the bays' environmental health. This work is part of a larger EPA-funded CISNet (Coastal Intensive Sites Network) project entitled "CISNet: Nutrient Inputs as a Stressor and Net Nutrient Flux as an Indicator of Stress Response in Delaware's Inland Bays Ecosystem."