INVESTIGATING THE DYNAMICS OF COASTAL GROUNDWATER SALINIZATION IN EAST DOVER, DELAWARE

Competing demands for fresh groundwater in a low-lying coastal area increases the risk of salinization of shallow groundwater from both sea level rise and saline intrusion caused by pumping. The population of Dover, Delaware has been steadily increasing over the last several decades and along with it, a rise in demand for fresh water. Groundwater is the main source of fresh water for irrigation and drinking water in central and southern Delaware. The City of Dover, most farming operations and many private residences east of the city toward the Delaware Bay rely on the Columbia aquifer for fresh water. Anecdotally, we know that local farmers have noticed a poor crop yield after irrigating with brackish water from their irrigation ponds or wells.

We have begun a multi-year project to investigate the long-term water resources of east Dover, Delaware – an area that is economically growing while also holding onto its agricultural roots. This project incorporates a combination of fieldwork and MODFLOW and SEAWAT simulations to help us understand the flow dynamics of the system and assess the effects of increased pumping from multiple sources. We are monitoring water levels and salinity in groundwater, marshes, tidal streams and irrigation ponds throughout the area with a focus on potentially affected farms. Water levels indicate flow patterns change in response to irrigation, tides, and precipitation. We observed different salinity responses associated with diurnal tidal cycles, monthly lunar tidal cycles, and irrigation pumping. Concentrations of salinity in marshes and irrigation ponds, which intersect the water table, show fluctuations during the irrigation season and peak salinities of up to 6 ppt during lunar high tide events. Salinity in tidal channels and the marshes reaches 15 ppt. Salinities at this level in irrigation water can result in a moderate to significant loss of crop yield, depending on the salt-tolerance of the crop. Although currently unaffected by salinity, many irrigation wells are only tens of feet from the marsh. A thin layer of clay beneath marsh sediments may act as a confining unit, preventing brackish water from infiltrating the aquifer. We expect an increase of salinity in wells and irrigation systems as higher tides, intensified by sea level rise, drive the fresh-salt water interfaces further inland.