Kent County Groundwater Monitoring Project: Hydrogeology and Salinization Dynamics of Eastern Kent County

Salinization can be a critical water quality problem in coastal aquifers. This is especially acute in coastal Delaware, where groundwater is the main source of fresh water and increasing population and irrigation are placing growing demands on water resources. Most farming operations and many private residences east of Dover toward the Delaware Bay rely on the shallow unconfined Columbia aquifer for fresh water. Additionally, the City of Dover operates a Columbia aquifer water supply wellfield in this area. To investigate locations in eastern Kent County, Delaware that were previously identified as being at high risk for saltwater intrusion, we collected groundwater level and salinity data from newly installed and existing monitoring wells and test wells, tidal surface water gages, and irrigation ponds throughout the area. Our work identified several processes that contribute to salinization in the east Dover area, including irrigation pumping near tidal creeks and marshes and ground surface inundation during high tides and storms. Most observations were a combination of more than one process occurring simultaneously. This information has been communicated to stakeholders, some of whom have modified their water supply system or pumping schedule to reduce the risk of salinization. The results of this study will help predict future conditions in light of expected sea-level rise. We found that the hydrogeologic framework plays an important role in determining the location and movement of the saltwater-freshwater interface in eastern Kent County. Discontinuous muddy beds are more prevalent at shallow depths in the Columbia aquifer and divide the aquifer into both shallow unconfined and deeper semiconfined portions. At the marsh-upland boundary and near tidal streams, groundwater in the shallow portion of the Columbia aquifer is a mixing zone of fresh groundwater and saline surface water. Salinity in groundwater here depends on a number of dynamic forces that affect the hydraulic gradient and subsequently the amount of bayward freshwater flux. During this study, we found that pumping for irrigation from wells and ponds shifts the freshwater-saltwater interface in the shallow portion of the aquifer inland and can allow saltwater to migrate into the well or pond. Water levels indicate flow patterns are affected by irrigation, tides, and precipitation. In areas where there were no or few shallow confining layers, we observed salty water migrating into the shallow Columbia aquifer during storm events where surface water flooded the ground surface and infiltrated to the water table. However, shallow muddy layers appear to prevent salty water from migrating downward into the deeper semiconfined aquifer during inundation events such as spring high tides and storm surge. We did not encounter the freshwater-saltwater interface in wells screening deeper portions of the Columbia aquifer.

Keywords

groundwater, salinity, Kent County, Dover, Delaware, DNREC, groundwater monitoring

URI

https://udspace.udel.edu/handle/19716/36621

Collections

DGS Reports of Investigations

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