In 2019, the Delaware Geological Survey, in cooperation with the Delaware Department of Natural Resources and Environmental Control, completed a groundwater-monitoring, infrastructure-construction, and data-collection project in Kent County, Delaware. This work, recommended by the Governor’s Water Supply Coordinating Council and funded by a capital appropriation from the state, addressed data gaps for the shallower aquifers commonly pumped by water-supply wells that serve domestic, public, irrigation, and commercial users and provided additional data to characterize the relationships between the aquifers and streamflow. The aquifers investigated in this study are, from shallowest (closest to the surface) to deepest, the Columbia, Milford, Frederica, Federalsburg, Cheswold, Piney Point, Rancocas, and Mt. Laurel. The groundwater-monitoring infrastructure and data created during this project will facilitate follow-up projects targeted to specific issues for the water resources of Delaware.
The Piney Point aquifer has a characteristic uncommon among other aquifers in the Coastal Plain of Delaware, in that it receives recharge only through slow, diffuse leakage through overlying and underlying confining beds. As a result, pumping of the Piney Point aquifer in the Dover area has reduced water levels more than 80 feet over the past 50 years in several wells in the Dover area. Given current rates of decline, static water levels in long-term observation well Id55-01 will reach the top of the aquifer within 30 years. Pumping water levels in two supply wells operated by Dover Water are projected to reach the top of the aquifer within 10 years if current rates of decline continue. Given that the Piney Point aquifer matrix contains glauconite, a compressible clay pellet, there is significant risk for aquifer compaction and reductions in well yield should water levels continue to decline.
Water-level and water-quality data from nested wells (e.g., multiple wells at the same site finished at different depths) in the Milford, Frederica, Federalsburg, and Cheswold aquifers are recharged primarily in areas where they are in close hydraulic connection with the overlying water table aquifer. Similarities in hydrographs, potentiometric surface maps from these aquifers, and time series of head differentials between the Frederica, Federalsburg, and Cheswold aquifers indicate that they function as a single, leaky, layered aquifer. Pumping has reduced water levels in the Frederica, Federalsburg, and Cheswold aquifers below sea level over large areas of Kent County, and has caused flow directions to change from a general southeasterly direction in pre-development times to flow directed toward pumping centers. Water quality data that show no significant correlation between dissolved solids and well depth support the interpretation that flow directions have changed in response to pumping.
Long-term declines in annual minimum total flow and baseflow at streamflow gaging stations in the Beaverdam Branch and Marshyhope Creek watersheds and associated long-term increases in annual precipitation, number of growing days, irrigated acres, and number of irrigation wells in those basins are consistent with the interpretation that the combined effects of irrigation pumping and climate change are reducing groundwater discharge to those streams.
Results of testing major groundwater constituents in the water-table portion of Columbia aquifer are consistent with previous studies in Delaware, with calcium and sodium the major cations, and different mixtures of the anions chloride, nitrate, and bicarbonate depending on land use and composition of the aquifer near each well. Major constituents of groundwater in the Milford, Frederica, Federalsburg, and Cheswold aquifers over most of Kent County are dominated by calcium, bicarbonate and sulfate.
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