The Delaware Geological Survey (DGS) has released a lengthy report providing a comprehensive analysis of aquifer geology and groundwater usage in central and southern Delaware. Bulletin 22, Aquifers and Groundwater Withdrawals, Kent and Sussex Counties, Delaware, was authored by staff members Peter P. McLaughlin and Jaime L. Tomlinson, along with former staff member Amanda K. Lawson. Groundwater is the sole source of drinking water and the main source of water for agriculture and industry in central and southern Delaware. Bulletin 22 documents a large volume of data assembled in a multi-year study of these aquifer systems and analyzes groundwater withdrawals by aquifer, by geography, and by type of water use, to better understand the nature of supplies and utilization of this important natural resource. This project was made possible by funding from the Delaware Department of Natural Resources and Environmental Control and by that agency’s cooperation on the compilation of water-use data.

The geology of the Delaware Coastal Plain provides a framework for the occurrence of groundwater in any of a series of permeable sand bodies that occur tens to hundreds of feet below the land surface. A broad complex of surficial Quaternary deposits unconformably overlies Cretaceous to Cenozoic sediments containing confined aquifers that dip gently to the southeast. The analysis of aquifer geology was focused on mapping the depth and thickness of thirteen aquifer sands in Kent and Sussex Counties using geographical information system software and geologic data from approximately 6,600 boreholes. These maps allowed analysis of the volumes of water being withdrawn from each aquifer based on comparison of well depths to aquifer depths.

Depending on the type of water use, volumes of groundwater withdrawals were calculated from pumping data reported to regulators (2004-2008 data) or estimated on the basis of geographic and demographic factors. Annual groundwater withdrawals for all uses in the study area ranged from approximately 89 to 144 million gallons per day annually for 2004 to 2008. Withdrawals could be tabulated from reported pumping data for public, industrial, and golf courses wells. Withdrawals for water use categories that could not be assigned to individual wells – irrigation, domestic self-supplied, and livestock – were calculated by census block based on documented estimation methodologies. The greatest volume of groundwater was used by agricultural irrigation wells, followed by public water wells for community use and private wells for self-supplied domestic use. Irrigation withdrawals were estimated to be as much as 91 million gallons per day (Mgal/d) for a dry year and as little as 50 Mgal/d in a year with abundant, well-timed rainfall. Withdrawal data report for public wells totaled between 22.8 and 26.2 Mgal/d, with slightly greater withdrawals in Sussex County than in Kent County. Domestic self-supplied usage was estimated as 11.6 Mgal/d, with nearly two-thirds of that use in Sussex County. Smaller categories of use analyzed include smaller public water systems below the reporting threshold (community, transient non-community, and non-transient non-community), non-irrigation agricultural withdrawals for poultry use, golf course irrigation, and self-supplied lawn irrigation. Analysis of groundwater withdrawals from each aquifer indicates that the unconfined aquifer was the source of more than half of the groundwater pumped; the remaining withdrawals were made from the 12 confined confined aquifers, with specific aquifers used depending on location and well type.

DGS Bulletin No. 22 is part of the Delaware Geological Survey’s ongoing mission to understand geologic and hydrologic systems and to advise, inform, and educate Delawareans about the results of such investigations for use in issues regarding surface and groundwater resources, agriculture, economic development, land-use planning, environmental protection, resource evaluation, engineering applications, hazard identification and mitigation, and recreation. It is available in PDF format from the DGS web page at www.dgs.udel.edu/publications.