Groundwater is the sole source of drinking water and the main source of water for agriculture and industry in central and southern Delaware. This study mapped the depth and thickness of thirteen aquifers in Kent and Sussex Counties, used these maps to assign groundwater withdrawals for 2004 to 2008 to the appropriate aquifer, and analyzed withdrawals for each type of water use by geographic area.
The geology of the Delaware Coastal Plain is characterized by a broad complex of surficial Quaternary deposits unconformably underlain by Cretaceous to Cenozoic sediments that dip gently to the southeast. Permeable sands within this succession are used as groundwater sources. The hydrogeologic framework of the study area was characterized by maps of the elevation and thickness of thirteen aquifers. The maps were created in a geographical information system by interpolating aquifer depth data extracted from a database encompassing approximately 6,600 boreholes. The unconfined aquifer occurs in surficial Quaternary and Neogene sands. It is generally less than 100 feet thick in Kent County but varies from a few feet to more than 200 feet thick in Sussex County. The confined aquifers mapped include one Cretaceous (Mount Laurel), two Paleogene (Rancocas and Piney Point), and nine Neogene sand units (lower Calvert, Cheswold, Federalsburg, Frederica, Milford, Middle Choptank, Upper Choptank, Manokin, and Pocomoke). These aquifers are typically tens of feet thick and occur at progressively greater depths southeastward from their recharge areas.
The study found that 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 were assigned to aquifers using the aquifer maps and well-screen elevation data. For water-use categories where withdrawals could be attributed to specific wells – public, industrial, and golf courses – aquifers were determined by analyzing well-screen elevations relative to aquifer raster surfaces. For categories in which withdrawals could not be assigned to individual wells – irrigation, domestic self-supplied, and livestock – available well depth data in each category were analyzed by census block and compared to the aquifer raster surfaces; for each block, the proportion of wells in each aquifer was used as the basis for apportioning withdrawals to aquifers.
The results indicate that the unconfined aquifer accounted for more than half of groundwater withdrawals. Three shallow, confined aquifers primarily used in Sussex County (confined Columbia, Pocomoke, and Manokin) each provided approximately between 8 and 11 percent of total withdrawals. Withdrawals for the three most important confined aquifers in Kent County (Cheswold, Frederica, and Piney Point) each represented 3 to 5 percent of total withdrawals. Estimated withdrawals were also computed by aquifer for each water-use category and each census block.
University of Delaware
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