APPLICATION OF 3-D AQUIFER MAPPING TO ANALYSIS OF GROUNDWATER WITHDRAWALS BY SOURCE IN THE DELAWARE COASTAL PLAIN

Groundwater is the sole source of drinking water and the main source of water for agriculture and industry in the Coastal Plain of Delaware. This study analyzed groundwater withdrawals in the context of new 3-D aquifer maps to assess water use from all groundwater sources in Kent and Sussex Counties.

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. Thirteen aquifer surface elevation grids were calculated using data from approximately 6,600 boreholes, providing a 3-D framework of the subsurface aquifer geology. The unconfined aquifer includes surficial Quaternary and Neogene sands. It is generally less than 100 ft thick in Kent County but is more variable to the south in Sussex County, from a few feet to more than 200 ft thick. The confined aquifers mapped include one Cretaceous, two Paleogene, and nine early Neogene sand units. These aquifers are typically tens of feet thick and occur at progressively greater depths southeastward from their northern recharge areas.

Total reported and spatially estimated groundwater withdrawals for all uses were calculated between 89 to 144 million gallons per day annual rates for 2004 to 2008. Withdrawals were assigned to aquifers using the 3-D aquifer maps. 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 grids. For categories where withdrawals could not be assigned to individual wells – irrigation, domestic self-supplied, and poultry – available well depth data in each category were analyzed by census block and compared to the aquifer grids; 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 accounts for more than half of groundwater withdrawals. The confined Columbia and Pocomoke are estimated to each represent around 11% of total withdrawals, the Manokin 8%, the Cheswold, Frederica, and Piney Point each 3 to 5%, and other confined aquifers less than 2% each. Estimated withdrawals were also computed by aquifer for each water-use category and each census block.