Atlantic Coastal Plain Aquifer System

The Atlantic Coastal Plain aquifer system extends North-South along much of the Eastern portions of New Jersey, Delaware, Pennsylvania, Virginia, and North Carolina (Figure 19). It consists of a sequence of layered sedimentary aquifers (sands and gravels) separated by series of aquitards, all deposited starting around 100 million years ago and continuing today. The layers slope, or dip, to the East and extend offshore for tens of km beneath the continental shelf (Figure 21).

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Figure 21. Layered sequence of major aquifer and aquitard units in the Atlantic Coastal Plain system. The sedimentary units dip and thicken to the East.
Source: USGS Water Atlas

Recharge occurs by both natural and managed infiltration on land across much of the coastal plain; groundwater flow in the subsurface is mainly to the East along the sediment layers. One interesting consequence of this flow pattern is that there may be a sizable freshwater resource offshore that could be accessed by drilling in relatively shallow water on the continental shelf. During the last ice age, when conditions were substantially wetter than today and a nearly mile-thick ice sheet covered the northern extent of the aquifer system, recharge was probably even larger - and thus may have forced fresh water several tens of km offshore, where that "fossil" water may remain today!

The Atlantic Coastal Plain system is an important water source for domestic/municipal supply and industry in population centers throughout coastal North Carolina, Maryland, Virginia, Delaware, and New Jersey. However, concentrated, localized pumping has led to a reversal of flow direction (toward the wells instead of Eastward) in some of the aquifer units throughout the region. In addition to overarching concerns about the sustainability of withdrawals that exceed recharge rates, the flow reversal has led to local salt-water intrusion, whereby saline ocean water infiltrates the aquifer and in some cases renders it non-potable.