Northern Atlantic Coastal Plain

The quiet aquifer system supporting 18 million people from Long Island to North Carolina — with saltwater advancing on its coastal margins

States

NY, NJ, DE, MD, VA, NC

Type

Multi-layered confined sandy aquifers (Cretaceous-Tertiary)

Status

Long-term water-level decline at major pumping centers; saltwater intrusion in coastal sub-regions

The Northern Atlantic Coastal Plain Aquifer System is the multi-layered groundwater stack underlying the Atlantic coast from Long Island, New York, southward through New Jersey, Delaware, Maryland, Virginia, and into northeastern North Carolina. It is one of the more productive aquifer systems in the eastern United States and supplies drinking water to roughly 18 million people — including Long Island in its entirety (no surface water supply), much of southern New Jersey, the Delmarva Peninsula, and the Virginia Coastal Plain including Norfolk and Hampton Roads.

The story here is rarely dramatic — it isn't a Front Range depletion crisis or a Mississippi Delta cancer-cluster story. It is a long, slow drawdown happening underneath one of the most densely populated parts of the country, and it is being matched, on the coastal margins, by a slow advance of saltwater into the aquifer. Both effects are gradual enough that they don't make the news, and serious enough that you should know what they mean for your well.

What it is, geologically

The aquifer system is a wedge of marine and fluvial sediments — sands, gravels, silts, clays — deposited during the Cretaceous and Tertiary as sea levels rose and fell along what's now the Atlantic Coast. The wedge thickens to the southeast: in the inner Coastal Plain (the western edge near the Fall Line) the system is shallow and the aquifers are unconfined; in the outer Coastal Plain (toward the coast) the wedge thickens to thousands of feet and the aquifers are deeply confined by intervening clay layers.

Major aquifer units, generally from oldest/deepest to youngest/shallowest:

• Potomac aquifer system (Patapsco, Patuxent, Magothy units) — Cretaceous sands; the deepest and most heavily pumped in VA and MD
• Aquia aquifer — Paleocene, Maryland and southern NJ
• Piney Point-Nanjemoy — Eocene, MD/VA
• Castle Hayne aquifer — Eocene-Oligocene limestone, NC (often treated separately)
• Surficial aquifer — Quaternary unconfined, present everywhere

For private wells, the relevant question is which layer your well taps. Shallow domestic wells typically draw from the surficial; deep production wells from confined Cretaceous units hundreds of feet down.

Long-term decline

Water levels in the deep confined aquifers have dropped steadily for over a century at major municipal pumping centers:

• Norfolk and Hampton Roads, VA — water levels in the Potomac aquifer dropped over 200 feet between 1900 and 2010. The state created the Eastern Virginia Groundwater Management Area to constrain pumping; partial recovery has been observed since 2015 with managed-aquifer-recharge programs.
• Camden and southern NJ — significant cumulative drawdown in the Potomac-Raritan-Magothy system; NJ's Critical Water Supply Management Areas regulate withdrawals.
• Long Island, NY — entirely dependent on the underlying Magothy aquifer; pumping stress combined with reduced recharge from urbanization has dropped water levels in some areas.
• Atlantic City and the NJ shore — heavy summer pumping for tourism stresses the Cohansey and Kirkwood aquifers; mandatory restrictions during dry years.

Saltwater intrusion: the coastal margin problem

Where the aquifer reaches the coast, the same physics applies as on the Floridan: drop the freshwater head and the saltwater wedge advances inland. Specific fronts:

• Long Island — saltwater intrusion documented since the 1930s; the Brooklyn-Queens Magothy system was effectively abandoned for water supply by mid-century. Active monitoring continues.
• Southeastern Virginia — the Potomac aquifer near Hampton Roads has chloride concentrations rising in some monitoring wells; the state's managed aquifer recharge program is partly motivated by this.
• Parts of the NJ shore — Cape May County, parts of Atlantic and Ocean counties.
• Lower Eastern Shore of Maryland — chloride elevation in coastal Worcester and Somerset counties.

If your well is within roughly 10 miles of the coast, test for chloride and sodium as routine.

Water quality

• Iron and manganese — common in the deeper confined aquifers where reducing conditions prevail. Most homes in southern Maryland and eastern Virginia have softeners and iron filters. See iron and manganese.
• Radium — naturally elevated in parts of the New Jersey Coastal Plain (especially Burlington, Camden, Gloucester counties) where it leaches from the Cohansey and Kirkwood sands. NJDEP maintains a Radium Areas program.
• PFAS — multiple military bases in the region (NAS Patuxent, Joint Base Andrews, Dover AFB, Joint Base McGuire-Dix-Lakehurst, Naval Weapons Station Yorktown) have documented AFFF firefighting-foam plumes in the surficial and shallow confined aquifers. See PFAS.
• Saltwater markers — sodium, chloride, sulfate. See above.
• Bacteria — shallow surficial wells in agricultural Delmarva and rural NC are vulnerable; nitrate co-occurrence is common in poultry-operation regions. See bacteria.

Known contaminant concerns

Communities on this aquifer