Groundwater in the Northern Atlantic Coastal Plain: How Long Can It Last?

Groundwater is the nation's main freshwater reserve: about half our drinking water, vital to food production and industry, and to the health of rivers, wetlands and estuaries. Falling water levels and other effects of heavy pumping have raised concern about the future supply. USGS regional studies of the nation's principal aquifers assess how much is available and how land use, water use and climate affect it.

The Northern Atlantic Coastal Plain aquifer system: confined aquifers and confining units under a surficial aquifer, east of the Fall Line. Groundwater flows mostly west to east toward the Atlantic. USGS.

The aquifer system

• Extent: from Long Island to northeastern North Carolina — mainly New York, New Jersey, Delaware, Maryland, Virginia and North Carolina.
• Structure: 10 regional aquifers and 9 confining units in a seaward-thickening wedge of loose to partly consolidated sediment, thousands of feet thick at the coast and up to about 10,000 feet near the edge of the Continental Shelf.
• Importance: one of the smallest of the nation's 66 principal aquifer systems, yet 7th in population served and 13th in total withdrawals.
• Demand: it supplies nearly 20 million people. Withdrawals in 2013 were about 1,300 million gallons a day (Mgal/d), some 40 percent of the area's drinking water. Water levels in some aquifers have fallen more than 100 feet since before 1900, and effects cross state lines and run under large bays.

How water moves through the system, from recharge to wells, streams and the coast. Modified from DeSimone and others (2014). USGS.

The water budget, 2005–2008

A USGS groundwater model balanced what enters and leaves the system: about 18,300 Mgal/d passed through it.

The system's water budget, 2005–2008, in million gallons per day. Modified from Masterson and others (2016a). USGS.

• In: 18,070 Mgal/d of recharge, most of which flows through the shallow aquifer to streams or the coast without reaching the deep aquifers; about 230 Mgal/d of wastewater from septic systems; about 60 Mgal/d released from storage as pumping lowers pressure in confined aquifers.
• Out: 93 percent to surface waters — 58 percent straight to the coast, 35 percent to streams — and 7 percent to wells.
• The confined aquifers, where most pumping happens: about 92 percent of what is pumped is replaced by downward flow from the surficial aquifer; the rest comes from storage (5 percent) and saline groundwater flowing in from under the sea (3 percent).

Where the pumped water came from, 1900–2013

Pumping since large-scale use began in 1900 removed 27.8 trillion gallons (Tgal).

Where the pumped water came from, 1900–2013. USGS.

Long Island versus Virginia

The two areas answer pumping very differently, because of their geology.

Cross sections of (A) Long Island and (B) Virginia. USGS.

In the Potomac aquifer itself, about 54 percent of the response is downward flow from the layers above. Pumping there doesn't directly cut discharge to streams — it draws water down from above, which eventually reduces surface outflow, but far more slowly than on Long Island, perhaps not fully for many years.

Storage depletion

Depleting stored groundwater can hurt supply, cause land subsidence, reduce streamflow and springs, and dry wetlands. Only about 8 percent of what was pumped system-wide came from storage — but unevenly:

Share of all withdrawals (red) and of all storage depletion (green), 1900–2013: 27.8 Tgal withdrawn, 2.2 Tgal depleted. USGS.

• Long Island–New Jersey: about 68 percent of withdrawals, only 35 percent of depletion — shallow aquifers, fed by captured surface-bound water.
• Virginia–northern North Carolina: 14 percent of withdrawals, but almost 46 percent of depletion — deep confined aquifers, where storage is a major source.

What it means for sustainability

Sustainable use can go on indefinitely without unacceptable environmental or social harm. Here, the challenge is meeting drinking-water demand while leaving enough groundwater for ecosystems such as the New York–New Jersey harbor estuary, Barnegat Bay, the Hackensack Meadowlands, Delaware's inland bays, the Delaware River Estuary and the Chesapeake Bay, the nation's largest estuary.

• Shallow aquifers (as on Long Island) respond quickly and reach a new balance, so depletion matters less — but their water is more exposed to land-use pollution, and pumping cuts the flow that sustains rivers and estuaries.
• Deep confined aquifers (Virginia, North Carolina) are better shielded in the short term, but aren't recharged directly from the water table, so pumping may outpace replenishment over time.
• Confining clays: water squeezed from clay layers can't be replaced as they compact, causing subsidence. Where those layers are thick and sea level rises on low ground, as in the lower Chesapeake Bay, flooding could worsen.
• Saltwater intrusion: pumping that captures coast-bound water lets the saltwater interface move inland. A USGS assessment found little regional risk in 2013, but local concern in parts of Long Island, New Jersey and Virginia with high-capacity wells near the interface.

Long-term monitoring and models like the USGS model of this system can help managers forecast responses and keep the resource sustainable.

Sources

Based on Sustainability of Groundwater Supplies in the Northern Atlantic Coastal Plain Aquifer System, by John P. Masterson and Jason P. Pope, U.S. Geological Survey Fact Sheet 2016–3046 (DOI; publication page), Water Availability and Use Science Program. Figures restored from the fact sheet's PDF. A work of the United States government in the public domain.