Study Assesses Threats to Groundwater Availability and Sustainability in Northern Atlantic Coastal Plain

Scientists Look at Issue from Long Island to North Carolina

Threats to groundwater availability and sustainability in the Northern Atlantic Coastal Plain are dependent to a large degree by the type of aquifers used for water supply, according to a new regional assessment by the U.S. Geological Survey.

The water challenges faced in the highly populated area, which ranges from Long Island, New York to North Carolina, vary greatly, as do the causes.

Block diagram showing the Northern Atlantic Coastal Plain aquifer system, which includes the areas east of the Fall Line from Long Island to northern North Carolina. This multilayer system consists of confined aquifers and confining units capped by an unconfined, surficial aquifer. Groundwater flows predominantly from west to east from the Fall Line to the Atlantic Ocean, except where high-capacity pumping wells alter this regional flow pattern., USGS

“We’re looking at water situations in the north and south of the region that might be seen as two sides of the same coin,” said John Masterson, a USGS hydrologist and lead author of this assessment. “In the north, the greatest threat to sustainability is the ecological impacts that may result from overpumping, whereas in the south, depletion of the groundwater resource is the biggest sustainability concern.”

In places like Long Island and New Jersey, in the northern part of the study area, groundwater depletion is not a serious concern. There, shallow aquifers used for drinking-water supply are well connected to the land surface and easily replenished by rainwater that seeps into the ground as aquifer recharge.

“The tradeoff to pumping this water is that, although there appears to be plenty of it, removing any of it from the aquifer comes at the expense of the freshwater needed to keep streams flowing and to support the marine life that depend on fresh groundwater discharge to coastal estuaries,” said Masterson. “Pumping these wells for human use captures the groundwater that otherwise would have become streamflow or gone into the coastal waters. Reducing flows from aquifers to these surface waters, if great enough, can result in adverse ecological effects.”

In the southern part of the study area, in places like Virginia and North Carolina, the situation is reversed. The aquifers used for drinking-water supply typically are deep and not well connected to land surface.  Pumping in this area therefore does not have a large effect on surface waters; however, the restricted connections between the deep aquifer and surface waters can potentially lead to groundwater depletion. Although only 14 percent of the total pumping from all aquifers in the Northern Atlantic Coastal Plain occurs in Virginia and North Carolina, it accounts for almost half of the total groundwater depletion in the entire aquifer system.

“In this area, most of the pumping occurs in the deep confined aquifers that are not well-connected to land surface and the water pumped from these wells isn’t groundwater that otherwise would have discharged to streams or to coastal estuaries,” said Masterson.  

Masterson characterized this groundwater depletion as a real concern, saying it may lead to land subsidence, which is the gradual lowering of the land surface, and intensify the effects of local sea-level rise, particularly in the Lower Chesapeake Bay area in southern Virginia.  An additional concern for this part of the aquifer system is that in coastal areas, groundwater depletion can result in the landward encroachment of salty groundwater, diminishing the quality of the drinking water in those areas.

Block diagram showing conceptual model of the flow system with the Northern Atlantic Coastal Plain aquifer system. The diagram illustrates how water enters the flow system as streamflow across the Fall Line and as recharge from precipitation; this water then leaves the flow system as groundwater is withdrawn by pumping wells or discharges to streams and to the coast. Red arrows indicate inflow from recharge and wastewater return flow, and blue arrows indicate outflow from groundwater discharge to streams and to the coast as well as water removed from private and public-supply wells. Modified from DeSimone and others (2014).

USGS water availability studies like this one integrate information about groundwater flow, aquifer storage and water use for the region. These types of studies are designed to quantify the effects of past development and examine the effects of future growth on groundwater flow and storage in the aquifer system. They show how competing uses and demands for water interact over time across a region.

“Although this analysis was conducted at a regional scale, the information produced in this study is valuable for managers at state and local levels in making informed decisions regarding the potential effects of future water use on existing water users, aquatic ecosystems and the public,” said Masterson.

This study and related groundwater availability studies are being conducted nationally by the USGS through the Water Availability and Use Science Program as part of its ongoing assessments of the principal aquifers of the Nation. Regional aquifer studies are intended to provide citizens, communities, and natural resource managers with knowledge of the status of the Nation’s ground­water resources and how changes in land use, water use, and climate have affected and are likely to affect those resources now and in the future.

Reports from this project, as well as additional information on USGS water availability studies in the NACP, are available online at https://pubs.er.usgs.gov/publication/pp1829.

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