NJ Clean Stream
njcleanstream.org
Saltwater intrusion is creeping into South Jersey’s drinking water — and it’s accelerating
Along a boundary with no signs or visible markers, a slow battle is being fought in the groundwater beneath South Jersey’s Coastal Plain. The saltwater-freshwater interface in the Coastal Plain aquifer system is moving — measurably, persistently, and in the direction of the freshwater supply that South Jersey communities, private well users, and agricultural operations depend on.
The geology of vulnerability: South Jersey’s Coastal Plain aquifer system
South Jersey sits atop one of the most productive groundwater systems in the northeastern United States — a layered series of sandy aquifer formations separated by clay aquitard layers. The major producing formations include the Kirkwood-Cohansey aquifer, the Atlantic City 800-foot sand, the Wenonah-Mount Laurel, the Piney Point, and the Vincentown formations at varying depths.
These formations are hydraulically connected to the ocean: saltwater in the offshore portion grades to freshwater in the inland portion, with the freshwater-saltwater interface at some depth and distance inland reflecting the balance between freshwater recharge pressure and saltwater density. This balance is dynamic. As freshwater recharge decreases or saltwater pressure increases — from sea level rise, from reduced precipitation, or from pumping that lowers freshwater head — the interface moves inland and upward. The concept of hydraulic head explains the mechanism: when pumping reduces the freshwater head in the aquifer below the equivalent saltwater head at the coast, the interface moves inland even if saltwater is not being directly drawn into wells.
Where intrusion is occurring: the most vulnerable communities
Barrier island and coastal township communities of Ocean and Atlantic counties are among the most immediately vulnerable. Communities on the barrier islands — Long Beach Island, the Barnegat Peninsula — are essentially surrounded by saltwater. Their freshwater lens is maintained by recharge from precipitation. Any imbalance between recharge and withdrawal can cause saltwater to encroach from below or the sides. Sea level rise is reducing the effective depth to saltwater in these communities.
Cape May County has documented saltwater intrusion among the most significant in New Jersey. Surrounded on three sides by saltwater, its freshwater Cohansey aquifer has experienced measured saltwater encroachment from the south and west, driven by decades of heavy groundwater withdrawal and accelerating sea level rise. The Cape May County Municipal Utilities Authority has for years been developing alternative water supply strategies in response.
Bay-side communities of Burlington and Ocean counties along Barnegat Bay and the Mullica River estuary face saltwater intrusion from bay systems rising in elevation as sea levels rise. Coastal storms that push bay water inland — increasingly frequent as sea levels rise — can inundate the shallow water table with saline water that disperses laterally through the aquifer system.
Inland agricultural areas drawing heavily on shallow Coastal Plain aquifers for irrigation are at risk both directly — from saltwater intrusion into irrigation wells — and indirectly through the effect of excessive agricultural pumping on freshwater head, which accelerates saltwater advance from the coast.
The measurement challenge: documenting a slow-moving crisis
Saltwater intrusion is difficult to document in real time without a dense network of monitoring wells measuring water quality across the freshwater-saltwater interface. New Jersey’s monitoring network, while more developed than in many states, is not sufficiently dense to characterize the interface position and its rate of change with high spatial resolution. Documentation of saltwater intrusion in specific locations often occurs reactively — when a utility’s production well begins showing elevated chloride concentrations indicating the interface has advanced to the point of affecting supply. At that point, remedial options are limited.
NJ Clean Stream is advocating for the NJ DEP and the NJ Geological and Water Survey to substantially expand the saltwater intrusion monitoring network and to make monitoring data publicly accessible in real time. This monitoring investment is modest relative to the cost of the water supply disruptions it could prevent.
How pumping decisions drive intrusion — and why this is partly controllable
Unlike sea level rise — driven by global greenhouse gas emissions — the contribution of groundwater withdrawal to saltwater intrusion is a local, controllable factor. New Jersey’s Water Allocation Permit program regulates groundwater withdrawals, specifying maximum permitted withdrawal volumes intended to ensure withdrawals do not exceed sustainable aquifer yield. In practice, some permits in coastal areas were issued decades ago based on historical aquifer data and historical sea levels that no longer represent current conditions, and may be permitting withdrawals that are accelerating saltwater intrusion.
NJ Clean Stream is pushing for systematic NJ DEP review of water allocation permits in saltwater intrusion-vulnerable formations, incorporating current sea level rise data and updated aquifer vulnerability assessments, and modifying permits where permitted withdrawal rates are contributing to or accelerating saltwater intrusion.
Adaptation strategies
Wellfield management and optimization. Utilities with multiple supply wells can optimize pumping schedules to minimize drawdown at the most vulnerable wells — those closest to the interface — while maximizing pumping from wells farther inland. A short-term operational measure that can delay saltwater advance.
Aquifer storage and recovery (ASR). Injecting treated water or desalinated water into the aquifer near the interface can push it seaward. ASR has been implemented in some coastal South Jersey communities as a saltwater intrusion management strategy.
Source switching and diversification. Developing alternative supply sources — deeper, more confined aquifer formations; surface water supplies; desalination; or interconnections with utilities with less vulnerable sources — reduces vulnerability to intrusion but requires substantial capital investment.
Regional water supply planning. The scale of saltwater intrusion as a threat — affecting multiple utilities across multiple counties — calls for coordinated regional assessment and strategy rather than utility-by-utility responses. NJ DEP and NJ Geological and Water Survey should lead this regional planning.
Desalination as a last resort. For communities where other adaptation options are insufficient, reverse osmosis desalination can provide a climate-resilient water supply independent of freshwater aquifer conditions. Expensive and energy-intensive, but for communities losing freshwater sources to saltwater intrusion, ultimately the only viable long-term solution. NJ should be developing the planning and regulatory frameworks for desalination now, before crisis makes hasty implementation necessary.
The climate justice dimension
Saltwater intrusion does not fall equally on all communities. Cape May County’s municipal utilities have the institutional capacity and revenue base to pursue sophisticated adaptation strategies. The private well users in a rural South Jersey township who discover their domestic wells are increasingly salty have none of those resources. They have a failing water supply and no mechanism to compel anyone to help them.
NJ Clean Stream is advocating for state financial assistance specifically targeted at private well users and small community water systems in saltwater intrusion-vulnerable areas, providing technical assistance and funding for alternative water supply development before existing supplies fail rather than after.
This is Article 2 of 3. Article 1 explains the four mechanisms through which climate change is stressing NJ’s water infrastructure. Article 3 examines whether NJ’s water systems are prepared for the next major storm — and what genuine storm resilience requires.