Water agency renews concern that sea-level rise will flood drinking-water intakes in Philadelphia, Southern N.J.

Inside Climate News

This article originally appeared on Inside Climate News, a nonprofit, non-partisan news organization that covers climate, energy and the environment. Sign up for its newsletter here.

Fears that seawater may one day flood drinking-water intakes serving millions of people in the Philadelphia region and southern New Jersey rose again when an interstate regulator said a current program to stop salty water moving up the Delaware River might be overwhelmed by sea-level rise and drought.

The Delaware River Basin Commission said its existing plan to release water from upstream reservoirs if needed to prevent a so-called salt front from approaching the intakes might no longer work, given the steady rise in sea level in the river’s tidal stretch and the expectation that downstream flows will sometimes be reduced by droughts resulting from climate change.

The plan, called the Trenton Flow Objective, aims to keep enough fresh river water flowing at a specified rate past New Jersey’s state capital at the northern limit of the river’s tidal section to ensure that the salt water from the Delaware Bay to the south stays away from the drinking water intakes in northeast Philadelphia and Burlington County, New Jersey. The program also aims to protect industrial water intakes along the river’s tidal section.

“Preliminary studies indicate that the Trenton Flow Objective may not be adequate for salinity repulsion with sea level rise; therefore, new management measures and additional freshwater may be needed for salinity management,” the agency said in its latest State of the Basin report, a wide-ranging assessment of the region’s water resources, released last week for only the third time since 2008.

Under the flow program, the DRBC can release water from two Pennsylvania reservoirs in order to maintain the agreed volume of water in the river at Trenton. The DRBC, formed in 1961, represents the water interests of Pennsylvania, New Jersey, New York and Delaware plus the federal government.

Concern about possible upstream movement of the salt front in a river that supplies about 60 percent of drinking water to Philadelphia’s 1.5 million people has underpinned the flow objective since it started in 1983. Now, it has been given new urgency by the steady local rise in sea level and the expectation that drought driven by climate change will make it harder to sustain the downstream flow that keeps the salt front at bay.

Sea level near Philadelphia rose by 6.7 millimeters (about a quarter-inch) a year from 2000 to 2023, more than twice the long-term rate, the report said. By 2100, the level of the nearby Delaware Bay is expected to be between 0.79 and 2.08 meters higher than it was in 2000, according to a 2022 forecast by the National Atmospheric and Aeronautical Administration.

But future forecasts by NOAA, a leading source of climate information, may be undermined by deep cuts in the agency’s workforce that began on Thursday. The Trump administration fired some 800 out of roughly 13,000 employees. Several hundred more workers were expected to leave on Friday as a result of the administration’s deferred resignation program, while 2,500 jobs could be lost through the agency ending agreements with contractors, The New York Times reported.  

Project 2025, a blueprint by the conservative Heritage Foundation advocating policy for the second Trump administration, has called for NOAA to be broken up, and its forecasts privatized. 

Meanwhile, January rainfall in Philadelphia was 2.49 inches below normal, making it the third-driest January since records began 131 years ago, according to Drought.gov, a NOAA website. That followed a record 29-day stretch without rain in the city in October 2024.

“Elevated sea level is the new normal,” said John Jackson, senior research scientist at the Stroud Water Research Center, a Pennsylvania-based nonprofit group that advocates for the stewardship of freshwater systems. “Then you layer on top of that a reduction in upstream water from a severe drought. Bigger rains keep downstream pressure on the salt front during a storm, but if the frequency of 30- or 60-day droughts continues or increases, then it’s countered.”

The salt front is defined as a seven-day average of 250 milligrams per liter of chloride, too salty for safe drinking water. It reached as far upstream as River Mile 102—only eight miles below drinking-water intakes for Philadelphia and southern New Jersey—during the “drought of record” in the 1960s, DRBC records show.

For now, the salt front is broadly stable between River Miles 67 to 76, near the Delaware Memorial Bridge between New Jersey and Delaware, the agency said in a public presentation last September. That’s some 35 miles downstream from the intakes. 

“Elevated sea level is the new normal.”— John Jackson, Stroud Water Research Center

But DRBC projections for sea-level rise put the salt front increasingly closer to the intakes. For example, sea-level rise of only a third of a meter, about a foot, from its level in 2000 would push the salt front as far upstream as River Mile 99.2—some 25 miles closer to the intakes than its current position, the DRBC’s presentation showed.

By comparison, the mid-point of another sea-level rise forecast led by Drexel University is that the level in the Philadelphia region will be nine inches higher in the 2030s than it was between 1995 and 2014.

Since seas are now higher than they were when the current flow-management program was designed, the DRBC is now “developing tools and performing studies” to see if the program still protects drinking water intakes in Philadelphia and southern New Jersey, said Amy Shallcross, the agency’s manager of water resource operations.

Without additional downstream flows, a 1-meter rise in seas would push the salt front to River Mile 104.4, or only six miles from the intakes. And a 1.6-meter rise—well below the upper range of NOAA forecasts for 2100—would drive the salt front to within a mile of the intakes, according to the DRBC report.  

“Conditions are becoming more favorable for upstream movement of the salt front,” the report said. “Sea level rise increases the salt being pushed into the estuary, and salinity from land-based sources is increasing.”

If seawater reached the intake that supplies Philadelphia, or another at Delran, New Jersey, on the opposite side of the river, it could corrode pipes, affect the taste and odor of drinking water and possibly cause health problems for residents sensitive to sodium, the agency said.

Carol Collier, a former executive director of DRBC, said the prospect of an increasingly saline Delaware River Estuary is also a problem for the basin’s ecological balance, which includes fish migration, the critically endangered local population of Atlantic sturgeon and its tidal marshes.

How best to keep the salt front from advancing upstream is complicated by increasing salt levels coming from land runoff—another problem identified by the new report—and by a 1954 ruling from the U.S. Supreme Court. That requires New York City to release water if needed from its reservoirs to meet a flow target in the Delaware River, an agreement that is now negotiated by the DRBC.

“I think this complex problem is an excellent example of why we need to support solid scientific studies upon which water management policies can be built. There are no simple answers. I’m just glad DRBC is there and leading this important work with many partners,” Collier said. “It should be an issue of concern for all of us.”

In the report, the agency said it is developing “sophisticated modeling and analyses” to establish the relationship between sea level rise and salinity, and to develop a better understanding of the potential impacts. “With models and other tools, different management options for salinity repulsion can be evaluated, including but not limited to, new infrastructure, optimization of infrastructure, and flow management.”

In other comments on the effect of climate change on the Delaware River Basin, the report noted that the air temperature is rising overall, prompting a decline in water quality and reducing the quantity of snowpack during the winter. Given forecasts for more warming in coming decades, residents of the highly urbanized region can adapt by planting trees, building green roofs and installing reflective surfaces, it said.

The Philadelphia Water Department, a city-owned utility, did not respond to requests for comment. New Jersey American Water, an investor-owned utility, said it monitors the salt front, which has so far not “crept up into an area where we would be concerned for our intake.”