The Delaware River, a major source of drinking water for Philadelphia, is facing an emerging threat as rising seas push saltwater farther upstream. It’s a problem that other places, from Miami to Shanghai, will also confront, especially as increasing drought lowers river flows.

At the Delaware Memorial Bridge, about 35 miles southwest of Philadelphia, the tidal waters of the Delaware River estuary push upstream with every incoming tide but are opposed by the river’s downstream flow. For years, this balance has kept salty water well away from intakes that supply drinking water to millions of people in Philadelphia and southern New Jersey. With the so-called salt front in its normal range, some 40 miles downstream from the intakes, any threat to the region’s water supply has seemed distant. But the combination of sea level rise and the expectation of reduced downstream flow as a result of climate change-related droughts have raised new fears that the region’s biggest source of drinking water could at some point become contaminated with seawater. Other regions around the U.S. and the world, from Florida to Bangladesh, are facing similar threats. The issue has long been discussed by water planners at the Delaware River Basin Commission (DRBC), an interstate regulator that represents the interests of the four basin states of New Jersey, New York, Pennsylvania, and Delaware, plus the federal government. Now, the risk is being given closer attention by other stakeholders, including the U.S. Army Corps of Engineers, water suppliers on both sides of the river, and New York City, whose reservoirs help to maintain the river’s flow during dry periods. Planners are asking whether they should be ready, during periods of low flow, to release more water from upstream reservoirs to sustain the volume of water that keeps downstream pressure on the salt front. Other possible options include building desalination plants, adding storage in new or existing reservoirs, curbing water consumption by industrial users such as nuclear power plants, or even moving drinking intakes farther upstream — an expensive alternative of last resort.

Concern about the Delaware River’s salt front is mirrored in other U.S. coastal areas, including the Savannah River.

“There is concern with climate change that we could get another record drought when we would run out of storage, in which case there would be added force pushing salt upstream from the ocean and less water to push it downstream,” said Amy Shallcross, manager of water resource operations at the DRBC. In the 1960s, a six-year drought pushed the salt front to River Mile 102, its highest-ever point, about eight miles downstream from a key water intake in Northeast Philadelphia. Since then, sea levels have been gradually rising, and the rate of increase is expected to surge in the coming decades. Concern about the Delaware River’s salt front is mirrored in other coastal areas, such as the Savannah River between Georgia and South Carolina, where an advancing salt front is threatening drinking water intakes as well as freshwater wetlands. Researchers from the University of South Carolina modeled the effects of 1 foot and 2 feet of sea level rise on salt concentrations over about 40 miles of the river’s estuary, starting at the city of Savannah, and found that those sea level increases would threaten drinking water intakes as well as natural systems.

Salt fronts form when freshwater moving downstream meets tidal water moving inland.

In Florida, seawater is moving rapidly inland along coastal rivers as sea level rises and water is withdrawn from the rivers at an “excessive” rate, according to researchers at Florida State University. As long ago as 2007, the scientists found that about half of the water planners they contacted were predicting that a sea level rise of 6 to 18 inches over the coming 50 years would likely pose a threat to some of their wells. “The ‘salt front’ of the tidal saltwater wedge in coastal rivers will move further upstream with the potential to affect both surface water intakes and well fields that are recharged with river water,” they said. In addition to advancing up tidal rivers, saltwater is also contaminating aquifers as seas rise and coastal communities pump unsustainable amounts of groundwater. In southeastern Florida, saltwater driven by sea level rise is increasingly intruding into the porous limestone of the Biscayne Aquifer, which supplies drinking water to about 6 million people. The canals that for decades drained fresh water from Florida’s interior to the ocean are no longer working as well because the gradient between the interior and the ocean has been reduced by the higher sea level, said Douglas Yoder, deputy director of the Miami-Dade County Water & Sewer Department, the state’s largest utility. Control structures built on the canals to hold back inbound seawater are expected to protect the aquifer from saltwater contamination until about the mid-2030s, Yoder said. But after that, rising seas will force policymakers to consider massive investments in desalinization plants. Harold Wanless, a professor of geography at the University of Miami, said Florida’s predicament of low-lying land, rising seas, and threatened fresh water supplies is replicated in many other parts of the world that are also heavily developed. He cited Mumbai, where some 18 million people live on barrier islands and reclaimed marshes, or low-lying Shanghai, which is also threatened by rising seas. In the coastal deltas of Vietnam and Bangladesh, drinking water for some 25 million people is vulnerable to saltwater intrusion, mostly from storm surges that will be exacerbated by sea level rise, according to a 2016 study.

A 3-foot rise in sea level would push the salt front to within 12 miles of Philadelphia water intakes.

“Almost all the deltas of the world, they are frightfully low,” Wanless said, “and they are primed for saltwater intrusion.” Kenneth Miller, a professor in the Department of Earth and Planetary Sciences at Rutgers University, said that New Jersey — with its dense coastal development, low-lying barrier islands, and high exposure to the ocean — is another poster child for the saltwater intrusion that can be expected in many comparable locations around the world in the future.

“It’s a natural laboratory for studying sea level change and its impacts,” he said, noting that more than 100 water wells have already been abandoned on the Cape May peninsula of southern New Jersey because of saltwater intrusion. And he predicted that the rising salt front will eventually force the authorities to move the Delaware River’s drinking water intakes or find other sources of drinking water. On the Delaware River, planners are especially concerned with seas that are rising about 25 percent faster than the global rate because the mid-Atlantic coast between New York and Virginia is sinking at the same time as the ocean level is rising. The latest projections for sea level rise along the New Jersey coastline are for a gain of up to 1.1 feet between 2000 and 2030, up to 3.5 feet by 2070, and up to 6.3 feet by the end of the century, according to a Rutgers University study. In 2017, the National Oceanic and Atmospheric Administration predicted that the global mean sea level could rise as much as 2.5 meters (8.2 feet) by 2100.

The Delaware River's current salt front (dark blue) lies 40 miles south of key drinking water intakes. A sea level rise of 3 feet would move it upstream to within 12 miles of the intakes. Yale Environment 360

The highest projections have not yet been correlated to the predicted position of the salt front in the Delaware River, according to DRBC’s Shallcross. But in a presentation last October, she said that a 3-foot rise in sea level — roughly equal to the Rutgers forecast for 2070 — would push the salt front about 34 miles upstream from its current position to River Mile 98, only 12 miles downstream of the Philadelphia and New Jersey water intakes. Those forecasts explain why New Jersey American Water, the state’s largest investor-owned water utility, is studying how to make sure the salt front doesn’t reach its Delran, New Jersey intake, which supplies about half-a-million people. “It’s a major concern for us,” said Matt Csik, the utility’s director of water quality and environmental compliance. “We need to continue to work through our partnerships to ensure that there’s no saltwater at the intake.” He argued that the salt front’s current position about 40 miles downstream of the intakes doesn’t mean utilities should soft-pedal their search for ways of preventing it moving closer to the intake. “We’re acting with urgency,” he said. “All the wrong things combine, and you find out you have a problem almost overnight. Then you don’t have the time to do the engineering and the modeling that you need.” The prolonged 1960’s drought is never far from planners’ minds, especially considering sea level increases over the last half-century. In response to that drought, the Delaware River Basin Commission created a flow-management plan that ensures a variable water flow down the river, depending on the time of year. If the natural river flows are too low, officials boost the volume of water coming downstream with releases from three reservoirs in upstate New York.

While an advancing salt front may threaten drinking water supplies, it can also damage the ecology of estuaries.

The salt front can best be kept downstream by the continued use of this flow-management program, Csik said, rather than more radical solutions such as moving the water intake, a massive project that Csik characterized as a “last resort.” Philadelphia’s intake is roughly opposite Delran, on the west side of the river, and it supplies about 60 percent of the water for the city’s 1.6 million people. The Philadelphia Water Department, the city’s utility, did not respond to a request for comment. For its part, the Army Corps is studying whether to expand the designated uses of a Pennsylvania reservoir to allow for its water to be released into the Delaware during times of drought — a potential addition to flows from the three New York reservoirs. While an advancing salt front may threaten drinking water supplies, it can also damage the ecology of the whole estuary, said Danielle Kreeger, senior science director for the Partnership for the Delaware Estuary, a science-based nonprofit that collaborates with other conservation groups to protect and restore the tidal section of the river. “With all these animals, plants, and people dependent on the freshwater-tidal zone, any increase in saltwater is considered a vulnerability,” she said.

Fishermen on the banks of the Delaware in Palmyra, New Jersey, across from Philadelphia. Tom Gralish/The Philadelphia Inquirer via AP

In addition to sea level rise, Kreeger cited evidence that salinity levels are boosted by dredging in the river — a project that has recently deepened the shipping channel to allow bigger ships to call at ports like Philadelphia and Wilmington, Delaware. “When we deepen the channel, we increase the tidal flushing volume of the estuary, and that potentially brings more saltwater up on the daily tides,” Kreeger said. Keeping the salt front at bay should be achievable by the careful management of existing upstream water resources, managers say, but it may become challenging during severe drought when officials are faced with a choice of supplying drinking water or protecting the environment. “Eventually we’re going to hit another drought, and you are not going to hit those [water flow] targets very easily,” Kreeger said. “Holding back water capacity in a reservoir to protect ecological systems is going to be down the priority list for when New York City or Philadelphia needs more water.”