Salt water creeping up Delaware River in worrying sign for big fresh water source

Source of Philadelphia’s drinking water sees salt line pushed closer to city by drought and sea level rise

Salty ocean water is creeping up the Delaware River, the source for much of the drinking water for Philadelphia and millions of others, brought on by drought conditions and sea level rise, and prompting officials to tap reservoirs to push the un-potable tide back downstream.

Officials say drinking water is not imminently at risk, but they are monitoring the effects of the drought on the river and studying options for the future in case further droughts sap the area, amid the climate crisis.

The salt front or salt line is where salt water from the ocean and freshwater meet in the river. That boundary is typically somewhere around Wilmington, Delaware, but the recent drought in the north-east has pushed it about 20 miles (32km) north, around Philadelphia international airport.

The farther the line moves upstream, the closer it gets to drinking water intakes, which officials have worked for decades to avoid.

The Delaware River provides drinking water for some 14 million people, including most of Philadelphia but also New Jersey and New York. Still, the line is south of those intakes and below the level it traveled in the 1960s during record drought conditions.

Desalination of saltwater is costly, energy intensive and can create new issues like where to dispose of the highly concentrated salt brine pulled from the water. It is also not a feasible option, officials say.

“There are alternative sources, but we don’t want to be trucking in bottled water for people,” said Amy Shallcross, the water resource operations manager at the Delaware River basin commission. “We get nervous when it starts to get up near Philadelphia. It’s only 18 miles right now from the drinking water intakes. And sometimes it can shoot upstream really quickly.”

Officials control the salt line by releasing water from two reservoirs, which pushes the front downriver. The commission monitors the flow at Trenton, which is the furthest upstream point affected by the tide. The flow officials target is roughly equivalent to the amount of water in two Olympic-sized swimming pools flowing by per minute. If the rate dips below that, then more water is released.

The salt front last reached roughly where it is now in 2016 during another drought, officials said.

The Delaware River basin is not alone in fending off intruding saltwater, which is exacerbated by rising sea levels and dredged riverbeds to aid navigation, Shallcross said. The Mississippi River similarly saw what officials call a “salt wedge” in 2023, resulting in heightening underwater levees and bringing in drinking water.

A rainless start to fall brought on a drought in parts of the north-east, including the Delaware’s basin. The river needs about an inch of rain a week for a time to move the line back to its normal location, Shallcross said.

The basin commission is studying the impact of the climate crisis on water resources.

Water managers are starting to consider more serious conservation measures as well. “I would say the east is not water-rich, we’re water adequate, and we need to recognize that,” she said.

Cover photo: By The Guardian