Special Series
This is Part 3 of a three-part series “Salt of the Earth” that examines the creeping threat of saltwater on the health of Delaware. Read Part 1 here. Read Part 2 here.
Why Should Delaware Care?
Saltwater is moving farther north and inland, and while that may mean more immediate impacts for habitats, farmers, and drinking water resources along Delaware’s more southern coasts, it also poses a potential risk for northern Delaware where about 75% of people rely on surface waters for clean drinking water. Research has also shown that contaminated sites may face risk of spreading pollution if they’re impacted by salty floodwaters.
If saltwater reaches hidden stores of arsenic or other chemicals found at polluted sites along northern Delaware’s waterways, it’s possible that a chemical reaction could cause those contaminants to mobilize in the water.
And anyone who’s aware of Delaware’s industrial roots knows that there’s no lack of contaminated sites along New Castle County’s waterways.
“Decades ago, there were no regulations on how you dispose of these things,” said Holly Michael, director of the Delaware Environmental Institute and a professor at the University of Delaware who has been studying saltwater intrusion threats statewide for years.
Michael has co-authored research exploring what would happen when rising water levels reach toxic sites or other places it hasn’t reached before.
“With rising sea level, the groundwater levels are rising, too,” she noted.
Those increasingly wet conditions, combined with salt, can create havoc for habitats as well as key infrastructure like drinking water facilities or industrial piping.
For substances that are bound to soil sediments, like arsenic, chemical reactions that occur when saltwater is mixed in can dislocate those toxins and allow them to travel wherever the water goes. While scientists have flagged the threat, what it means locally for the environment and public health remains to be seen.
The basic chemistry at hand is similar to what went wrong when public officials decided to cut costs in Flint, Michigan, and switched water supplies without understanding that a basic chemical reaction would cause lead to leach into the public drinking water supply: It’s all about corrosion.
While it’s clear there will be contaminated sites facing the risk of pollutants mobilizing as sea level rises, it’s unclear what Delaware’s environmental regulators are doing about the potential threats. The state Department of Natural Resources and Environmental Control declined multiple interview requests to discuss the potential scenario for this story.
“[Our expert] wasn’t familiar with any New Castle County work by DNREC on projects that could be impacted by saltwater intrusion,” spokesman Michael Globetti said in an email.
Globetti confirmed that there is at least one private property contaminated by arsenic that has been studied — published research shows DNREC’s remediation section, which manages contaminated sites, helped facilitate sampling for the project. Globetti also noted another nearby Superfund site at the Port of Wilmington as a contaminated site that’s under federal remediation and monitoring.
He did not respond to additional requests for clarification and more information.
UD’s Michael said that while some arsenic is already seeping from that privately owned property, it does not pose any immediate threat. But her work and other studies indicate there may be risks in the future as sea level continues to rise.
“Once it happens, it’s a big problem,” Michael said of saltwater intrusion threats like this, particularly one so toxic. “Until it happens, you don’t notice.”
The risks we know
One of the known sites threatening arsenic contamination in the Christina River as water levels rise is located on private property. But it likely isn’t the only one, Michael noted.
New Castle County is home to a number of significantly contaminated sites known as “Superfund” sites, which require federal oversight and remediation, as well as current industrial operations that handle hazardous chemicals and materials.
“There’s so much legacy contamination in New Castle County along the rivers that if you get an increase in salinity in those tidal rivers, there is a lot that can happen in terms of mobilizing those contaminants,” Michael said.
Arsenic may have been the poison of choice in the 19th century, but it’s a naturally occurring element that is also widely used in industrial processes. It is a toxic substance, and people are most often exposed to it through contaminated drinking water, according to the National Institute of Environmental Health Sciences.
Just as there are unknowns about how and where arsenic might mobilize, it’s also unclear how the more corrosive nature of saltwater might impact other industrial operations along the Delaware River, such as the water intakes at facilities like the Delaware City refinery or the Salem nuclear power plant in New Jersey.
“I imagine corrosion could be a major issue,” depending on the type of material those pipes and processing facilities rely on, Michael said. Plastic PVC pipe, for example, would likely be unaffected while something made of concrete or certain metals might suffer from corrosion.
“It’s a problem even at really, really low levels of salt,” she said, specifically noting impacts to concrete that was built for a freshwater environment that then becomes salty. “It could cause problems, too, with water treatment.”
If salty water reaches drinking water systems, that could cause even more problems than corroded intake pipes. Removing salt from water, a process known as desalination, is an expensive and challenging process — one that no municipality in Delaware is currently equipped to do.
Unlike in Kent and Sussex counties, where most residents rely on wells from layered underground aquifers to provide water for drinking, irrigation, and other uses, New Castle County’s major population centers rely on surface water for drinking. In southern Delaware, wells are simply relocated when salt gets too close.
In New Castle County, where 75% of residents rely on surface waters like the Brandywine, White Clay and Red Clay creeks for drinking water, switching sources may not be so easy. There’s a 7-square-mile area near New Castle that’s now listed as a Superfund site because of high levels of contamination from toxic and carcinogenic “forever” chemicals found in the groundwater.
As for the other contaminated property pointed to by DNREC, the former Halby Chemical Co. site at the Port of Wilmington, pollution there has been remediated and capped and the U.S. Environmental Protection Agency has prohibited the use of any groundwater. Elevated levels of arsenic, as well as over a dozen other metals and chemicals, were found at the site decades ago. Today, it’s basically a paved lot at the port, with any remaining contamination sealed beneath the surface.
Upstream solutions
While many people may think of saltier waters much farther south, there’s actually a mixing zone in the lower Delaware River that teeters around Claymont. This salt line, or salt wedge, is closely monitored by the Delaware River Basin Commission, a multistate regulatory agency that oversees management of the Delaware River’s natural resources.
This area of mixing between freshwater coming down from the northern reaches of the Delaware River and the saltier water moving in from the Delaware Bay creates the salt front. This is how the DRBC monitors “salinity intrusion” — mainly the risk of saltwater reaching the City of Philadelphia’s drinking water intakes. That was a hot topic throughout 2024 during an intense drought, and while the buzz has subsided, the challenges still remain well toward the end of 2025.
“We’re starting to get pretty concerned,” DRBC Manager of Water Resource Operations Amy Shallcross said. “But our drought management program works really well … to bring freshwater flows downstream.”
When the salt line creeps past its usual spot around Claymont, the DRBC has a management plan that can help keep the salty water at bay. That can include releasing freshwater from upstream reservoirs as needed.
“It’s not unusual, but it’s not frequent either,” Shallcross said.
However, she noted, the salt line has reached the most upstream place it’s been since the 1960s at least three times in the last decade. Philly seems far upstream, but the Delaware River is connected to the Delaware Bay, which is connected to the Atlantic Ocean. And the dredged channel that allows ship traffic to move up that maritime corridor is deeper, meaning the denser saltwater can find another way to wedge itself farther upstream than what may be realized at the surface.
That’s why the salt front the DRBC measures isn’t a static point — it’s actually a seven-day average of the salting mixing zone’s location.
Whether similar management could be needed in First State waterways is unclear. The northern Delaware water provider Veolia operates an inflatable dam, known as a “tidal containment system,” on the White Clay Creek, deploying it when the stream gets low or when the salt level in the Delaware River creeps too high.
It’s unclear how often that emergency system is needed.
“In terms of Delaware, I think the real area of concern is that intake on the Christina River,” DRBC’s Shallcross said. The DRBC doesn’t measure the salt front there, she said, it’s the local authorities that track salinity by measuring chloride levels at a static location.
“There’s a lot of complexity to the issue,” she said.
Long-term management needed
A key point of those arsenic and sea level rise studies at the University of Delaware was to help prepare people for potential solutions.
“Our results demonstrate the threat of sea level rise stands to impact arsenic release from contaminated coastal soils by changing redox conditions,” another study’s authors note, pointing to the chemical reaction caused in part by saltier waters.
But when UD’s Michael first started proposing studies on saltwater intrusion, particularly in more southern parts of the state, she said colleagues said saltwater intrusion wasn’t a problem. But now, as the threat has become visible — ghost forests along coastal waterways in Kent and Sussex counties show the creep of saltwater — salt is moving into places where it never had been seen before.
“As those effects go farther inland, it’s happening in more areas,” Michael said. “I think it’s creeped high enough that it’s just more and more into where we are.”
Even the DRBC, which has been managing the salt front for decades, is still studying future projected impacts of sea level rise and looking for alternatives when today’s solutions dry up. In 2023, the agency issued a report on some back-up options to store freshwater, if the need ever arises.
“Long-term problems, you can anticipate,” Shallcross said, noting that sea level rise is one of those examples. “But you can only prepare so much.”