When President Biden issued a federal emergency for New Orleans’s saltwater intrusion threat in late September, it signaled a wake-up call for the rest of the coastal United States. People living closest to the Gulf of Mexico near the Mississippi River Delta lost access to safe drinking water as early as June, prompting officials to closely track the saltwater’s encroachment upstream. 

It’s no light concern. Saltwater can corrode the city’s lead pipes and pose health concerns for more than 1.2 million residents. But thanks to an underwater levee called a sill that helped block saltwater—and some much needed rainfall in the Red River upstream—saltwater isn’t expected to reach New Orleans until the end of November at worst, giving authorities more time to implement emergency measures. 

Although the city is safe for now, saltwater intrusion remains a looming concern for New Orleans and low-lying coastal towns across the country. As the effects of climate change—such as drought and sea level rise—increasingly compound, intrusion threatens more than just freshwater in rivers like the Mississippi: groundwater flooding, storm surge and salt contamination in aquifers are all ongoing concerns in other coastal regions. 

How it happened

Drought across the Midwest over the past year has led to lower-than-average water levels in the Mississippi River, which empties into the Gulf of Mexico just south of New Orleans. When water levels are high enough, freshwater flows out of the river and into the Gulf, pushing saltwater away and preventing it from creeping upstream. But low river levels decrease the amount of freshwater flowing outward toward the ocean, allowing saltwater to creep upstream. (That’s the “saltwater wedge” you may have heard about.) 

Scientists have also pointed to Louisiana’s rapid sea level rise, now at about one inch per year, as another key factor. Combined, drought and sea level rise have put low-lying New Orleans at risk of saltwater intrusion into the city’s main water supply. It works like a seesaw, explains Jeeban Panthi, a postdoctoral fellow and hydrologist at Kansas State University: As drought worsens and groundwater levels decrease in tandem with rising sea levels, saltwater intrusion becomes more likely in low-lying areas.

These symptoms of climate change aren’t new. The Mississippi recorded its lowest-ever water levels last year, and sea level in the Gulf has been steadily increasing since the mid-20th century. That’s why the Army Corps of Engineers began construction of its underwater levee over a year ago in preparation for this summer’s extra-dry conditions. 

But New Orleans is far from unique in its situation. Studies have highlighted other dire examples of sea level rise—worse than what was previously predicted—in Miami and Houston, among other low-lying coastal cities along the East Coast, Gulf of Mexico, and southwest California. These hotspots have long grappled with the ongoing threat of saltwater intrusion as sea levels rise, coastlines erode, and temperatures skyrocket. Coastal erosion exacerbates the effects of sea level rise, and higher temperatures increase freshwater evaporation—all factors that accelerate freshwater loss and saltwater encroachment on land. Experts warn it’s bound to get worse if cities don’t prepare. 

The scope of the problem

There are two main ways saltwater intrusion occurs, explains Paul Barlow, a longtime hydrologist with the United States Geological Survey. One way is via large-scale drainage, such as in South Florida, where water was diverted from the Everglades into a string of canals starting in the early 20th century. This disrupted the natural eastward flow of water to the Atlantic Ocean, causing a widespread decline in freshwater levels. The second way is by intense groundwater pumping, allowing for what scientists call lateral intrusion from the coast: Saltwater infiltrates the freshwater aquifers underground, contaminating the most common source of drinking water in coastal towns. In Maine and other parts of the northeast, decades of well-water pumping have led to conditions that invite saltwater inland.

These incidents of saltwater intrusion are no surprise. Some of the earliest North American cases date as far back as the 1850s. One such example was in the 1960s, when drought in the Delaware River basin near Camden, New Jersey led to conditions similar to the recent intrusion in New Orleans. This would seem unexpected: Camden sits just across the river from Philadelphia about 102 miles inland from the mouth of Delaware Bay, the nearest source of seawater. But as the water table fell from drought, less freshwater flowed toward the coast, Barlow explained, leading to higher levels of chloride—an element in salt that can be unhealthy if there’s too much of it. Drought coupled with intense drainage similarly exacerbated saltwater intrusion into the Biscayne Aquifer in Miami in the 1930s and 1940s, spurring a decades-long struggle with water quality issues that the Army Corps and local water management entities are still grappling with today. These examples have long been in the scientific literature, including Barlow’s report from 20 years ago, and yet these conditions still threaten coastal regions.

As climate change alters coastlines, towns in those regions are growing more concerned about the possibility of saltwater intrusion from multiple avenues—not just laterally from sea to river, as in Louisiana, but also below from groundwater and above from extreme rainfall.

As the sea rises, it brings the groundwater table in coastal areas up with it, increasing the risk of water welling up and submerging septic tanks and pipelines. This is called groundwater flooding. It’s especially a problem in New York City and Long Island Sound, where water tables are high and rainfall is becoming more frequent and intense from climate change. Heavy groundwater pumping makes things worse: Freshwater is pulled toward pumps instead of pushing seaward against the denser saltwater—the same type of wedge in New Orleans. This gives saltwater a way into rivers and aquifers. “The concern is that with sea level rise, the saltwater will have an opportunity to move landward,” Barlow said. These are examples of what Panthi calls diffused events, like a “slow poison” over time. 

Acute events, such as hurricanes and tsunamis, pose more sudden threats, inundating land with saltwater that infiltrates the soil from the top down into the water table—and these events are expected to become more globally frequent. Areas with sandy or silty soils, such as Florida, are especially prone to this type of saltwater intrusion because water can easily and quickly trickle downward. 

Frail infrastructure and health concerns

As climate change continues to exacerbate the risk of intrusion, communities could lose access to potable water. Saltwater can also damage water-related infrastructure—such as pipes and service lines—beyond repair, creating the possibility for health crises in the future. 

According to a 2019 study, ingesting water with high salt content was linked to an increase in hospital visits for cardiovascular and gastrointestinal issues in Bangladesh. The study revealed that drinking saltier water is associated with higher blood pressure, which can increase the risk of cardiovascular disease. Drinking this water can also lead to diarrhea and gastrointestinal distress, especially in infants and children, leading to a higher risk of dehydration. 

Water with a higher salt content is also more corrosive, meaning it has the ability to break off electrons from the atoms in the materials used to make water lines and pipes. In some cities, this means iron and polyvinyl chloride (PVC) could end up in drinking water. But in cities with older infrastructure, saltwater intrusion can lead to lead contamination. Despite scientific evidence about the harmfulness of ingesting lead mounting steadily throughout the 20th century, cities across the US continued installing lead pipes and plumbing materials until Congress banned their introduction in 1996. Today, millions of those lead pipes and service lines are still in use, including in New Orleans.

Residents might not even be aware they have lead service lines unless they’ve had their water tested. Had this month’s saltwater wedge reached New Orleans’s drinking water supply, residents could have been unknowingly exposed to these elevated lead levels. 

The city doesn’t currently have a map outlining all of its lead service lines, but it is working with water company BlueConduit to document all lead pipes in New Orleans for the first time by October 2024. The map may allow a broader swath of residents to become aware of their risk, and in the years to come will allow officials to figure out a plan of action for replacing lead infrastructure. But until those replacements are complete, another saltwater intrusion event could make way for lead to leach into drinking water. 

The EPA, World Health Organization and other agencies agree there is no safe level of lead exposure, especially for children. Lead exposure is linked to brain damage and can negatively impact normal growth and development in children. 

High levels of chloride from saltwater can also interact with the water treatment process itself. Municipal water systems often use chlorine to disinfect drinking water, which is safe when the untreated water has low salinity. But even a small proportion of seawater in groundwater—just 2 percent—can interact with chlorine to produce harmful disinfection byproducts that researchers link to adverse pregnancy outcomes and cancer with large and long-term exposures. So far, health officials have not said that disinfection byproducts are an urgent concern for residents of New Orleans. 

What’s next for New Orleans?

While immediate threats to major water treatment facilities upstream of the sill are put to rest, the Army Corps and New Orleans officials continue to monitor water quality along the Mississippi River. Officials expect saltwater to reach Belle Chasse, a town just south of New Orleans proper, by late November. Several last-ditch efforts to secure freshwater for residents are in the works: In Louisiana’s southernmost parish, where the Mississippi River flows into the Gulf of Mexico, officials are completing reverse osmosis units at several water treatment plants to remove salt that has infiltrated drinking water since June. New Orleans and neighboring Jefferson Parish, which extends into the Gulf of Mexico, are also building emergency pipelines to deliver freshwater from farther upstream, each of which has cost millions of dollars to construct and will require millions more to maintain. 

These retroactive solutions are expensive and fail to address the problem in the long term. A more robust saltwater intrusion monitoring network across the US is necessary to identify hotspots and allocate resources, argues Panthi, the hydrology postdoctoral fellow. Then, cities can develop mitigation tactics, such as building underground seawalls and identifying key aquifer recharge points. But first, monitoring wells need to be streamlined, he said. Currently, some wells collect data every 15 minutes and others only once every five years, creating an inconsistent dataset. 

Another long-term management solution is to pump groundwater seasonally, Panthi said. In winter and spring, places like New Orleans could intensify pumping toward the coast, but in the summer and fall, they should pump inland. “It’s a big question for planners,” he said. “Can we make a schedule of where to pump and how much to pump from these aquifers?” 

Growing populations in coastal areas that are already dense presents another major challenge for future saltwater intrusion prevention. “We need water to feed these growing populations,” Panthi said. “We need to plan in such a way that we can optimize pumping.”