This story is a collaboration among Sierra, WWNO, and the Mississippi River Basin Ag & Water Desk, an independent reporting network based at the University of Missouri.

Just off the Mississippi River in southeast Louisiana, an 18-wheeler trailer filled with white cylinders whirred as workers prepared to turn the machine off for the day. The truck houses a reverse osmosis system, and it’s stopping salt water from contaminating drinking water for more than 20,000 people. 

This is the third year in a row that salt water from the Gulf of Mexico has crept up the Mississippi River. This phenomenon, known as saltwater intrusion, occurs when the rate of flow in the river dips below 300,000 cubic feet per second, leaving the river’s fresh water unable to push back against salt water at its mouth. Previously, this only happened about once per decade. 

For cities within 100 miles of the river’s southern terminus—most of whom depend on the river’s fresh water for their drinking water—saltwater intrusion is an existential threat. Last year, the saltwater wedge almost reached as far as New Orleans, 80 miles upriver, causing panic until rains pushed it downriver.

But it did reach Plaquemines Parish, which surrounds the last 70 miles of the Mississippi.

“When you go across the ferry and you look out at the water, when it's blue-green-type water, I can guarantee you Boothville and the lower end of our parish is salt water,” said Keith Hinkley, the president of Plaquemines Parish. 

In 2023, the presence of salt water that far up the river affected the drinking water for months, forcing residents to buy bottled water and replace appliances damaged by the salinity. Even after the water became safe, residents were still afraid to drink from their tap. Consuming salt water can cause dehydration. The chloride could have also damaged outdated pipes, releasing lead and other contaminants into the water. 

“If you'd have been around us last year, it was brutal. It was bad. Our residents were suffering because of it,” said Hinkley. “And this year, our goal was to not let them even feel the impact of the salt water.” 

For now, the effective solutions are the water treatment upgrades and the salt water sill—a temporary underwater structure, made of material dredged from a sandbar in the river, that’s currently blocking the saltwater wedge from moving much farther. “Without the sill, the wedge would travel maybe at least 20 miles, if not more, upriver,” said Ehab Meselhe, a professor of River-Coastal Science at Tulane University. It wasn’t until this week that the wedge began to retreat downstream from the sill since it started moving upstream in September.

Yet as Plaquemines Parish grapples with threats to their water supply for the third consecutive year, many of the region’s leading hydrologists say the Mississippi River Valley needs more than just a temporary fix for this issue. When the salt wedge appears, the only way to push it back out into the Gulf of Mexico is either a shift in long-term weather patterns or a comprehensive infrastructure solution at the river’s southernmost point.

“The scales on which [the wedge] operates are so large … that I think, at least for me personally, there's a feeling of almost helplessness,” said Paul Miller, an assistant professor of coastal meteorology at Louisiana State University. “There are some solutions, but a lot of times they almost feel like Band-Aids. It’s just buying yourself time.”

The atmosphere’s “thirst for moisture” 

When the wedge first appeared in 2022 and 2023, most experts pointed to an ongoing drought in the Midwest as the culprit, observing how low rainfall in the Mississippi River Basin and its tributaries led to low flow throughout the river. The basin is the largest in the United States, supplying the 10 states it traverses with crucial drinking water, irrigation, and income from the river’s $12 billion shipping industry. But a consensus is emerging among researchers who have been studying the phenomenon over the past three years that saltwater intrusion is tied not just to drought but also to more complex multiyear weather patterns.

Miller researches how the saltwater wedge is affected by evaporative demand—a metric he described as the atmosphere’s “thirst for moisture.” By analyzing factors such as temperature, humidity, and wind speed, meteorologists can pick up on early signs of drought by gauging how much water the earth’s surface is likely to lose to evaporation. 

Just before the appearance of the saltwater wedge in 2023, Miller said, his team found that evaporative demand throughout the Mississippi River Basin was reaching 40-year highs. Historically, periods of high evaporative demand have lined up almost exactly with periods when the saltwater wedge appeared, he found.

“We saw evidence that there are these years-long cycles,” Miller said. “We're in a saltwater-wedge-favorable regime right now.”

Rainfall can relieve low flow, but recent extreme weather patterns have shown that this isn’t always the case for the Mississippi. In October, Hurricane Helene devastated western North Carolina and eastern Tennessee, bringing an estimated 40 million gallons of rainfall. When the runoff emptied into the Ohio River and made its way to the Mississippi, it only boosted the river’s flow for a few weeks before slipping back down to critical levels.

“When you get these big rain events, the soil can only absorb so much at one time, so you get a lot more runoff,” said Matthew Wilson, a hydrologist at the National Weather Service’s Lower Mississippi Forecast Center. “The rivers get higher faster, but they don't stay high—their base flow isn’t lifted.”

What typically keeps the river high is “effective rain,” said Wilson. This type of rain soaks into groundwater beneath the soil, building a natural supply of water that filters into the river over a longer period of time. But extreme rainstorms instead create much more runoff—excess water that flows into the river from the surface. Runoff can boost flows in the short term, but because it doesn’t replenish groundwater reserves that keep the river consistently high, it’s rarely a relief to chronic low flow. Wilson said the latter could become more common as global temperatures continue to rise, bringing more extreme weather.

“A decade or so ago, you would've had three to five rain events each month that had two or three inches over the events,” he said. “Now, you're getting two or three events that'll have five to seven inches of rain in the event.” 

So while most places in the basin aren’t necessarily seeing a rainfall deficit across the year, Wilson said, these major rainstorms only temporarily boost the river’s flow. And even if a series of rainstorms did increase flow farther upstream, Miller said, it “really wouldn't move the needle on discharge down in south Louisiana for at least two months.”

The same is true with releasing water from upstream reservoirs. North of Louisiana, the US Army Corps of Engineers manages localized flow with a network of dams and reservoirs along the Mississippi and its tributaries. But the amount of water in these reservoirs isn’t enough to have a major impact on flow thousands of miles south. 

Hurricanes that hit Louisiana with massive rainfall directly aren’t a source of relief either. Flow levels at the mouth of the Mississippi aren’t controlled by local rainfall but by the amount of water that enters from the Ohio River, the Missouri River, and others farther north. Until conditions change upstream, Louisiana could be left to tackle local solutions to the salt wedge on its own, since hydrologists working in those tributaries are prioritizing managing low flow issues they too face regionally. 

“How much water is flowing past New Orleans, I don't think that ever comes into their calculations,” said Chris Dalbom, who heads the Tulane Institute on Water Resources Law and Policy. “And it would be really hard to ask them to do that.”

The cost of salt water

Over the past three years, the saltwater sill has been southeast Louisiana’s main line of defense against the salt wedge. The structure essentially acts as a temporary underwater dam, blocking heavier saltwater molecules from traveling past it while allowing fresh water to flow over it. The Army Corps funds and maintains the sill, using 28-day forecasts of projected rainfall and river flow to determine when it needs to be built and when it can be taken down. In recent years, the wedge has typically appeared around August or September and retreated by November, although the 2023 saltwater wedge didn’t retreat until January 2024.  

The sill isn’t without its problems. Each year, it’s built around 64 miles upstream from the mouth of the river—usually far enough south to block salt water from reaching New Orleans, but too far upriver to save Plaquemines’s drinking water. According to Army Corps spokesperson Matt Roe, building the sill closer to the mouth of the river would require the Army Corps to build the sill more frequently, even in years when saltwater intrusion doesn’t pose a viable threat to Plaquemines.

In 2022, the Army Corps spent just over $5 million building the sill. In 2023, that cost surpassed $20 million, mostly due to the Army Corps heightening the existing sill a few months after its construction. “If you're having to redo it every year, that cost could end up adding up and being more than if somebody were to spend more money now for a situation that would have longer-lasting effects,” Dalbom said.

What remains unclear is what that longer-lasting solution could be. Meselhe, who has worked with the Army Corps to combat saltwater intrusion, said he’s been looking into ways to help make the river’s flow more efficient. In order to maintain high enough flow for the river to push the saltwater wedge back out into the Gulf of Mexico, it’s crucial to keep as much water in the river as possible, building up freshwater force. 

One point where he found the river lost a lot of its momentum was in crevasses—breaks in New Orleans’s levees where the river forced through and deposited sediment in nearby bays. “All these crevasses made the river much weaker and unable to push that salt water out back toward the Gulf,” Meselhe said.

While fully blocking these crevasses isn’t feasible, Meselhe is looking into potentially putting sand at their entrances during times of low flow. “That will keep the fresh water in the river, allowing the river to push against the salt water,” he said. “Once high water comes in, it will wash out that sand and build land with it in the outfall areas.” 

Ultimately, though, the responsibility for providing clean water and curbing contamination falls on local water utilities, said Dalbom.

Plaquemines Parish installed reverse osmosis systems at their Port Sulphur, Pointe à la Hache, and Boothville water treatment facilities almost as soon as it became clear another wedge was coming toward them in September. That’s because after last year, when the parish had to barge in millions of gallons of water, former Louisiana governor John Bel Edwards pushed for a better solution, said Hinkley. 

Plaquemines Parish is using this reverse osmosis system, housed inside a semitrailer, to keep salt water out of residents' drinking water. | Photo by Eva Tesfaye/WWNO

The reverse osmosis systems are not a simple solution, while slightly more long term. They require a lot of electricity, so the facilities needed upgrades. Micro-filtration systems also had to be installed to clean the muddy river water.

“The river, it's got a lot of sand, a lot of mud, just a lot that goes through there, so this water has to be filtered first before we can even bring it through the reverse osmosis,” said Hinkley. 

The systems are effective. Sitting in a conference room with a map of the river and the treatment plants behind him, Hinkley ran his finger over the numbers that show the presence of salt as milligrams of chloride per liter of water. 

Before hitting the reverse osmosis systems, the Mississippi's water in the Boothville treatment plant contains 570 milligrams of chloride. After reverse osmosis, the water contains just 80. The advisory to avoid unsafe drinking water kicks in at 250; this year the parish has managed to avoid issuing such an advisory.

But whether Plaquemines, a parish with a population of only about 23,000, can continue to carry the cost of treating its drinking water every time the saltwater wedge returns remains uncertain. The total cost of the reverse osmosis systems for all three plants is between $850,000 and $1 million per month. 

“These are small towns, small populations,” Dalbom said of the communities along the mouth of the river. “They don't have exactly a huge ratepayer base, and so that investment by public utilities down there to afford saltwater removal on a permanent basis, I'm almost positive, is beyond their capacity.”

The parish has received money from both the state and the Federal Emergency Management Agency to help pay for the costs of saltwater intrusion. But Hinkley said long term, these facilities will need upgrades that will cost the parish millions more.  

“We're searching for it all over the place: the Louisiana Department of Health, the federal government, state government,” he said. 

The state government has stepped in to help local utilities in times of crisis before. In the aftermath of Hurricane Katrina, the state opted to form the Southeast Louisiana Flood Protection Authority, combining each levee district’s individual flood authorities into just two regional arms. Dalbom envisions a similar solution for the saltwater wedge: regionalizing southeast Louisiana’s water utilities so that communities can collaborate on long-term solutions to water supply issues.

“Whether or not they have the capacity to actually engage with each other on these things is another question,” Dalbom said. “It might be the kind of thing that needs to happen at the state level, where the legislature, at the very least, passes a resolution pledging support for these utilities to work together.”

For Plaquemines residents, the hope is that greater regional unity could bring greater equity. Hinkley said he’s noticed there’s much more alarm and action around the saltwater wedge when it travels far enough upriver to threaten Jefferson Parish and New Orleans. He wants to see this same action when the wedge contaminates water supplies farther south.

“We're not neglected. Just the sense of urgency doesn't happen when it's just us,” he said. “We get our lion's share, but we get our lion's share when they're affected.”