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Saltwater wedge moving slower than projected; unlikely to reach New Orleans until late November

In this aerial photo, dredging operations to build an underwater sill are seen in Plaquemines Parish, La., Tuesday, Sept. 26, 2023. A salt water wedge slowly moving upriver from the Gulf of Mexico, due to the unusually low water level in the river, may threaten municipal water supplies, potentially even New Orleans. (AP Photo/Gerald Herbert)
Gerald Herbert
/
AP
In this aerial photo, dredging operations to build an underwater sill are seen in Plaquemines Parish, La. on Sept. 26, 2023. A salt water wedge slowly moving upriver from the Gulf of Mexico, due to the unusually low water level in the river, may threaten municipal water supplies, potentially even New Orleans.

Salt water from the Gulf of Mexico is moving up the Mississippi River much more slowly than initially expected.

According to the U.S. Army Corps of Engineers’ latest projections, released Thursday, the “saltwater wedge” isn’t forecast to reach New Orleans’ west bank until late November — a month later than earlier projections suggested. That’s due, in part, to a 120-foot-deep drop-off in the bottom of the river south of the naval air station where the salt water has sat at a standstill for more than a week, officials said.

The wedge is made up of a dense strip of ocean water that sinks beneath the fresh water of the river when less water is flowing down the Mississippi. This happens because salt water is denser than fresh water — and because the bottom of the Mississippi River is below sea level throughout the entire length of Louisiana.

U.S. Army Corps of Engineers Col. Cullen Jones, who commands the New Orleans District, provides an update on his office's measures to address a "saltwater wedge" moving up a historically low Mississippi River during a news conference on Sept. 15, 2023.
Halle Parker
/
WWNO
U.S. Army Corps of Engineers Col. Cullen Jones, who commands the New Orleans District, provides an update on his office's measures to address a "saltwater wedge" moving up a historically low Mississippi River during a news conference on Sept. 15, 2023.

Back-to-back years of drought throughout the Mississippi River valley have led to salt water encroaching on the drinking water supplies of some southeastern Louisiana communities for two straight years. Typically, that natural phenomenon happens about once a decade.

The wedge stagnated, too, because more water than expected has been flowing down the river. The combination of factors has bought the region more time before salt water starts affecting drinking water for more people, said Col. Cullen Jones, who commands the Corps’ New Orleans District.

Though his agency is still building up an underwater barrier, or sill, Jones said the project has already contributed to the wedge’s sluggish pace. The barrier stretches across the width of the river’s bottom about 13 miles south of Belle Chasse, blocking some of the salt water behind it, and weakening the wedge.

“When that salt water goes over top of that sill, you're only getting the top portion, which has less salinity and density, which makes it very susceptible to those higher (Mississippi River) flows,” Jones said during a news conference Thursday. “We're getting an additional speed bump there.”

Once construction is finished, the sill will sit about 30 feet below the river’s surface except for a 625-foot-wide notch that will remain 55 feet deep to allow ships to pass. In that location, the river is about 2,700 feet wide.

Without more rainfall across the Mississippi River basin, Jones said the salt water could still affect the Belle Chasse water plant by the end of next week, and reach St. Bernard Parish by the end of the month.

U.S. Army Corps of Engineers

But the region has already begun preparing for that, now ready to ship in fresh water from higher upstream to dilute the river water before treating it should it turn briny. They’re also planning to deploy technology that separates salt from the water. Together, Jones said, that should be enough to keep the salinity level low enough to be safe to drink – avoiding any disruption to drinking water service in either parish.

“At the end of the day, it's going to be able to maintain water supplies for the population,” Jones said.

Water utilities issue health advisories when the salinity reaches 250 parts per million. That’s when water starts to taste salty. In New Orleans, water typically comes out of the tap with a salinity ranging from 40 to 80 ppm, according to the city’s Sewerage and Water Board.

The lower end of Plaquemines Parish went without drinking water for three months before the parish’s water treatment plant in Port Sulphur went back online, boosting the capacity to send fresher water into the system. That plant has also received its first shipment of barged-in water, plus some reverse-osmosis purification units.

At its peak, the salt level in residents’ water exceeded 1,600 ppm — more than six times the amount the Environmental Protection Agency deems healthy. In the past two weeks, the salinity has dropped below 250 ppm, though a health advisory is still in place as the salt is flushed from the system.

In Thursday’s updated projection, the larger water treatment plants in Jefferson Parish and New Orleans aren’t expected to see any impact from the saltwater intrusion through at least November.

Leaders from both parishes had already begun rapidly planning new pipelines that will port fresh water from upriver to those treatment plants when the threat to drinking water seemed imminent. On Wednesday, Jefferson Parish started construction of a temporary, 15-mile-long pipeline after receiving approval from the Corps.

Parish president Cynthia Lee Sheng explained that the plan is to lay out the first seven miles of the pipeline and test the system to ensure it works. The flexible, “lay-flat” piping would bring millions of gallons each day to water treatment plants on the east and west bank of the river if completed.

Jefferson’s contractors will prepare the next eight-mile length of pipe and have it ready to deploy, but will wait to connect it while they monitor the progress of the saltwater wedge.

“That, we think, is the most prudent way to handle this, the most scalable way to handle this, and the way to position ourselves so [in the] very very worst-case scenario, we’re not caught off guard,” said Cheng.

In New Orleans, the city’s sewerage and water board secured a contractor earlier this week to lay a 11-mile-long, 48-inch PVC pipeline. The agency’s interim general superintendent, Ron Spooner, said the pipeline can be built in 30 days, if needed, but didn’t say if — or when — the city might begin construction.

Spooner said he’s focused on ensuring the city’s Algiers plant is prepared for the possibility of salt water reaching that far north by Nov. 23, as the new projections predict.

Jones said the Corps will update its projections weekly, based on forecast data from the National Weather Service. As of this week, its meteorologists only anticipate two days of rainfall in October, and no rain in November.

“So if we get better than two days of rain in October, and we see rain in November, these projections will improve,” Jones said.

Though the current projection appears less dire, officials across the region said they were still committed to finding long-term solutions to preventing salt water from disrupting water supplies.

Halle Parker reports on the environment for WWNO's Coastal Desk. You can reach her at hparker@wwno.org.
Aubri Juhasz covers K-12 education, focusing on charter schools, education funding, and other statewide issues. She also helps edit the station’s news coverage.

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