Salt Water Wedge In Retreat

When the New Orleans Engineer District updated its salt water wedge tracker October 9, the leading edge—or toe—of the wedge intruding from the Gulf of Mexico had retreated back downriver an estimated 5.5 miles from its previous location near Mile 69.4 above Head of Passes (AHP). The Corps last surveyed the wedge October 5 and found the toe at Mile 65.3 AHP, and as of October 9, the district now estimates the toe to be at about Mile 63.9.

The positive development follows an October 5 press conference where the New Orleans District announced an update to its projections for when the wedge could reach municipal water intakes in upper Plaquemines, St. Bernard, Orleans and Jefferson parishes. As of that October 5 update, the Corps said chloride levels at the region’s largest water intakes in Orleans and Jefferson parishes would likely not exceed 250 parts per million after all. Lower Plaquemines Parish has been dealing with elevated chloride in the river since June.

Ricky Boyett, public affairs chief for the district, said the improved conditions are due to a number of factors, both weather-related and engineered.

“The reason for the regression is we have higher flows—the Red River is increasing so less water is being diverted at Old River—coupled with better than anticipated performance of the augmented sill design,” Boyett said.

Above the Old River Control Complex, the Corps takes the total flows of the Red and Mississippi rivers and sends 30 percent to the Atchafalaya River and 70 percent down the Mississippi. Higher flows on the Red, Boyett said, means the Mississippi keeps more of its flow.

The Corps began work on a sill, or underwater mound of river sand, near Mile 64 in July, building it to an initial height of -55 feet. Late last month, the Corps and its dredging contractor began raising the sill to a height of -30 feet, with a notch in the middle at -55 feet to allow for deep-draft navigation, in hopes of slowing the upriver movement of salt water. At the same time, rain in the Red River and Mississippi River valleys has led to bumps in both rivers, increasing flows and holding the salt water at bay.

Pushing back the salt water in the river means that Orleans and Jefferson parishes likely will be able to shelve plans to pipe in water from farther upriver. As of October 12, the district estimates salt water could affect water treatment facilities on the following dates: Belle Chasse on October 27, Dalcour on November 1, and St. Bernard on November 8. The district no longer expects chloride levels to exceed 250 ppm at the Algiers, Gretna, West Jefferson, Carrollton and East Jefferson facilities.

LowerPlaquemines Parish, though, continues to battle chrloride levels above 250 ppm..

Boyett said the Corps continues to play a role in that effort.

“We continue to barge water to Port Sulphur and Pointe à la Hache, as well as securing reverse osmosis water purification units for the lower water treatment facilities,” Boyett said.

The issue of salt water intruding into the lower reaches of the Mississippi River is one the Corps has studied and mitigated against. Ahead of deepening the river to 45 feet in the late 1980s, the Corps concluded that deepening the ship channel near the mouth would lead to increased salt water intrusion.

One document on the New Orleans District’s Mississippi River Ship Channel webpage, titled “Saltwater Intrusion Mitigation,” looks closely at salt water intrusion in Plaquemines Parish and establishes the parameters for the Corps barging fresh water to the parish’s water plants.

“Increased durations of saltwater intrusion caused by the 45-foot channel during the years 1989, 1990 and 1991 did not overwhelm the ability of the local parish government to handle the situation,” a comment dated December 10, 1991, reads. “Because of this and the ongoing effort between the New Orleans District and local parish officials to develop a permanent saltwater mitigation plan, Plaquemines Parish elected not to request the Corps for reimbursement at that time.”

Boyett, though, said the advance of salt water farther upriver is more coincidental to ship channel deepening than caused by it.

“Over time, saltwater intrusion has occurred more often than I believe most people realize,” Boyett said.

According to Corps records, the salt water wedge moved near or above Mile 64, the established location of the sill, in the following years: 1939-1941, 1952-1964, 1987-1988, 1999, 2012, 2022 and 2023. The farthest north the salt water has traveled on record was Mile 119 in 1939 and 1940. The Corps estimates the minimum flow in the Mississippi River for those years was 87,500 cubic feet per second (cfs.) and 99,000 cfs, respectively.