Too much saltwater is flowing up the Mississippi River, impacting drinking water in Plaquemines

Due to dry weather causing a lack of freshwater outflow, saltwater from the Gulf of Mexico is encroaching into the Mississippi River — and it’s concerning state and local officials.

Saltwater heading upstream is a naturally occurring and periodic condition that happens about every ten years, according to the U.S. Army Corps of Engineers. In a typical year, the heavy outflow from the Mississippi River is enough to keep saltwater from encroaching.

This condition, called a saltwater “wedge,” is when denser saltwater flows upstream along the bottom of the river under less dense river freshwater.

Saltwater far enough upstream can ultimately affect municipal drinking water and industrial water supplies in Plaquemines Parish and the greater New Orleans area.

NPR reported previously that Plaquemines Parish paid to have water piped in from New Orleans and other municipal systems and also sent barges upriver to gather freshwater back when the saltwater wedge occurred in 2012.

This time around, the parish has secured two reverse osmosis machines that will run at the Boothville and East Pointe à la Hache water facilities to mitigate the issue. This will cost the parish about $40,000 per machine, per month.

“These plants will take out the chloride in the water and still produce approximately 1 million gallons of water for everyday use,” Plaquemines Parish President Kirk Lepine said.

Lepine declared a state of emergency before the press conference Wednesday, and by doing so, hopes to share the cost with state and federal partners.

Plaquemines Parish has also issued a drinking water advisory due to higher levels of sodium and chloride. The parish press release advised residents who have sodium sensitive diets to consult with their health care provider on how much sodium per day is safe to consume.

“We're at the mercy of the river every 10 years — we have to go through this,” Lepine said.

To combat this, USACE Emergency Manager for the New Orleans’ District Heath Jones said that they are installing an underwater sill to slow the wedge enough to keep the salt levels low enough for water treatment and protect the water treatment plants in Belle Chasse and further upriver.

The USACE is building the sill on the bottom of the Mississippi River at about mile 63 above the Head of Passes, which is between Belle Chasse and Myrtle Grove. Jones said they expect construction to begin in about three weeks and be done by early November.

“What we're trying to accomplish this time is get out in front of it a little bit better than we have in the past,” Jones said.

The sill has been constructed three times previously in 1988, 1999 and 2012 and is expected to cost about $10 million to build. Jones said that they will build the barrier in five-foot increments and check salinity levels along the way. It could become a 45-foot-tall sill.

Hurricane Ian’s storm system will not affect the saltwater wedge in any capacity, according to Jones. The storm system is too far east to end up in the Ohio River Basin, where the influx of water then traveling through the Mississippi River would push the saltwater wedge back out to the Gulf.

The problem will be resolved once the dry spell ends and more freshwater makes its way downstream, but it is unknown when this will happen.