Saltwater is again intruding up the Mississippi River from the Gulf of Mexico, the result of a reduced flow of freshwater from the Midwest because of a lack of rainfall, posing a possible threat in the weeks ahead to Plaquemines Parish’s southernmost water treatment plants.
“We are closely monitoring the water flow, height and the position of the saltwater wedge,” said Shannta Carter, a spokesperson for the parish. “Currently, none of our intake facilities are affected.”
On Tuesday, the Army Corps of Engineers determined the “toe”, or leading deepwater edge of saltwater in the river, had reached river mile 22.1 above Head of Passes, just below the town of Buras and about 40 miles north of the river’s mouth. That’s still several miles south of the parish’s southernmost water treatment plants and well below its Belle Chasse plant, which supplies much of the water for the parish.
This map shows the estimated location of the leading toe of saltwater traveling from the Gulf of Mexico upriver in the Mississippi River. The …
Saltwater from the Gulf is heavier and more dense than the freshwater moving downriver, so it hugs the bottom until there’s not enough freshwater to keep it from reaching the surface. The movement upriver has been assisted during the past few years by the recent deepening of the river’s navigation channel to 50 feet to allow larger ocean-going vessels to move upriver.
For 209 days between July 2023 and late January 2024, saltwater levels at water treatment plants in the lower river were measuring chloride levels above 250 parts per million, the limit for public water supplies. That prompted an emergency declaration by President Joe Biden and a promise of millions of dollars in aid to support efforts by the Corps to deliver water from upstream by barges for Plaquemines and other localities with high chloride levels in their water, and to help other cities and parishes with other emergency water supply alternatives.
From the Army Corps of Engineers
It’s too soon to determine whether the present reduced river flow will match last year’s unusual event, officials say.
The Corps this week restarted twice-weekly monitoring of the saltwater toe, which stopped in January when freshwater flow in the river was finally rising.
The Corps tracks the potential saltwater intrusion by modeling its movement upstream based on the speed of freshwater flowing at Red River Landing, above Baton Rouge, which has been slowing significantly over the past few weeks.
FEMA will reimburse 75% of Plaquemines costs for using barges to collect and store freshwater during 2023-24 saltwater intrusion event. More reimbursements could be coming for several other parishes.
On Wednesday, the flow rate was 331,000 cubic feet per second, a 9,000 cfs drop from the day before. The Corps has been using a rate of 300,000 cfs at that location as an indicator that saltwater may be threatening water supplies along the lower river.
In New Orleans, the river drop also is being measured by the National Weather Service’s Lower Mississippi River Forecast Center. On Wednesday, the water level at the Carrollton gauge in the river in front of the Corps’ headquarters building was at 4 feet, and was forecast to drop to 2 feet by Sept. 11.
But Corps spokesperson Matt Roe said the official trigger point for the agency’s emergency operations is determined by modeling that predicts the northward movement of the saltwater toe to river mile 65 above Head of Passes, a few miles below Belle Chasse, within 28 days.
At that point, the agency will work with state and local officials to determine which water treatment plants will be affected, and what assistance they might need.
“Based on modeling of the forecast, which only includes the next 48 hours of rainfall, the toe of the saltwater wedge is expected reach around river mile 45 on Sept. 10 if nothing changes in the forecast, so, the toe is not expected to be in a position to trigger emergency operation procedures within the next 28 days,” Roe said.
When the model shows the wedge will reach river mile 80 in 28 days, or is 10 days away from river mile 65, the Corps would begin construction of an underwater earthen sill across the deepest part of the river to block the saltwater from flowing upstream.
This graphic shows how saltwater from the Gulf of Mexico was expected to move upriver in late September 2023, at the height of the low freshwater flows on the Mississippi River.
BY DAN SWENSON | Graphics Editor
Last year, construction began in mid-July on a sill near river mile 64, just north of Myrtle Grove. That initial sill’s top was 55 feet below the water’s surface, allowing ocean-going ships with a draft of 50 feet to still travel upriver. In late September, when the river continued to drop and saltwater topped the sill, the Corps increased its height so it was only 30 feet below the water surface, except for a notch in its center for ship traffic that was 55 feet deep.
Continued drops in the river’s freshwater flow — and movement of the saltwater north of the sill — resulted in the presidential disaster declaration that also helped fund planning for alternative freshwater supplies by St. Bernard and Jefferson parishes and the city of New Orleans. By early October, however, freshwater flow on the river began to increase, and only Plaquemines water plants were affected by the intrusion.
The Corps arranged for freshwater to be shipped in barges to those plants, with additional barges stationed at the plants for storage. Plaquemines also installed temporary “reverse osmosis” treatment equipment to assist in reducing salt in water in its plants.
Plaquemines’ Carter said the parish has already begun planning for the potential of saltwater intrusion.
“During the previous saltwater intrusion, we implemented mitigation measures that enable us to pump water from the northern part of the parish to the southern end if needed,” she said. “We also have tie-in connections with Jefferson and New Orleans. We are in ongoing communication with federal, state, and local partners to ensure the necessary steps are in place should our facilities be impacted. Reverse osmosis is an option, though we have not yet discussed barging, as that decision will be made by the (Corps).”
David Welch, a hydrologist with the National Weather Service’s Lower Mississippi River Forecast Center in Slidell, said “we have been dry on the Ohio River going into the summer and the high flows from the Upper Midwest floods that have sustained the Lower Mississippi through June and July are dropping out now back to seasonal levels.”
Long-term rainfall outlooks are calling for normal to below normal conditions over both the Ohio and upper Mississippi rivers.
“The September-October-November timeframes are typically our lowest flow periods of the year and will warrant monitoring to see whether we approach the critical lows we experienced the last couple years,” Welch said.
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Publish date : 2024-08-14 22:00:00
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