Low water in the Mississippi River has allowed a “wedge” of saltwater from the Gulf of Mexico to work its way up to mile marker 43, just above the Plaquemines Parish community of Jesuit Bend, but is not yet considered a threat to New Orleans, St. Bernard or Jefferson water supplies, officials with the Army Corps of Engineers said Thursday. Plaquemines Parish officials have measured elevated salinity levels at water intakes in Boothville and Venice, but the lower end of the parish has access to freshwater from a pipeline from Belle Chasse, said Will Veatch, a corps hydrologist. The pipeline was installed after low river events in 1988 and 1999.
Denser, heavier saltwater flows upriver beneath fresh water flowing downstream when the river’s flow drops below normal. The federal drinking water standard for salt is 250 parts per million, which could be violated if the wedge’s upper level reaches the water intakes.
If officials believe the wedge is four weeks away from fouling the upriver freshwater intakes, the corps will block the saltwater from moving upstream by building an underwater sill of dredged sediment at mile marker 63.7, 31 miles below the Canal Street ferry.
But that’s not a threat until the leading edge of the wedge has moved 15 to 25 miles upstream of the intakes, Veatch said, and he said corps officials still don’t believe that will happen this summer.
The surface of the river was at only 2.5 feet above sea level at the Carrollton Gage in New Orleans on Friday, which was slightly higher than a reading of 2.1 feet over last weekend.
But hydrologists with the Lower Mississippi River Forecast Center, based in the Slidell office of the National Weather Service, predict the water level will drop to 1.8 feet by Aug. 6, based on rainfall to date.
And the weather service’s Climate Prediction Center is forecasting mostly dry weather in the Midwest, upstream of New Orleans, over the next two weeks, which could result in even lower water levels in New Orleans later this summer.
Veatch said the corps’ sill decision will be triggered by the river’s height and speed of its flow at Red River Landing, above Baton Rouge, since tidal flow at the Carrollton Gage complicates its use for long-term estimates of the wedge’s movement.
On Friday, the Red River Landing water level was 17.1 feet, and was forecast to drop to 13.5 feet by Aug. 8. Veatch said a forecast of 10 feet would be required to trigger the sill construction.
The corps has a standing contract with a dredging company to build what amounts to an underwater dam that fills in the lowest part of the river bottom where the saltwater is moving upstream, said Michelle Spraul, project manager for the Mississippi River’s operation from Baton Rouge to the Gulf.
She said about 2.5 million cubic yards of sediment would be dredged from two disposal areas located just upstream to create the sill. She could not estimate the cost of building it.
The sill will raise the bottom of the river to between 50 feet below sea level and 45 feet below sea level, which will still allow ocean-going vessels to move upstream, she said.
No additional dredging will be required to remove the sediment once river levels rise and the flow of fresh water flushes the saltwater out, Spraul said.
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