New Orleans Saltwater Intrusion

Saltwater from the Gulf of Mexico intruding upriver is threatening the water supply of Louisiana residents dependent upon the Mississippi River for potable water. Neglected infrastructure is crumbling and climate change is worsening the problem from both the upstream and downstream sides.

It’s not a new problem, but its scope is rapidly escalating past the usual seasonal emergencies Louisiana communities have become accustomed to.

Those communities can’t wait for long term investments in climate change initiatives to, hopefully, have a positive effect on their plight.

The Saltwater Wedge Problem Explained

Salt water is more dense than fresh water, so any interface between salt and fresh water will result in two layers with salt water beneath the fresh. Rivers with higher rates of flow have more distinctly recognizable layers between salt and fresh. The downstream pressure of fresh water will impede the intrusion of the salt water but will not entirely stop it. Because of the density differences and the bottom contours of the river, a distinct wedge of salt water will mark the leading edge of the saltwater intrusion. The more volume of downstream water, the more distinct is the wedge shape and the distance of the intrusion.

Many factors determine how far upstream the salt water from the Gulf will travel. As noted, the volume of water flowing downstream is the major determinant. Lesser factors including tides (generally mild in the Gulf), wind, and water temperature come into play.

Historical data indicates that the “toe” of the wedge hugs the channel bottom of the river as the downstream flow rides over the top until the sheer volume of the more dense salt water raises the slope of the intrusion shaping it into the wedge.

The bottom of the Mississippi River riverbed is actually BELOW sea level all the way from the Gulf of Mexico 364 miles upriver to Natchez, Mississippi. That eases the way for the Gulf to impose its will on communities far upstream.

Levels of chloride in the water historically exceed the 250 parts per million (PPM) levels 15 to 25 miles downstream of the toe of the wedge.

The Army Corps of Engineers (USACE) has had a century long responsibility for maintaining the channel of the Mississippi River to keep 18% of US waterborne commerce operating smoothly. The depth of the channel has traditionally been maintained at 45 feet. In 2022, Congress authorized a USACE project to increase that depth to 50 feet to give larger ships access to the Mississippi River ports.

A byproduct of that project was to allow the wedge of saltwater easier access upstream. While that result was anticipated within the USACE’s 2018 environmental study, the data used to plan the project didn’t accurately take into account the dramatically decreased downstream flow of the Mississippi being caused by ongoing drought conditions in the Mississippi and Ohio River drainages.

Sinking land and rising water

The last two parts of greater travel of the saltwater wedge to impact drinking water supply are land subsidence and rising sea level in the Gulf of Mexico.

Land subsidence is a fact of life for every community built on or near the Mississippi delta. As the land sinks, the water intrudes further into the sodden ground and floods are worse. We can debate the wisdom of building below sea level some other time.

As climate change deniers will point out, there are natural cycles of sea level rise and fall as well as storm intensity cycles documented since the 1800’s. Tropical and hurricane storm intensities also affect sea levels in the Gulf on an identified cycle independent of climate change, documented as the Atlantic Multi-Decadal Oscillation which is within the upward curve of the “normal” cycle now.

Climate change is, however, provably a factor in rising sea levels in the Gulf of Mexico. While the impact of melting glaciers and ice pack at the poles is not currently a major factor, ocean warming and changes to prevailing currents are. Worse, because the Gulf is neatly enclosed by the US gulf coast states and Mexico, it naturally retains more warmth as fuel for worsening storms pushing water up against the shore.

Sea level along the U.S. coastline is projected to rise, on average, 10 – 12 inches (0.25 – 0.30 meters) in the next 30 years (2020 – 2050), which will be as much as the rise measured over the last 100 years (1920 – 2020). Sea level rise will vary regionally along U.S. coasts because of changes in both land and ocean height.

Rise in the next three decades is anticipated to be, on average: 10 – 14 inches (0.25 – 0.35 meters) for the East coast; 14 – 18 inches (0.35 – 0.45 meters) for the Gulf coast; 4 – 8 inches (0.1 – 0.2 meters) for the West coast; 8 – 10 inches (0.2 – 0.25 meters) for the Caribbean; 6 – 8 inches (0.15 – 0.2 meters) for the Hawaiian Islands; and 8 – 10 inches (0.2 – 0.25 meters) for northern Alaska.

NOAA – 2022 Sea Level Rise Technical Report

Based upon NOAA projections, the Gulf coast can expect to experience 14-18 inches of sea level rise. New Orleans and surrounding areas along the Mississippi will continue to sink and have shorelines eroded by worsening storms.

All of that will increase the impact of seawater intrusion on the water supply.

Army Corps Builds A Dam (again)

The USACE has built a “sill” on the Mississippi River bottom three times before in 1988, 1999, in 2012. The illustration depicts how a sill impedes the intrusion of the saltwater wedge. It’s pretty simple.

Sand is dredged from the river bottom upstream and deposited across the river to a height that won’t impede navigation. This time, the Corps built the sill at river mile 64, 64 miles upstream from the current mouth of the Mississippi. The Corps assumes this sill would have to be rebuilt every five years as currents erode the sill.

Unfortunately, the sill barely worked this time. That sill, completed in July, 2023, was “overtopped” two months later September 20, 2023. Construction is underway to increase the existing underwater sill from a depth of -55 feet to a depth of -30 feet.  A 620-foot-wide navigation lane will be kept to a depth of -55 feet to ensure deep-draft shipping continues along the nation’s busiest inland waterway.

So the USACE is now adding onto the top of the exiting sill, raising the top another 25 feet to bring the top of the sill to within 30 feet of the surface. However, to allow continued navigation of large ships, there will be a 620 foot wide gap in the sill down to the original 55 foot depth.

Because of that gap, the existing intrusion of salt water upstream will continue, if at a slower pace than before. Until drought conditions improve, the situation is unlikely to improve until spring snow melt might help increase the flow enough to push the wedge back where it belongs.

The USACE doesn’t offer much solace for communities upstream.

“As a result of continued falling conditions, this existing sill was overtopped and the toe of the saltwater wedge has reached River Mile 69, near the community of Jesuit Bend,” said Col. Cullen Jones, USACE New Orleans District commander. “Our modeling indicates that by augmenting the existing sill, we can support state and local preparedness and response efforts by delaying further upriver progression of the salt water by approximately 10 to 15 days.”

USACE underway with sill augmentation to delay upriver progression of saltwater – Sept. 25, 2023

Until the flow of the Mississippi increases enough, what will people do for drinking water?

A Bucket Brigade on the Mississippi

While some relief from low river levels has been provided by increased flow from the Red River drainage, there’s no time to relax preparations for continued worsening of the seawater intrusion problem that prompted the declaration of an emergency in September, 2023.

The USACE began barging water sourced upstream of the wedge to water treatment facilities downstream for mixing with water too saline for human consumption. Plans are in place to acquire “… the capability of delivering 36 million gallons of water per day in the event additional facilities are impacted by the saltwater intrusion in the Mississippi River.”    

That doesn’t seem an effective long-term solution to the salt water problem which extends beyond the potability of tainted water.    

Salt And Metals Don’t Play Well Together

Louisiana and Mississippi have the highest state poverty rates in the country. Even a brief tour through either state is enough to clarify the decaying infrastructure in both states in the mind of an observer.

That older infrastructure includes old pipes within dwellings and beneath the streets leading to those dwellings.

Any significant rise in sodium levels in the water will begin the process of corrosion of those pipes and, before that, the coating inside those pipes.

The water problems and resulting health issues in Flint, Michigan proved the need for and effective and immediate resolution to the salinity problem.

Are There Permanent Solutions?

There’s little hope for actual improvement in the situation with the incoming Governor-Elect of Louisiana Jeff Landy being a climate change denier who has pledged to stack coastal and environmental related positions with petrochemical industry people. Until Landry is voted out, the water situation is unlikely to improve without lucky weather events.

Desalination would be a permanent solution to many water related problems worldwide. The current state of large scale desalination technology consumes too much energy to be a viable alternative. Of course, really smart people with great tools are working on desalination.

Piping in water from upstream would be a permanent solution. There are rights of way for all sorts of pipes to the shores of the Gulf of Mexico, eliminating at least one large political hurdle to the importation of water.

The problem of a reliable water supply is far from unique to the Mississippi River delta and it’s becoming LESS unique every day. Living in Arizona within just a few miles of more than one community that lost its water supply drives that home. That’s a discussion for another day.

Supporting information links

• An Overview of the Mississippi River’s Saltwater Wedge – US Army Corps of Engineers
• LOWER MISSISSIPPI SALTWATER INTRUSION – NOLA Ready, City of New Orleans
• Where is the Saltwater Wedge Now? – US Army Corp of Engineers
• Mississippi River Channel, Gulf to Baton Rouge, LA – US Army Corp of Engineers
• The Climate Data Guide: Atlantic Multi-decadal Oscillation (AMO) and Atlantic Multidecadal Variability – Trenberth, Kevin &, Zhang, Rong & National Center for Atmospheric Research Staff
• US Seasonal Drought Outlook – National Weather Service Climate Prediction Center
• Louisiana Seawater Intrusion – FEMA, EM-3600-LA
• Unprecedented sea level rise seen on Gulf Coast – NOLA.com
• Biden Approves Emergency Declaration for Louisiana – FEMA