I live in New Orleans, the low-lying “Big Easy” city on the Mississippi River. This fall, local news was dominated by government officials warning residents to brace for a disruption in drinking water supply because of salt water intruding into the river.
Interventions by the Army Corps of Engineers and increased rainfall upriver gave us a reprieve. Local news moved on to Republican Jeff Landry winning the race for governor and LSU’s Jayden Daniels earning the Heisman Trophy. But this will not be the last news cycle dominated by the saltwater wedge. The problem has most assuredly not gone away–and New Orleans isn’t the only city that needs to be thinking about saltwater intrusion.
What exactly is happening?
The location of the boundary between salt and freshwater is determined by two opposing forces–the mass of the ocean pushing against land and the outflow of freshwater. It’s like an arm-wrestling match between the river and the ocean. When there is more outflow, the ocean is pushed further out to sea at the delta. This year (and last), drought conditions influenced by climate change throughout the Mississippi River basin have resulted in less freshwater making its way to the delta. The ocean is pushing on land harder and forcing its way up the highly engineered river channel. (I talked more in-depth about the issue with Nancy Walecki of the Atlantic.)
Why isn’t the problem going away?
Salt water is advancing inland and up the river due to increasing mean air temperatures, melting ice sheets and sea level rise. Sea level rise has accelerated over the last century and is projected to continue to accelerate, even with aggressive mitigation efforts. We have seen only 20 cm of sea level rise in the last 120 years or so. Up to an additional 50 cm of sea level rise is projected over the next 75 years or so. That means, even in normal water years, the freshwater push will be up against a larger mass of seawater than in the past. We should expect to see saltwater advance farther upstream–and proximate to drinking water intakes.
So how do we protect New Orleans, its vital ports and their commerce, and other communities vulnerable to saltwater intrusion?
There are a lot of ideas floating around–everything from importing water on barges for short-term supply to constructing expensive pipelines. One answer is to try to fight the ocean with built infrastructure. The U.S. Army Corps of Engineers built a sill–an underwater dam–to deflect salt water back downstream while still allowing for ship traffic upriver. This is an engineering marvel, but as sea level continues to rise, it will be a less and less reliable solution, especially if climate change continues to deliver or fails to deliver rainfall in a manner that defies long-term expectations. We can expect higher variability from year-to-year across the Mississippi River basin–stronger droughts and stronger floods and alternation between them more frequently.
The Mississippi River basin is becoming an increasingly volatile place. The specific timing and scope of floods and droughts or their impacts on the river can’t be predicted with any real accuracy. On the other hand, one change we do know with medium confidence is that sea level rise will continue, even under the rosiest of scenarios in the IPCC’s recent AR6 report. The additional push of salt water into the river from climate change is almost certain, and modeling offers a good picture of how much, where and over what period of time. For example, NOAA has a sea level rise viewer that illustrates the saltwater wedge moving upstream under different sea level rise scenarios.
That’s why I’m suggesting it’s time to shift our mindset–to see New Orleans as a saltwater, not a freshwater, city. And what we learn can influence other places with some of the same challenges.
That might sound strange coming from a freshwater scientist. And I realize that in so many ways–culturally, economically–-New Orleans and the Mississippi River are intertwined. But if we reframe our thinking, we stand to make decisions that are more forward-facing. An adaptive response approach based on more concrete understanding will prioritize solutions differently.
Shifting our mindset to accept that New Orleans will experience saltwater intrusion in the river EVERY YEAR rather than once a decade can open our eyes towards cost savings over 30 years in place of expensive band-aids every 10. For example, I generally don’t promote desalination because it is expensive and energy-intensive. It would initially cost more than a pipeline and admittedly lead to higher water costs. By my back-of-the envelope calculations, the cost of a pipeline is between one seventh of, and the full cost of permanent desalination. But that calculation assumes we don't eventually need to move the pipeline intake farther upstream in coming years, at additional cost. Desalination is more stable and sustainable in the sense that we know there will be abundant salt water into the future. Accepting a saltwater fate and planning for that future are more permanent steps towards climate adaptation.
Are there other solutions? Potentially. New Orleans is sinking because we currently manage our aquifer as a flood control reservoir. The city is a fortress, walled off from the river’s floods and surge of the sea. All the rain that falls inside the fortress has no natural way out. We manage this dilemma by drawing down the aquifer to anticipate rainfall. This underground reservoir absorbs and buffers the city from storms. Excess water is pumped out to the Gulf of Mexico through canals. One consequence of drawing down the aquifer for internal flood protection is soil compaction and land subsidence. Strategically designed green space–aboveground natural infrastructure–could reduce the need for belowground natural infrastructure to provide flood protection. Relieving New Orleans’ aquifer from this flood control duty would also open up the possibility of managing it as a recharge asset. We could store treated wastewater underground, where it would be available as drinking water via indirect potable reuse and help manage saltwater intrusion to groundwater resources as the ocean begins to more completely surround the walled fortress.
This is not a tomorrow problem–it is happening right now in real time. Globally, if we don’t cut greenhouse gas emissions enough to limit overshoot of 1.5 degrees C warming by 2040, we will have another 20 to 23 cm of sea level rise by 2050. That translates into miles of coastline lost, river miles that the ocean has traveled–and communities with salty water coming out of their sinks, in the Mississippi River basin and around the world. It also translates into known information that we can use to motivate forward thinking about a future in which we are pre-adapted to changes we know are coming.
