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Lower Mississippi Saltwater Intrusion

About Saltwater Intrusion

What’s the problem?

Extended drought conditions in the Mississippi River and Ohio River watersheds have brought the River’s flow in south Louisiana to near historic lows. The lack of flow from the river is allowing salty water from the Gulf of Mexico to push its way upriver toward the metro area.

New Orleans at the other regional parishes get our municipal water supply from the river, so if the saltwater reaches our water intakes it will threaten our ability to provide drinkable water.

How does this work?

Much of the Mississippi River’s channel is below sea level in south Louisiana. This means if the river were to stop flowing, its channel would fill up with salty water from the Gulf of Mexico and become an estuary. Normally, the large volume of fresh water coming down the Mississippi River has enough force to push out the saltwater and prevent this from happening. With the river’s flow as low as it is now due to the drought, this effect is diminished and salty water is able to encroach in from the Gulf.

Saltwater is denser than freshwater, so as it pushes in the saltwater tends to creep along the bottom of the water column first, forming a “wedge” of saltwater. The US Army Corps of Engineers (USACE) graphics below illustrate how this works.

The front portion of the wedge is called the “toe”. While the toe arrives first, the water at the surface of the river may remain mostly fresh for approx. 15 miles before similar salt (chloride) levels are measurable at the surface.

The USACE conducts sampling to monitor the location of the wedge. Their estimate of the current wedge location is available here. This site also includes links to other relevant river flow data.

Is there any way to stop the wedge?

Plaquemines Parish began seeing impacts from this situation in June. As a mitigation measure against this, the USACE constructed an underwater sill (essentially a levee at the bottom of the river) at river Mile Marker 65 (Plaquemines Parish, south of Belle Chasse) to slow the advance of the wedge.

Based on the current projections, the USACE is working to raise the current sill higher. Without completely shutting down river traffic, this will not stop the wedge permanently, but is expected to delay its progress by 10 to 14 days and buy communities upriver more time to prepare. Eventually, the wedge will overtop the sill and continue progressing upriver.

The only other thing that would stop the wedge is an increase to the river’s flow. Unfortunately, drought conditions are expected to persist up north and no relief is in sight. Upcoming winter presents a further challenge as precipitation may fall as snow up north and remain there until it melts in the spring. Long-term forecasts can always change, but we have been told that the low river situation could persist until January or later.

The US Seasonal Drought Outlook from the Climate Prediction Center is available here.

State and local partners have inquired with USACE about increasing the flow by closing outlets or releasing water from any reservoirs further north. The Corps advised that this is not a viable solution – given the drought situation there is not enough water available to make a difference.

How much time do we have?

USACE released an estimated forecast on Friday, September 22nd, which shows saltwater impacts at Water Intakes for regional parishes at the following dates and locations along the river:

MGD = Million Gallons per Day
Region 1 - Major Water Intake Sites
River Mile Marker	Community Served	Est. Water Consumption
RM 75.5	Belle Chasse	6.5 MGD
RM 88	St. Bernard	8.8 MGD
RM 95.7	Algiers	9.9 MGD
RM 96.7	Gretna	3.9 MGD
RM 99.1	Jefferson Westbank	23 MGD
RM 104.7	Orleans Eastbank	141 MGD
RM 105.4	Jefferson Eastbank	40 MGD

What does this mean for our water?

There is a chance that SWBNO may begin to see increasing chloride (salt) levels in the water they take in from the river at their Carrollton and Algiers water plants. SWBNO, LDH, and the EPA are working to develop an enhanced sampling and testing regimen to monitor for impacts. LDH will recommend SWBNO issue a public advisory to consumers if chloride levels rise above the drinking water standard of 250 ppm (parts per million).

A secondary concern is that the elevated salt content could lead to corrosion of pipes in the water distribution system. This is similar to what happened in Flint, Michigan – incoming water was not treated properly and led to corrosion of lead pipes throughout the system. This contaminated Flint’s water system with heavy metals such as lead and copper, which left water undrinkable and required a major effort to replace corroded pipes. As chloride levels increase, SWBNO may need to add additional alkaline or anticorrosive agents to the water in order to mitigate this type of situation. Water sampling tests will also be monitoring for increases in Lead and Copper content in order to identify any warning signs of corrosion in the system.

It is possible that salt levels could reach a point where water from the tap is no longer drinkable. As long as the corrosion problem can be avoided, it would still be safe for washing, bathing, and other uses except for consumption.

Region-wide mass distribution of potable bottled water could become necessary for the duration of the event if the situation reaches this point. Out of an abundance of caution, NOHSEP is already exploring options.

What are the options?

There are a few different treatment options to reduce the salt content of water entering the system:

Blending

Blending involves bringing in fresh water from an alternate source (further upriver, for example) and mixing it with the salty river water to reduce the overall salt content. This currently appears to be one of the most likely courses of action available and is already being done in Plaquemines Parish. Fresh water would be brought in by barge or by tanker ship and pumped into a reservoir, which would then be drawn into the water system.

Given New Orleans’ water demands (approx. 150 MGD), and the potential for a months-long event, it is not realistically possible to import enough fresh water to fully meet the demand. Regardless of its salt content and drinkability, some amount of river water will be necessary to provide enough water to maintain critical systems such as fire suppression and sewerage.

Barging and blending presents its own challenges. The number of barges required will be substantial, and pumping water from barge to reservoir takes a large amount of time. Designing and constructing the necessary reservoir and pumping system is also likely to be a significant engineering challenge.

Alternate Water Source

This would involve drawing from another source of fresh water. In our case, that means further upriver. SWBNO and USACE are exploring the potential to draw water from further upriver beyond Mile Marker 115, which is just past Kenner. This is near the maximum wedge extent from the past low river event in 1988. If the wedge does not reach past that point, fresh water could be pumped from there into the east bank water systems of Jefferson Parish and New Orleans.

Desalination / Reverse Osmosis

Reverse osmosis (RO) is a process of converting saltwater to freshwater by filtering out the salt content. RO works by passing the water through a semi-permeable membrane that separates the water from any other molecules (salt, mineral content, impurities, etc). The membrane system has the consequence of significantly reducing water pressure. This has been explored as an option for smaller water systems downriver, but do not appear to be available at the scale or within the timeframe to be a viable solution for a water system the size of New Orleans.

Household-level RO systems are commercially available but can be prohibitively expensive for many homeowners. They require technical expertise to install and would only treat a single faucet such as the kitchen sink. Unlike other contaminants, there does not appear to be any user-friendly filtration system (Brita, PUR, etc.) for salt that could be distributed to and installed by residents.

Connections to Other Systems

In some cases, it may be possible for one jurisdiction to connect their water system to that of another system further upriver beyond the location of the wedge. This is being pursued wherever available for the parishes downriver. St. Bernard intends to connect with New Orleans’ east bank system, and Belle Chasse will connect with Algiers and/or Jefferson Westbank. This will allow them to draw fresh water from neighbors upriver as long as possible, but ultimately those intakes may also become impacted.

The next system upriver from Orleans and Jefferson east bank communities is St. Charles Parish. Connection does not appear a viable solution in this situation due to the difference in scale. With a total population of approx. 52,000 across both sides of the Mississippi, the much smaller water infrastructure of St. Charles’ east bank is not built to handle the huge demands of Jefferson and Orleans.