Salt wedges (or saltwater wedges) occur in estuaries where the fresh river water meets the saltwater from the sea. If the river flow rate is not sufficient to drive the saltwater back, it moves upstream in the shape of a wedge. The denser saltwater sinks to the bottom creating a thinner ‘toe’ at the edge extending underneath the freshwater.
Problems arise where the saltwater wedge extends so far upriver that it reaches the areas where drinking water is obstructed. This means that the water has such a high salt content that it is no longer suitable for us to drink which presents the authorities with huge problems.
What Problems Do Saltwater Wedges Cause?
If saltwater wedges reach water obstruction points, they significantly affect the quality of the drinking water. The Environmental Protection Agency’s (EPA) maximum standard for chlorides (salts) in drinking water is 250 mg/L. Beyond these limits, the water can start to corrode water distribution pipes, machinery, and appliances. It also makes the water unsuitable for drinking.
Saltwater Wedge on the Mississippi
The intrusion of salt water into the Mississippi is now so close to water treatment plants that emergency procedures have had to be put in place. This has included dredging of water intakes and the installation of reverse osmosis equipment. These use a semi-permeable membrane to remove dissolved salts from water.
As the social media post below shows, this equipment is being installed in locations along the Mississippi including the Boothville water treatment plant.
Why Is the Mississippi Saltwater Wedge So Extensive?
Normally, the flow of freshwater out of the Mississippi is enough to keep the saltwater wedge a long way downstream. However, exceptional drought conditions have existed across large parts of the river’s catchment area for some time. According to the US Geological Survey, flow rates are around half of normal. This allows the salt wedge to move much further upstream than we would normally see.
What Is Being Done to Tackle the Problem?
In addition to the installation of reverse osmosis equipment, the US Army Corps of Engineers is taking several other measures to assist. They are adding 25 feet to a 1,500-foot underwater levee to slow down the progression of the wedge. Bulk bottled water deliveries are planned in the New Orleans area.
The most effective solution for this issue would be rain. However, the long-term forecast does not seem to indicate that sufficient rain will fall during October to alleviate the issue.
The photo featured at the top of this post is © Justin Wilkens/Shutterstock.com
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