The Impact of Seawater Intrusion on Coastal Communities

Monday - October 23 2023, 12:56 UTC - 1 year ago

Seawater intrusion is the movement of saline water from the ocean or estuaries into freshwater systems which can potentially contaminate drinking water sources, kill crops and cause soil salinization. Climate change is expected to increase the severity and frequency of seawater intrusion events, making it a major threat to coastal communities in the future.

Seawater intrusion is the movement of saline water from the ocean or estuaries into freshwater systems. The seawater that has crept up the Mississippi River in the summer and early fall of 2023 is a reminder that coastal communities teeter in a fragile land-sea balance. Fresh water is essential for drinking, irrigation and healthy ecosystems. When seawater moves inland, the salt it contains can wreak havoc on farmlands, ecosystems, lives and livelihoods.

I am a coastal hydrogeologist and have studied water across the land-sea interface for 25 years. I think of seawater intrusion as being like a seesaw: The place where fresh water and salt water meet is the balance point between forces from land and forces from the sea. A push from the land side, such as heavy rainfall or high river flows, moves the balance point seaward. A push from the sea side – whether it’s sea-level rise, storm surge or high tides – moves the balance point landward. Droughts or heavy use of fresh water can also cause seawater to move inland. As climate change and population growth stress freshwater supplies, one result will be more seawater intrusion.

When the ocean moves upriver .

The current seawater intrusion in the lower Mississippi River is due primarily to drought in the Midwest, which has reduced the river’s volume. Both the magnitude of reduction in river flow and the length of time that the river is low influence how far upriver the salt water moves. As of Oct. 2, 2023, the saltwater "wedge" in the Mississippi had moved nearly 70 miles upstream from the river’s mouth.

This isn’t the first time that low water on the river has allowed seawater to move inland. But as climate change raises sea levels and causes more severe weather anomalies, intrusion will become more common and will inch farther upstream.

And the problem isn’t unique to the Mississippi. In Delaware, seawater is traveling farther up small tidal streams during storms and the highest tides, flooding farmland and killing crops.

In the Sundarbans of India and Bangladesh – one of the largest coastal mangrove forests in the world – seawater is intruding into the mouth of the Ganges River. The main causes there are upstream dams and water diversions from the river for irrigation and navigability, plus encroachment due to sea-level rise. Seawater intrusion could threaten many types of plants and animals in this UNESCO World Heritage Site, which is home to countless rare and endangered species.

Invading underground .

Another interface between fresh water and salt water at the coast is less obvious because it’s underground. Many coastal communities draw their freshwater supply from groundwater – clean water that moves through pore spaces between grains of sand and soil.

Groundwater doesn’t just stop at the coastline: Under the ocean floor, the groundwater is salty, and somewhere between land and the ocean, there is an underground meeting point. It typically is landward of the coastline because salt water is denser than fresh water, so it has a greater force and naturally pushes in. But just as with a river, that interface moves when groundwater levels drop on land or water levels rise in the ocean, driving the salt water further inland.