and Aquifer Contamination The Coast to Come Saltwater Intrusion and Aquifer Contamination

Threatened Freshwater Rising seawater and changing patterns of precipitation, evaporation, and surface-water runoff - all expected consequences of climate change - will likely act together to draw or push saltwater into freshwater aquifers, contaminating them. Sea-level rise has already caused saltwater intrusion into estuaries and threatened freshwater in several parts of the mid-Atlantic. Saltwater intrusion is the displacement of fresh groundwater by saltwater in coastal water supplies. Saltwater is denser than freshwater and underlies fresh ground water in many USGS coastal areas.

Added Pressures As aquifers come under increasing pressure from development, they will also get less recharge due to pavement-diverted runoff in urban environments and more water being pumped from the system for human use. As a consequence, some aquifers may become unusable without desalinization, which can be expensive and laborious. Water treatment plants may either have to be moved or outfitted with desalinization equipment. If this happens, it will cause difficulties and enormous costs for farmers, well-owners, and utilities alike as they struggle to keep growing crops or providing potable water.

The Point of No Return Saltwater intrusion of freshwater aquifers is another system that is prone to tipping point behavior: rising seas may produce no discernible response in freshwater aquifers until a sea level is reached that produces a widespread, sudden influx of saltwater into coastal forests and groundwater. It is challenging to anticipate which aquifers would be most sensitive to saltwater intrusion, because several variables are involved: natural recharge rates to each aquifer; rates of groundwater withdrawal for human use; and the geologic structure of the aquifer, to name a few.

Encroaching Saltwater and Ecosystem Health Aquifer contamination isn't the only repercussion of encroaching saltwater on coasts, however. Saltwater intrusion, along with stronger storms and higher temperatures, could also harm coastal fisheries. Mangroves and estuaries can withstand high salinities in the short-term, but not permanently. Many coastal species can't tolerate salt in the soil, and "ghost forests" of dead trees have been seen in coastal areas already subject to saltier water in Louisiana, Maryland, southern New Jersey, North Carolina, and Virginia.

Encroaching Saltwater and Ecosystem Health (cont’d) Bald-cypresses in Louisiana and cabbage palm forests in Florida have likely declined due to saltwater intrusion. Because such ecosystems provide spawning habitat for fish, pollutant filtration, and sediment and storm surge control, losing them could be a serious problem for coasts. In the Albemarle-Pamlico Peninsula of North Carolina, stands of Atlantic white cedar are dying as canals and ditches built to drain land for agriculture or timber now funnel salty water inland, into the peats where the trees grow.

Encroaching Saltwater and Ecosystem Health (cont’d) Some farmers' crops have been affected by this influx, and North Carolina State University is working to develop salt-tolerant soybeans. Tide gates have also been installed in some ditches, but plugging them is also being considered.

Control Structures Saltwater intrusion in southeastern Florida has been caused by the construction of drainage canals in addition to ground-water withdrawals for water supply. Initially, the canals were uncontrolled, which resulted in overdrainage of the aquifer and periodic movement of seawater inland along the canals and subsequently into the ground-water system. Since 1946, control structures have been placed on the canals to prevent inland migration of seawater, as well as to provide flood protection and artificial recharge to the aquifer.