What is Saltwater Intrusion

Many coastal communities around the United States are experiencing saltwater contamination of water supply wells, this problem has been seen for decades it’s complicated by natural and man-made reasons that satisfy water demand. What is Saltwater Intrusion?

Saltwater intrusion is the movement of saline water into freshwater aquifers, which can lead to groundwater quality degradation resulting in the abandonment of wells used for drinking water & can naturally occur in coastal aquifers, owing to the hydraulic connection between groundwater and seawater.

Freshwater is defined as having a low salt concentration of fewer than 500 parts per million (ppm) of dissolved salts. Plants and animals in freshwater regions are adjusted to the low salt content and would not be able to survive in areas of high salt concentration like the ocean. Freshwater is found in ponds, lakes, rivers, streams, and wetlands.


How Does Saltwater Intrusion Occur


Freshwater habitats are classified as either lentic systems, which are still surface waters including ponds, lakes, swamps, and mires which are running-water systems, or groundwaters which flow in rocks and aquifers that are used mostly for drinking water systems like wells.

Saltwater intrusion has occurred in many of the coastal aquifers of the United States at different levels. Since saltwater cannot be used to irrigate crops or be consumed by people, saltwater intrusion can be very detrimental to coastal communities that rely on fresh groundwater supplies for their livelihood.

The USGS studies how excessive groundwater pumping, sea level rise, and other factors help contribute to the encroachment of saltwater into freshwater ground source supplies. Groundwater pumping can reduce freshwater flow toward coastal areas and cause saltwater to be drawn toward the freshwater areas of the aquifer.

Officials who manage water supplies, allow for better management strategies that ultimately protect people and their sources of freshwater. Because saline content water has a higher mineral ingredient than freshwater, it is denser and has higher water pressure. As a result, saltier water can push further inland beneath the freshwater.


Saltwater Intrusion Causes


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How is Saltwater Intrusion Measured


In freshwater, the concentration of salts, or salinity, is nearly zero and up to 500 parts per million ppm. The salinity of water in the ocean averages about 35 ppm. The mixture of seawater and fresh water in estuaries is called brackish water and its salinity can range from 0.5 to 35 ppm.

Monitoring and assessment provide the basic characterization and availability of groundwater resources in an area, with an understanding of the different pathways by which seawater might intrude an aquifer, giving a basis for sustainable management of water supplies.

Common scientific approaches for monitoring, often used in combination are:

  • measuring depth-to-water (groundwater levels) and hydrograph analysis
  • water quality sampling
  • geophysical logging. 

Water-quality monitoring networks are particularly important to serve as early-warning systems of seawater movement toward freshwater supply wells, as well as providing information on the rates of seawater encroachment.




Where is Saltwater Intrusion a Problem


Ground-water flow patterns in an idealized coastal aquifer
Ground-water flow patterns in an idealized coastal aquifer (Figure A)

Salinity is the dissolved salt content of a body of water. It is a strong contributor to conductivity and helps determine many aspects of the chemistry of natural waters and the biological processes within them.

Salts can be toxic to freshwater plants and animals and can make water unsafe for drinking, irrigation, and livestock watering. 

Saltwater intrusion decreases freshwater storage in the aquifers, and, in extreme cases, can result in the abandonment of wells altogether.

Saltwater is toxic to humans because your body is unable to get rid of the salt that comes from seawater normally removed by producing urine. In order to do that body needs freshwater to dilute the salt in your body for the kidneys to work properly.

Generally, saltwater intrusion into coastal aquifers is caused by two mechanisms:
  • Lateral encroachment- from the ocean due to excessive water withdrawals from coastal aquifers
  • An upward movement from deeper saline zones due to upcoming near coastal discharge/pumping wells.

Saltwater intrusion can and does naturally occur in coastal aquifers, because of the connection between groundwater and seawater. Because saline water has a higher mineral content than freshwater, it is denser and has higher water pressure. Because of this, saltwater can easily push inland beneath the fresh groundwater and aquifers.

(Figure A)  Under natural conditions, the seaward movement of freshwater prevents saltwater from encroaching on coastal aquifers, and the interface between freshwater and saltwater is maintained near the coast or far below the land surface.

This interface is actually a diffuse zone in which freshwater and saltwater mix and is referred to as the zone of dispersion (or transition zone) Ground-water pumping can reduce freshwater flow toward coastal discharge areas and cause saltwater to be drawn toward the freshwater zones of the aquifer.

A 2016 study in the Journal of Science predicted that 9 percent of the U.S. coastline is vulnerable to saltwater intrusion a percentage likely to grow as the world continues to warm. Scientists are just beginning to assess the potential effect on agriculture, Manda said, and it’s not yet clear how much can be mitigated.

Because of its lower elevation, land along much of the Northeast seaboard is at risk from saltwater intrusion. In this area, many acres of farmland are lost every year because they are the soil becomes too wet and salty for growing crops.

Where I live in Pennsylvania USA, along the Delaware river salt front, is defined as a concentration of at least 250 milligrams per liter of chloride which is monitored continuously especially when the area along the river is under drought conditions or abnormally dry conditions.

Fresh water is a renewable and variable, but finite natural resource. Fresh water is replenished through the process of the water cycle, in which water from seas, lakes, forests, land, rivers, and reservoirs evaporates, forms clouds, and returns inland as precipitation

Other Saltwater Intrusion Places in the US:

  • Saltwater is creeping farther inland into the soil and surface waters of North Carolina’s coastal plain
  • In South Florida, the aquifer is experiencing increasing levels of saltwater intrusion due to pumping, wells, overuse of water by coastal populations and agriculture
  • Groundwater pumping has led to extensive water-level declines and seawater intrusion in coastal Los Angeles, California.



What Causes a Dead Zone

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USGS- Saltwater Intrusion



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