Saltwater intrusion or encroachment is the movement of saltwater into underground sources (aquifers) of freshwater, which can occur in coastal regions or inland, and the surface movement of saltwater inland from the coast.
Saltwater intrusion is most common in coastal regions, where the freshwater is displaced by the inland movement of saltwater from the ocean. But it can also occur inland, far away from an ocean, as freshwater is pumped out from underground reservoirs and the salt-laden water from surrounding salty layers of the earth flow in.
The most common cause of saltwater intrusion is the pumping of freshwater from wells near coasts. Climate change can increase saltwater encroachment along coastal regions as sea level rises. Increased salinity of coastal freshwater can threaten the plant life and wildlife of coastal areas, destroy habitats such as marshes, and force the abandonment of drinking-water supplies.
Historical Background and Scientific Foundations
Normally, along coastal regions, freshwater flows downhill to the sea through aquifers. This outward flow prevents seawater from moving inland. Also, freshwater contains a lesser content of minerals than saltwater and so will float on top of saltwater. The boundary between the freshwater and saltwater tends to be near the coast.
The most common cause of saltwater encroachment is the pumping of freshwater from wells. Since freshwater runs down slope to the sea, even underground, intercepting and removing some of this flow causes less freshwater to reach the coast; this allows saltwater to migrate inland and can force the abandonment of wells. Another mechanism of saltwater encroachment on well is termed upconing. This occurs when a well draws freshwater from an aquifer that overlies saltwater. Pulling large amounts of freshwater can draw up a cone-shaped volume of saltwater into the porous rock of the freshwater aquifer; if the peak of this cone of saltwater reaches the draw point of the well, the well ceases to produce freshwater even though most of the freshwater aquifer is undisturbed. Rising coastal populations are accompanied by increased freshwater demand, which results in more water being pumped from wells; this, in turn, increases rates of saltwater intrusion.
An increasingly important cause of saltwater encroachment is sea-level rise due to anthropogenic (human-caused) climate change. Human beings are changing global climate by adding certain gases to the atmosphere, especially carbon dioxide (CO2) and methane (CH4), through burning fossil fuels and through agriculture. Destroying forests also contributes to global climate change. All these activities tend to warm Earth as a whole, although this warming is not evenly spread over time or space.
Measurements of tide levels that have been conducted for over a century and satellite measurements since the 1970s have shown a continuing rise in sea level. According to the 2007 Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), the average global sea level rose by 0.07 in (1.8 mm) per year from 1961 to the early 1990s, and, since 1993, has been rising by 0.12 in (3.1 mm) each year.
The cause of the sea level rise is twofold. First, warming ocean waters expand. Second, melting mountain glaciers and polar ice sheets increase the volume of the ocean by adding water to it. Both processes are results of anthropogenic climate change.
As the volume of ocean waters increases, sea level rises, causing the coast to move inland. A rise in sea level can cause saltwater to move farther inland in regions where the coastline is near sea-level in height, simply
WORDS TO KNOW
WATERSHED: The expanse of terrain from which water flows into a wetland, water body, or stream.
WETLANDS: Areas that are wet or covered with water for at least part of the year.
from the increased volume of water present. Also, the tendency of the saltwater to penetrate to underground freshwater reservoirs is increased.
If the involved freshwater reservoir is used as a source of drinking water, the pumping of the freshwater from wells can encourage the movement of the saltwater from lower in the aquifer to the vicinity of the wells. This can contaminate drinking water and lead to abandon ment of the drinking water source.
Impacts and Issues
The rising sea levels documented by the IPCC have already affected some coastal locales. One example is the Canadian province of Prince Edward Island. According to the Geological Survey of Canada, rising sea levels are contributing to increased damage caused by coastal storms. Erosion of the beaches is threatening the island’s nearly $3 million annual tourism industry. Almost half of the coastline could be submerged by 2100.
Another area being affected by saltwater encroachment is the peninsular state of Florida in the United States. The advancing sea level has already affected freshwater bodies, including coastal wetlands. The low elevation of the state even puts the Florida Everglades at risk. Wetlands such as the Everglades are home to many species of birds and wildlife. A threat to wetlands water quality threatens the entire ecosystem.
IN CONTEXT: SALTWATER INTRUSION
Following the 2005 Indian Ocean tsunami destruction, saltwater intrusion created substantial problems, including saltwater contamination of agricultural areas, freshwater wells, and septic systems. The United Nations Environmental Program (UNEP) estimated that 62,000 freshwater wells in Sri Lanka alone were contaminated by marine water.
The fact that many of Florida’s shorelines are gently sloping will increase the height of the sea-level rise, which could exceed 2 ft (0.6 m) by 2100. The state’s drinking water supplies will be threatened.
Developed regions such as Florida have the resources and infrastructure to at least attempt to deal with the changing ecology caused by saltwater encroachment. In less developed areas of the world, however, the threat to drinking water supplies and coastal life will be far less easily overcome, and could displace many people.
Freshwater supplies of small islands are particularly threatened by sea-level rise. On small islands, freshwater aquifers exist as relatively small lens-shaped bodies sitting on top of, and surrounded by, saltwater. Even small rises in sea level can drastically shrink, or even eliminate, such lenses of freshwater.
As the global climate warms and drinking water scarcity grows, the threat of saltwater contamination of aquifers will also grow. For example, in the United States, approximately two thirds of the known underground sources of drinking water are surrounded by formations containing saline water. Overuse of the reservoirs to supply drinking water to an increasing number of people and for agricultural use will lead to saltwater contamination.
arry, M. L., et al, eds. Climate Change 2007: Impacts, Adaptation and Vulnerability: Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.
Pugh, David. Changing Sea Levels: Effects of Tides, Weather and Climate. New York: Cambridge Universit Press 2004.
Valsson, Trausti. How the World Will Change with Global Warming. Oxford, UK: Oxbow Books, 2007.
U.S. Geological Survey. “Freshwater-Saltwater Interactions along the Atlantic Coast.” http://water.usgs.gov/ogw/gwrp/saltwater/salt.html (accessed April 3, 2008).
Brian D. Hoyle