Saltwater Encroachment

Introduction

Saltwater 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 encroachment 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.

Climate change can increase saltwater encroachment along coastal regions, particularly as the sea level increases. The increased salinity of coastal freshwater can threaten the plant life and wildlife of coastal areas, destroy habitats such as marshes, and threaten drinking water supplies.

Historical Background and Scientific Foundations

Normally, along coastal regions, freshwater moves out to the sea in rivers or streams. This outward flow prevents seawater from moving inland. As well, the boundary between the freshwater and saltwater (freshwater contains a lesser content of minerals than saltwater and so will float on top) tends to be near the coast rather than inland.

However, measurements of tide levels that have been conducted for over a century and satellite measurements recorded beginning in the 1970s have revealed a continuing rise in sea level. According to the 2007 Assessment Report of the Intergovermental 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 an increased volume of ocean water due to the expanded volume occupied by the warming ocean waters, melting glaciers, and melting

polar ice sheets—changes that have been linked to global climate change.

The ocean basins have a set volume that they can hold. As the amount of ocean waters increases, the ocean depth rises, causing the coastal ocean to move farther inland. A rise in sea level can cause saltwater to move farther inland in regions where the coastline is very near sea level in height, simply from the increased volume of water present. As well, 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 abandonment 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.

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.

WORDS TO KNOW

RESERVOIR: A natural or artificial receptacle that stores a particular substance for a period of time.

WETLANDS: Areas that are wet or covered with water for at least part of the year.

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.

See Also Beach and Shoreline; Coastlines, Changing; Sea Level Rise.

BIBLIOGRAPHY

Books

Houghton, John. Global Warming: The Complete Briefing. New York: Cambridge University Press, 2004.

Parry, 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 University Press, 2004.

Valsson, Trausti. How the World Will Change with Global Warming. Oxford, UK: Oxbow Books, 2007.

Brian D. Hoyle