Salt water intrusion

Aquifers in coastal areas where fresh groundwater is discharged into bodies of salt water such as oceans are subject to salt water intrusion. Intrusion occurs when water usage lowers the level of freshwater contained in the aquifer . The natural gradient sloping down toward the ocean is changed, resulting in a decrease or reversal of the flow from the aquifer to the salt water body, which causes salt water to enter and penetrate inland. If salt water travels far enough inland well fields supplying freshwater can be ruined and the aquifer can become so contaminated that it may take years to remove the salt, even with fresh groundwater available to flush out the saline water.

Salt water intrusion can also develop where there is artificial access to salt water, such as sea level canals or drainage ditches. On the coastal perimeter of the United States there are a number of areas with such intrusion problems. There are five methods available to control intrusion: 1) the reduction or rearrangement of the pumping draw-down; 2) direct recharge; 3) the development of a pumping trough adjacent to the coast; 4) maintenance of a freshwater ridge above sea level along the coast; and 5) construction of artificial subsurface barriers.

Developing a pumping trough or maintaining a pressure ridge are methods that do not solve the basic problem: the overdraft or excessive withdrawal of water from the aquifer. Only when this problem is solved by reducing draw-down, or isolated by the subsurface barrier method, can the intrusion be stopped or reversed. In some areas, it is simply not economically feasible to control intrusion.

Oceanic islands, such as Hawaii, have unique problems with salt water intrusion. Most islands consist of sand, lava, coral, or limestone; they are relatively permeable , so freshwater is in contact with salt water on all sides. Because fresh water is supplied entirely by rainfall in these places, only a limited amount is available. A freshwater lens is formed by movement of the freshwater toward the coast. Depth to salt water at any location is a function of rainfall recharge, the size of the island, and permeability. Tidal and seasonal fluctuations may form a transition zone between freshwater and salt water.

The close proximity of salt water in oceanic islands can introduce saline water into fresh groundwater even without excessive overdraft. An island well pumping at a rate high enough to lower the water table disturbs the fresh-salt water equilibrium, and salt water then rises as a cone within the well. To avoid this condition, island wells are designed for minimum drawdown, and they skim freshwater from the top of the aquifer. In general, drawdowns of a few inches to a few feet provide plentiful water supplies.

[James L. Anderson ]

RESOURCES

PERIODICALS

Gorrie, P. "Water From the Ground." Canadian Geographic 112 (September–October 1992): 68–77.

Wagman, D. "Protecting Hidden Assets: Many American Cities and Counties Are Designing Groundwater Regulations to Protect Underground Aquifers." American City and County 105 (March 1990): 38–41.