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...