Salt water intrusion occurs in coastal freshwater aquifers when the different freshwater and salt water densities allow the intrusion of ocean water into the freshwater aquifer. These areas typically support populations where the groundwater demand from these aquifers exceeds the recharge or replenishment rate of fresh water. This can become more likely when the water is being used for in-house use as well as lawn irrigation. Irrigating with salty water will destroy grass, trees and plants.
Aquifers are naturally replenished by precipitation and surface waters that saturate into the ground and work their way through the soil, rocks, etc. to the water table. The encroaching seawater will encounter an area known as the zone of dispersion, where the freshwater and saltwater mix. This interface moves back and forth naturally because of fluctuations in the recharge rate of freshwater back into the aquifer. The illustration below gives an illustration of what this salt water intrusion looks like.
There are many homes with private wells near coastal areas where there is no access to public water supplies, because of distance to the water distribution system or other reasons. When the well experiences salt water intrusion, the proven and tested technology to desalinate the water is reverse osmosis, a membrane technology that was invented in the 1950s specifically for sea water desalination. A system designed to treat the water at the point of entry into the home will correct this salt water intrusion. In fact, the water in the toilets will be of higher quality than what most people drink out of their taps!