Occurs when salt water enters a fresh-water aquifer. Rising sea levels can cause saltwater intrusion along coasts or on small islands. Another problem from salt can occur in soil. Overuse of water on crops (e.g., from irrigation ditches), can lead to salt build-up because the minerals (including salt) dissolved in water are left behind when water evaporates. This process is referred to as "salinization" and can harm soil, making it difficult or impossible to grow crops.
salt water is denser than fresh water, so fresh groundwater tends to form a “lens” on top of saltwater in coastal areas. Sea-level rise tends to increase the salinity of groundwater, tending to push the freshwater lens inland and toward the surface. Withdrawals of fresh groundwater can be faster than its replenishment, which would tend to pull saltwater into what had been freshwater areas. These push and pull effects can allow saltwater to intrude into freshwater aquifers and wells.
Saltwater intrusion is a natural process that occurs in virtually all coastal aquifers. It consists in salt water (from the sea) flowing inland in freshwater aquifers. This behaviour is caused by the fact that sea water has a higher density (which is because it carries more solutes) than freshwater.