Saltwater intrusion occurs mainly in:

Saltwater intrusion primarily occurs in coastal regions where the water table is falling, as this situation creates a pressure imbalance between the freshwater aquifers and the encroaching saltwater from the ocean. When the groundwater levels in these areas decrease—often due to over-extraction of freshwater for agricultural, industrial, or municipal use—the natural barrier that keeps saltwater at bay weakens. As a result, seawater can flow into the freshwater aquifer, leading to contamination.

This phenomenon is significant for the sustainability of freshwater resources in coastal communities. If the water table remains high, as indicated in one of the other choices, there is usually enough pressure in the freshwater aquifer to prevent saltwater from intruding. Understanding the dynamics of groundwater usage and the interface between saltwater and freshwater is crucial for managing and safeguarding water resources in coastal areas.