Final answer:
Saltwater intrusion primarily affects coastal areas, degrading the water supply and impacting both human usage and agriculture.
Step-by-step explanation:
Contamination of wells by saltwater intrusion is commonly a problem in coastal areas. This environmental issue occurs when the overpumping of freshwater aquifers near the ocean causes the saltwater to enter freshwater zones. Saltwater intrusion leads to the degradation of water quality and can make the water supply unfit for human consumption and agriculture. Coastal regions are particularly vulnerable because of their proximity to the sea and the potential for their freshwater sources to be overwhelmed by the encroaching saltwater.