What occurs during saltwater intrusion?

During saltwater intrusion, the mixing of saltwater into freshwater aquifers occurs due to various environmental and human-induced factors, primarily related to groundwater extraction and coastal development. When the water table near a coast is lowered through excessive pumping of freshwater, it reduces the hydraulic pressure that normally keeps the saltwater in the ocean at bay. As the pressure drops, seawater can move inland and upward into the freshwater aquifer, leading to contamination of the freshwater supply with salt. This process can significantly impact local ecosystems, agriculture, and potable water supplies, making it a critical concern for water resource management in coastal areas.

The other options relate to different environmental issues that do not specifically address the process of saltwater intrusion. For example, contamination by sewage involves pollutants entering freshwater systems but does not involve the physical mixing of saltwater and freshwater. Similarly, while the overuse of freshwater resources can contribute to various problems, it does not describe the specific phenomenon of saltwater encroachment into freshwater aquifers. Lastly, the freezing of surface water sources is a seasonal occurrence and does not pertain to the interaction between saltwater and freshwater aquifers.