Seawater intrusion (or salt water intrusion) is the underground flow of salt water into wells and aquifers. It occurs when fresh water is withdrawn faster than it can be recharged near a coastline. Jefferson County Public Utility District (PUD) Number 1 offers a webpage answering "Frequently Asked Questions about Seawater Intrusion."

Jefferson County has policies and development regulations intended to protect groundwater against the phenomenon of seawater intrusion. Three types of Seawater Intrusion Protection Zones (SIPZ) have been designated as Critical Aquifer Recharge Areas (CARAs): Coastal SIPZ, At-Risk SIPZ, and High Risk SIPZ. See the Maps and Map Key section for depictions. The level of regulatory protection is related to the type of designated SIPZ.

All residents, whether residing inside or outside a designated SIPZ, are encouraged to conserve water to help protect our water resources.