Groundwater is water contained in underground geological formations called aquifers, filling spaces between soil particles, sand, gravel, and rock fractures. It originates from precipitation that percolates down through soil and accumulates in saturated zones. Groundwater supplies drinking water to many Canadian communities through wells, sustains streams and wetlands during dry periods, and provides irrigation water for agriculture. Aquifer characteristics vary—some recharge quickly from surface infiltration while others contain ancient water that replenishes slowly or not at all. Groundwater contamination from landfills, septic systems, agricultural chemicals, industrial activities, or road salt is a serious concern because underground pollution is difficult and expensive to remediate. Municipalities relying on groundwater must protect recharge areas, monitor water quality, and manage extraction to prevent over-pumping. Provincial regulations govern groundwater use and protection, though municipal land use planning affects groundwater protection zones.