Groundwater is water that exists beneath the soil surface. When it rains, water infiltrates into the ground and percolates through the soil until it reaches a depth where all of the pore spaces are filled with water very much like water filling the void spaces within a sponge. Water in this saturated zone is called groundwater, and it can flow vertically and horizontally at a rate influenced by the glacial and bedrock geology of the area. Groundwater is found in aquifers, geologic formations where significant amounts of water can be stored, transmitted or supplied to well or a spring.

The quality of groundwater and surface waters are intimately linked. Eventually, some groundwater flows to the surface to feed into lakes and streams. In Michigan, groundwater typically discharges from aquifers to replenish rivers, lakes or wetlands. Conversely, surface waters recharge groundwater sources. Land-use activities affecting groundwater quality especially through infiltration of pollutants can affect surface water quality as contaminants are carried with groundwater discharge. Thus, in one way or another, drinking water quality in Michigan requires the protection of groundwater supplies.

1. Groundwater Tutorial
2. GW Glossary
3. GW Stewardship Manual
4. Maximum Contaminant Levels (MCLs)