The layers of earth under our feet determine the quantity and quality of groundwater in any particular spot. Two features, the consolidated layer and the unconsolidated layer, determine most of the characteristics.
Consolidated layer: Consisting of not just bedrock but sometimes multiple layers of rock, consolidated layers are made up of solid stone with varying degrees of water-holding capacity. The southeast corner of the state, from approximately Pine County down to the western side of Faribault County, sits on a foundation of limestone and sandstone formed from layers of sediment, which have relatively large spaces between the stone particles. It tends to hold water like a sponge, and the water moves through relatively freely. The rest of the state sits on what is mostly igneous, or volcanic, bedrock. The spaces between the particles in this hard type of rock are very small to non-existent. Aquifers in these layers tend to be small, hard to find, and isolated, found only in cracks and fissures in the rock.
Unconsolidated layer: Sand and gravel or clay make up the unconsolidated layers. Water-rich Central Minnesota, for example, is covered by a sand plain that sits on top of the hard, non-porous igneous bedrock, which is what gives the region plenty of aquifers and thousands of acres of wetlands. Water drains through the sandy soil quickly, however, meaning that areas above the water table tend to dry out fast (hence, they need irrigation) and aquifers can become easily contaminated. In western, north central, southwestern, and south central Minnesota, the soil is composed of heavy clay with pockets of sand. Water soaks into the clay soil slowly, which means underground aquifers recharge slowly, and rainwater sits on the soil or runs off into lakes and streams. When water does soak in, this type of soil holds water well, which is why farmers with this soil type often tile their fields. In addition, the unconsolidated layer in the northeast and southeast corners of the state is thin to nonexistent, meaning aquifers are almost nonexistent, too.