What is groundwater?
Nearly all of the residents within the District get their drinking water from groundwater. Groundwater is water held underground in soil, sand, or rock. The amount and movement of groundwater depends on the geology of an area. The geology of the District includes a layer of glacial drift on top of layers of bedrock. The glacial drift varies between 150 and 250 feet thick in most of the watershed. In the north, St. Peter Sandstone lies underneath. In the rest of the District bedrock is made up of Prairie du Chien dolomite.
The geology of groundwater
The groundwater in the watershed reflects this geology: glacial drift (or surficial) aquifers on the top, with bedrock aquifers below. Glacial drift aquifers are layers of sand and gravel saturated with water. They lie close to the surface and many private wells in the watershed pull water from these aquifers. Because they are close to the surface, they are more susceptible to pollution. They are recharged by rainwater, and can also interact with lakes, streams, and other water bodies. You can read more about these interactions in the watersheds 2017 Regional Groundwater Surface Water Interaction Study.
Bedrock aquifers lie below the glacial drift. There are four major beckrock aquifers in the District. From shallow to deep, they are: St. Peter Sandstone, Prairie du Chien-Jordan, Wonewoc Sandstone, and Mt. Simon-Hinckley Sandstone. Water in these aquifers is stored in cracks and pores within the rock and take longer to recharge the deeper they are. This means they are less susceptible to pollution, but also harder to fill back up if they are drained. The Prairie du Chien-Jordan aquifer is the one used by most cities in the District to supply water for their residents. It is easier to access than the deeper two aquifers, has good quality water, and underlies most of the District.