An aquifer is a geologic formation (rock, gravel, or sand) that contains useable amounts of groundwater and can contribute that water to wells, springs, lakes, or streams. Groundwater is simply water in the ground, where it fills and moves through pores and cracks in the aquifer.
Wisconsin has four main aquifers that are layered in varying thicknesses, one atop another. (The maps on this page use a different color to represent each aquifer; darker shades of each color indicate thicker deposits.)
Wisconsin’s aquifers, from shallowest to deepest:
Sand and gravel
We have the glaciers to thank for our abundant sand and gravel aquifers. Melting ice washed out and deposited sand and gravel in broad areas and in river valleys in Wisconsin. The glaciers were large and carried lots of ground-up rock. They crossed parts of Wisconsin several times over the last 2.5 million years, leaving behind layers of sand and gravel, and sometimes clay, up to 300 feet thick.
The sand and gravel aquifer is not distributed uniformly across the state and, compared to rock layers, it is often not continuous over large areas. However, the sand and gravel can produce enough water for private wells even if the deposit is small because sand and gravel is very permeable. Many Wisconsin villages and some cities have their water supply wells in sand and gravel aquifers, especially in river valleys.
The sand and gravel aquifer is commonly present at or very near the ground surface. It is particularly vulnerable to contamination from surface pollutants, especially if there is no clay layer on top of it to slow the movement of these pollutants into the aquifer.
In eastern Wisconsin, the uppermost bedrock is mostly a carbonate rock called dolomite, and these rocks form the eastern dolomite aquifer. This aquifer is also sometimes referred to as the Silurian aquifer or the Niagaran aquifer.
The eastern dolomite aquifer extends in Wisconsin from Door County in the north, where the dolomite forms cliffs along Green Bay, along the east shore of Lake Winnebago, to the Wisconsin-Illinois state line, and to Lake Michigan on the east. In places, especially in the northern counties, this rock is exposed at the surface; in some places, especially to the south, it is covered by hundreds of feet of glacial deposits. This aquifer is thickest along the Lake Michigan shoreline, and thins to the west. It is not present in other parts of Wisconsin.
The rocks forming the eastern dolomite aquifer are about 416 to 444 million years old. This aquifer lies above a fine-grained layer of shale (this layer is called an aquitard because it restricts movement of water between aquifers). The shale protects the underlying sandstone aquifer from contamination originating at the land surface.
Dolomite aquifers produce water from interconnected cracks, fractures, and pores, and sometimes even caves. The amount of interconnection varies across the aquifer and, for that reason, the yield of wells in the eastern dolomite can vary widely—even from one well to the next.
The eastern dolomite aquifer is especially vulnerable to contamination from the surface where the cover of glacial materials is thin. In addition, groundwater is transmitted quickly through cracks that can extend from the surface to nearby wells so contamination can move rapidly through this aquifer.
Sandstone and dolomite
The sandstone and dolomite aquifer, also called the sandstone aquifer, Cambrian aquifer, or Cambrian-Ordovician aquifer, lies beneath the dolomite aquifer and the shale aquitard in eastern Wisconsin, beneath glacial deposits in central Wisconsin, and at the surface in southwestern Wisconsin. It is absent in north-central Wisconsin. These rocks are 425 to 580 million years old.
The sandstone of this aquifer yields water from the spaces or pores between the sand grains; the dolomite produces water from cracks and fractures. Well yields depend on which rock type is dominant in the area. Layers of clayey sandstone and shale within the sandstone and dolomite aquifer also affect well yield.
The deep sandstone and dolomite aquifer is the primary source of groundwater for much of the state.
Crystalline bedrock underlies all of Wisconsin and in much of the state it is overlain by sandstone, dolomite, or glacial deposits. In north central Wisconsin, the crystalline bedrock “basement” is at the surface and may be the only available aquifer. This aquifer is sometimes called the crystalline aquifer or Precambrian aquifer.
These rocks formed between 1.0 and 2.8 billion years ago and are principally fine-grained igneous and metamorphic rocks that yield water from fractures and cracks. If the fractures and cracks are numerous and interconnected, this aquifer can provide water for household use. It rarely produces enough water for large municipalities or industry. In some areas, fractures in this rock are enlarged using hydraulic fracturing to make this aquifer produce more water (this type of hydraulic fracturing doesn’t involve injecting sand to hold cracks open).