Bedrock

Bedrock Type

The rock below the Earth's surface is the bedrock. If all bedrock consisted of a material like solid granite, then even gravity would have a hard time pulling water downward. This is not the case. Bedrock naturally has varying amounts of space in them where groundwater accumulates. Bedrock can also become broken and fractured creating spaces that can fill with water.

In Dunn County, the majority of the bedrock, about 95%, is sandstone with a small amount of dolomite bedrock in the western portion of the county. Typically, dolomite is highly fractured in Wisconsin. This allows water to move through it rather quickly allowing contaminants, such as nitrate, to enter the groundwater with relative ease. Sandstone tends to be less fractured which results in water moving more slowly through it.

 

Depth to Bedrock

Bedrock is generally considered a restrictive layer for groundwater. This means that as water flows through the soil, bedrock can act as a layer that slows or even stops downward movement of water. Water that is trapped above the bedrock transports contaminants to groundwater below it much more slowly. This does not mean that wells drilled below bedrock are impervious to nitrate contamination, but it does offer some level of protection in areas where the bedrock is not fractured. However the depth to bedrock is highly variable within Dunn County with most of the county having bedrock within five feet of the ground surface, but many places having bedrock deeper than 200 feet from the surface. Additionally, it is not always feasible for well drillers to go through the bedrock nor does the bedrock offer complete protection.

A map of the depth to bedrock within Dunn County is available here: Depth to Bedrock

An interactive depth to bedrock layer is also available through the “Dept Maps” tab at: https://dunncowi.wgxtreme.com/