30,000-year-old groundwater

Madeline Seveland
Carver County Water Management

The water you are drinking in your home may be 30,000 years old. That’s right, according to the Minnesota Department of Natural Resource’s recently completed study of aquifers in Carver County, many of the aquifers in the county hold water that is 100-40,000 years old.

This study, funded by Carver County, consisted of two parts: Part A, a Geologic Atlas that looked at bedrock and Part B, a Hydro-geologic Atlas or Groundwater Atlas. Part B includes information on the different aquifers, age of water present, water-table elevation and direction of groundwater flow, permitted groundwater use, groundwater chemistry, and pollution sensitivity of groundwater and aquifer recharge times. There were 3,900 wells sampled throughout Carver County to collect this data.

Recently Carver County hosted a workshop for anyone wishing to learn more about Part B. Here is some of the information attendees took away. Carver County has eight sedimentary bedrock aquifers (the deepest aquifers), six buried sand and gravel aquifers and one surficial sand and gravel aquifer (the most shallow aquifer).

The most used (pumped) aquifers in Carver County are the Prairie Du Chien and Jordan aquifers, followed the Upper Tunnel City and Wonewoc aquifers. The third most used are the Mt. Simon and Fond Du Lac aquifers. These are all bedrock aquifers which is where 78 percent of permitted groundwater use comes from. Groundwater in these aquifers has been there for thousands of years and is very slowly moving from the northwest to the south east towards the Minnesota River.

The water table is typically within 10 feet of the land surface in most of Carver County, but near the Minnesota River the water table can be more than 120 feet below the land’s surface.

Due to the soils in Carver County being clay rich and low in permeability, which is the ability of water to move down through the soil, these aquifers generally have limited water recharge from rain, snow or lakes and wetlands.

However, in the Minnesota River Valley where the soil is sandier, rain and snow can recharge some aquifers resulting in the groundwater being much more recent in age; 50 years or younger. A downfall from Carver County’s clay rich soils is that no major recharge areas exist. The upside of this is that the bedrock and buried sand and gravel aquifers have low risk of and sensitivity for contamination.

However, are a few scattered areas in the County of moderate sensitivity to groundwater contamination due to large organic peat and muck deposits, and wells constructed in the surficial aquifer (the most shallow) had evidence of high sensitivity showing more recent groundwater age, elevated chloride levels, and/or elevated nitrate-nitrogen levels.

Carver County has a couple areas where groundwater mingles with surface water, near Lake Zumbra and the Minnesota River Valley. Wells near Lake Zumbra were found to contain groundwater that had been partially recharged by lake water.

All of this information and more will be available in December through digital and print publications available on our website, in Carver County libraries, schools and more. Contact Carver County Water Management for more information www.co.carver.mn.us/water.