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Newly completed survey maps Renville County's groundwater resources

Tribune file photo Alan Knaeble of the Minnesota Geological Survey splits open a segment of clay that was part of the glacier-covered landscape of Renville County 400,000 to 500,000 years ago. The drilling core was taken at a rural Bird Island site in October 2010.

OLIVIA — In ancient times, a large valley once cut through what is today Renville County, very much along the path of present-day U.S. Highway 212.

Today, this subterranean feature, hundreds of feet deep, holds many of the aquifers so important to the county's municipalities, farms and industries.

And it is all mapped, thanks to years of work and more than 8,000 worker hours devoted to developing the now completed Renville County Geological Survey.

The information makes it possible to know where are the best prospects of finding water resources in the county, as well as the aquifers most in need of protection. Along with identifying the location, boundaries and depth of many of the aquifers, the Geological Survey determined the age of the water they hold and consequently how "sensitive'' each is to contamination.

These and other key findings of the county's newly completed Geological Survey were presented Tuesday to the Renville County Board of Commissioners by Dale Setterholm, associate director with the Minnesota Geological Survey, and Randy Bradt, hydrogeologist with the Minnesota Department of Natural Resources.

Both emphasized that the treasure trove of data is available to help with the county's economic development as well as protect its groundwater resources.

By tracking the presence of tritium, a radioactive isotope resulting from aboveground atomic bomb testing, the survey was able to identify those aquifers quickly reached by rain and snowmelt. These are the aquifers most sensitive to pollution from spills and surface activities.

Other aquifers have much slower recharge rates, and the risk for contamination is not as great.

Some of the groundwater consumed by Renville County residents is very old. In 10 of the 91 wells sampled for their chemical composition, the groundwater was determined to be anywhere from 3,000 to 35,000 years plus in age.

The survey was developed by analyzing 1,889 well records and drilling for core samples at sites scattered across the county. It provides a look at the varying depths of the crystalline bedrock underlying the county, and the layers of cretaceous shale — from an ancient seabed — and mix of glacial clays and till from several glacial epochs.

Overall, the county's groundwater resources are better than those found in much of southwestern Minnesota, according to Setterholm.

"Groundwater is challenging enough here in Renville County, but when you get down to the very southwestern Minnesota, it is even more difficult,'' he said.

The county's most productive aquifers are deposits of sand and gravel and found in the subterranean valley. Glacial melt flowed through the valley and that left the deposits of sand and gravel that serve as aquifers today.

In terms of quality, the shallow aquifers have the best tasting water, but are more susceptible to contaminants. Water chemistry changes with depth. Like a water softener, there's an exchange of sodium and calcium magnesium in water as it flows downward, Bradt explained. Sulfate and methane are found in some of the county's groundwater, and arsenic is scattered among aquifers as well. The survey found arsenic above the recommended level for lifetime consumption in 25 of 91 wells.

All of the survey information is available from the county, as well as online at the Minnesota Geological Survey site: " target="_blank">

Tom Cherveny

Tom Cherveny is a regional and outdoor reporter with the West Central Tribune in Willmar, MN.

(320) 214-4335