Groundwater emanating from a spring and flowing into a cold-water trout stream in Minnesota’s karst landscape. Photograph accreditation: Kevin Kuehner
St. Paul, MN: A newly published study by researchers at the Minnesota Department of Agriculture (MDA), the University of Minnesota’s Minnesota Geological Survey, and the Minnesota Department of Natural Resources reveals new information about the age of groundwater in the state’s distinctive Driftless Area of southeastern Minnesota.
The peer-reviewed study examined current concentrations of a discontinued row-crop herbicide, popular in the 1970s and 1980s, in springs and wells. Researchers then compared the data against the historical use of the herbicide. Researchers combined those results with independent age-dating methods to reveal a mixture of groundwater ages, ranging from 10 to 40 years old in many of the region’s shallower springs and wells, to thousands of years old in deeper aquifers.
Groundwater ages were then combined with historical land use data and climate information to help interpret nitrate concentration trends between 2000-2021 for nearly 1,200 well, spring, and stream monitoring locations.
The results showed that most monitoring sites with elevated nitrate had groundwater less than 20 years old. Nitrate levels in this water were either decreasing or relatively steady. Improved agricultural practices may have contributed to these results. Dilution from record-setting precipitation over the past two decades could also be a factor. Groundwater that is several decades older typically had lower nitrate concentrations, but a higher likelihood of increasing trends as the nitrate-contaminated water moves into deeper aquifer systems.
The authors highlight that although it may take decades to measure the impact of clean water activities in certain aquifers, the cumulative effect of best management practices implemented today will help reduce the amount of nitrate entering groundwater over the long term.
The study’s findings are available in the Hydrogeology Journal (https://link.springer.com/article/10.1007/s10040-024-02871-2). Funding for the project was provided by Minnesota’s Clean Water, Land and Legacy Amendment, MDA’s Pesticide Regulatory Account, Root River Field to Stream Partnership, the Minnesota Environment and Natural Resources Trust Fund, and legislative appropriation to the Minnesota Geological Survey, and the University of Minnesota.
by Kevin Kuehner, Hydrologist, Minnesota Department of Agriculture
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