Eric Burkness, Minnesota Department of Agriculture, Research scientist, reviewed by Tana Haugen-Brown, UMN Pesticide Safety and Environmental Education Extension educator, Sally Raymond, UMN Pesticide Safety and Environmental Education Extension educator, and Jolene Warnke, UMN Pesticide Safety and Environmental Education Extension educator
Groundwater monitoring is an ongoing project in Minnesota. To maintain a successful and productive agricultural economy while minimizing impacts on human health and the environment, the Minnesota Department of Agriculture (MDA) each year monitors surface water (e.g., rivers, streams, and lakes) and groundwater throughout the state to determine if any of 185 pesticides and their breakdown products (also known as degradates) are present. If a pesticide is detected, the amount of pesticide in the water is compared to a reference value to determine if action is necessary. In the case of groundwater, reference values are human health-based guidance values developed by the Minnesota Department of Health (MDH). In Minnesota, the majority of pesticide and breakdown product detections in groundwater are below their established reference values.
In 2024, a total of 225 samples were collected from 168 groundwater monitoring sites (Figure 1). Of the 185 pesticide-related chemicals monitored, 55 were detected.
The highest rates of detection were for the pesticides clothianidin, bentazon, fomesafen, sulfentrazone, and thiamethoxam (Figure 2). The highest detection rates for pesticide breakdown products were for metolachlor ESA, metolachlor OXA, acetochlor ESA, alachlor ESA (registration cancelled), and dimethenamid ESA (Figure2).
In 2024, the following pesticide compounds were detected for the first time: clethodim sulfone, clethodim sulfoxide (Cleanse, Volunteer, and others1), flutriafol (Rhyme, Topguard, and others), and halosulfuron-methyl (Stadia, Promote, and others). Monitoring for these pesticides began in 2018 for clethodim sulfone and clethodim sulfoxide, in 2013 for flutriafol, and in 2010 for halosulfuron-methyl. In 2024, the breakdown product pyroxasulfone M3 (Fierce, Anthem, and others), was detected for the first time in groundwater.
A breakdown product of the fungicide chlorothalonil, 4-hydroxychlorothalonil, (Echo, Bravo, and others), was the only pesticide compound that exceeded its reference value in 2024 groundwater samples, with nine samples showing a concentration greater than the drinking water Risk Assessment Advice level of 2,000 ng/L (parts per trillion). All detections occurred in coarse, sandy soils in Central Minnesota.
For information on pesticide monitoring in Minnesota’s surface water, see Part 1: 2024 River and stream results.
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| Figure 1. Groundwater monitoring well. |
In 2024, a total of 225 samples were collected from 168 groundwater monitoring sites (Figure 1). Of the 185 pesticide-related chemicals monitored, 55 were detected.
The highest rates of detection were for the pesticides clothianidin, bentazon, fomesafen, sulfentrazone, and thiamethoxam (Figure 2). The highest detection rates for pesticide breakdown products were for metolachlor ESA, metolachlor OXA, acetochlor ESA, alachlor ESA (registration cancelled), and dimethenamid ESA (Figure2).
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| Figure 2. Detection rate for the most commonly encountered pesticide compounds in MN groundwater in 2024. |
In 2024, the following pesticide compounds were detected for the first time: clethodim sulfone, clethodim sulfoxide (Cleanse, Volunteer, and others1), flutriafol (Rhyme, Topguard, and others), and halosulfuron-methyl (Stadia, Promote, and others). Monitoring for these pesticides began in 2018 for clethodim sulfone and clethodim sulfoxide, in 2013 for flutriafol, and in 2010 for halosulfuron-methyl. In 2024, the breakdown product pyroxasulfone M3 (Fierce, Anthem, and others), was detected for the first time in groundwater.
A breakdown product of the fungicide chlorothalonil, 4-hydroxychlorothalonil, (Echo, Bravo, and others), was the only pesticide compound that exceeded its reference value in 2024 groundwater samples, with nine samples showing a concentration greater than the drinking water Risk Assessment Advice level of 2,000 ng/L (parts per trillion). All detections occurred in coarse, sandy soils in Central Minnesota.
For information on pesticide monitoring in Minnesota’s surface water, see Part 1: 2024 River and stream results.
Additional sources of information
- Each year, the MDA produces a water quality monitoring report that provides results for the previous year’s monitoring efforts. Results for 2024 can be found at 2024 Water Quality Monitoring Report | WRL Digital Asset Management.
- Groundwater monitoring focuses on locations considered sensitive to contamination from activities on the land surface. Details for sampling locations and collection are available at Water Monitoring Reports and Resources | Minnesota Department of Agriculture.
- The MDA developed a new tool called Water Quality Monitoring StoryMaps to explore water quality monitoring data. The 2024 data is now being added.
- To help protect Minnesota’s water quality, always follow label application rates, timing restrictions, and other label recommendations. In addition, water quality best management practices (BMPs) are available from the MDA at the Pesticide BMPs webpage. BMPs are provided for general pesticide use and specific active ingredients.
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