The MDA monitors rivers, lakes, and groundwater in agricultural and urban areas for the presence of pesticides.
Metolachlor and its degradates (metolachlor ESA and metolachlor OXA) have been found by the MDA in both Minnesota’s surface water and groundwater. Finding metolachlor (the parent compound) in surface water and groundwater was not unexpected due to its wide use and its high solubility in water. The United States Environmental Protection Agency (EPA) conducts a risk-benefit analysis when it registers a new pesticide or existing pesticide with new use. In the case of metolachlor, it was determined that when used according to the label, metolachlor does not pose unacceptable risks to water resources.
Metolachlor water monitoring data are reviewed by the MDA for both the frequency and concentration at which metolachlor and its degradates are detected. Monitored concentrations are compared against risk-based guidelines or standards established by the Minnesota Pollution Control Agency (MPCA) and the Minnesota Department of Health (MDH). State statute and agency policy require the MDA to take action when pesticides are frequently detected or when detections occur at concentrations approaching or exceeding these numbers.
Minnesota water quality guidelines or standards for metolachlor
| Water Resource | Chemical | Reference Number | Basis for Number | Responsible Agency |
|---|---|---|---|---|
| Drinking water | Metolachlor | 300 µg/L | Chronic Health Based Guideline | MDH |
| Drinking water | Metolachlor ESA degradate | 800 µg/L | Chronic Health Based Guideline | MDH |
| Drinking water | Metolachlor OXA degradate | 800 µg/L | Chronic Health Based Guideline | MDH |
| Surface Water | Metolachlor | 23 µg/L | Chronic Water Quality Standard | MPCA |
| Surface Water | Metolachlor ESA degradate | 24,000 µg/L | Water Quality Benchmark | EPA |
| Surface Water | Metolachlor OXA degradate | 7,700 µg/L | Water Quality Benchmark | EPA |
Notes:
- μg/L stands for micrograms per liter; which in water is also equivalent to parts per billion (ppb).
- Chronic guidelines and standards provide protection from long-term exposure over a period of days (surface water) or years (drinking water).
- The MPCA Chronic Water Quality Standard for metolachlor was adopted into rule in 2008.
- The MPCA Chronic Water Quality Standard for metolachlor has a time component; to violate the standard, the average metolachlor concentration over a 4-day (96-hour) period needs to exceed 23 μg/L.
- The MPCA Chronic Water Quality Standard for metolachlor is based specifically on protecting aquatic plants, but is also protective of other aquatic life, water recreation, and drinking water sources.
Detection of the metolachlor parent compound in groundwater is rarer, and when detected, is found at very low concentrations relative to the standard. Metolachlor degradate, ESA, is found more frequently in groundwater, but not at concentrations that approach human health guidelines.
The MDA monitoring program has found both metolachlor and both of its degradates in surface water. Metolachlor has been detected more frequently and at increasing concentrations in certain parts of Minnesota. In general, metolachlor detections are low relative to the Minnesota Chronic Water Quality Standard value of 23 μg/L. Metolachlor degradates are found more frequently but at low levels relative to aquatic life or human health guidelines.
For more information on the MDA’s water monitoring efforts, refer to the MDA’s Monitoring and Assessment Program and the annual water monitoring reports.
Based on the frequency and concentration that metolachlor has been found in water from normal agriculture use, the Minnesota Commissioner of Agriculture, in 2002, ordered MDA staff to develop voluntary Water Quality Best Management Practices (BMPs) for metolachlor. The voluntary BMPs were first published in 2004, updated in 2008, and revised in 2010.
