Detections in Water
Chlorpyrifos detections in surface water occur primarily in August (see figure below), typically during base-flow conditions when water levels are low and runoff is minimal. From 2013 through 2025, 92% of all detections occurred in August, and 59% of those August detections were recorded during base‑flow periods.
Impairments Due to Chlorpyrifos
Water quality data is assessed every two years using the Minnesota chronic water-quality standard of 41 ng/L and maximum (acute) water-quality standard of 83 ng/L. If a detection of chlorpyrifos in surface water exceeds a Minnesota water-quality standard, the Minnesota Pollution Control Agency can designate the water body as “impaired” on the USEPA Impaired Waters List. A waterbody may be removed from the Impaired Waters List when the water-quality standard is met.
There are six waterbodies in Minnesota designated as “impaired” for chlorpyrifos on the 2026 draft Impaired Waters List.
Detections of chlorpyrifos in surface waters above Minnesota standards are a concern due to toxicity to aquatic organisms. Chlorpyrifos may enter water by several routes, including:
- Spray drift or vapor drift
- Runoff (primarily with eroded sediment)
- Direct spray/overspray (i.e., misuse)
Chlorpyrifos Response Plan
The MDA developed and implemented a chlorpyrifos response plan in 2019 in response to chlorpyrifos surface-water impairments. Implementation of the plan was paused following the cancellation of food and feed uses of chlorpyrifos in 2022–2023 and the subsequent delisting of several impaired waterbodies. Following the conditional re‑registration of chlorpyrifos for limited agricultural uses in 2024 and 2025, surface‑water detections again occurred starting in 2024. The MDA is now revising its chlorpyrifos response plan and renewing efforts to address elevated chlorpyrifos detections and currently listed impairments.
The plan focuses on three areas:
- Education and Outreach
- Inspections
- Monitoring and Assessment
The MDA will focus its efforts in growing regions throughout the state where chlorpyrifos products are used. Targeted efforts also will occur within watersheds or rivers that have or may have impairments. The education and outreach efforts will focus on educating applicators about legally required setbacks from surface waters.
