Private Well Pesticide Sampling Project: Results and Work Plans

NEW: Update on Cyanazine Monitoring

The PWPS Project is one component of the MDA's Monitoring and Assessment for Agriculture Chemicals in the Environment program. The larger program includes the monitoring of groundwater and surface water in the state. Data summaries for the PWPS Project are included in the annual reports for the Monitoring and Assessment program.

Pesticide Results

During the 2014-2015 sampling, pesticides were detected above the laboratory method reporting limits in six of the private drinking water well samples (0.3%). The wells with pesticide detections were located in a single well from Benton, Olmsted, Sherburne, and Stearns County and two wells in Washington County. Complete pesticide results can be found in the 2014-2015 Private Well Pesticide Sampling Project Report (PDF).

Based on the results of the 2014-2015 sampling, the MDA contracted with a different laboratory (Weck Lab) capable of analyzing at least 125 pesticides and pesticide degradates at lower detection levels. Homeowners who had their well sampled from 2014-2015 were given the opportunity to have their wells resampled using the new laboratory methods.

Approximately 5,700 wells were sampled in 50 counties between 2016 and the spring of 2021 (Table 1) during Phase 1 of the PWPS Project. All samples were analyzed for at least 125 pesticide and pesticide degradates. Results indicate that pesticides or pesticide degradates were detected in 76% of the wells tested. There were 75 different pesticides and degradates found overall (Table 1). Consistent with the MDA's ambient network monitoring, metolachlor ESA (a corn herbicide degradate) was the most frequently detected compound (Table 2). Table 2 also documents concentration statistics and health reference values for pesticide chemicals detected in greater than 14 percent of the samples analyzed between 2016 and the spring of 2021 during Phase 1 of the PWPS Project. All concentrations are reported in ng/L, which is equivalent to parts per trillion (ppt). It should be noted that, for some of the pesticide compounds, the MRL values changed over time during the PWPS Project. The existence of multiple analytical reporting limits over time for a single compound complicates data evaluation. To ensure comparability of results over time required censoring (or restricting) the data at the highest historical MRL for each compound. Concentrations below the highest historical MRL were set to zero (i.e., non-detect).

Based on sampling performed between 2016-2018 it was found that three wells exceeded a drinking water reference value (for diuron (herbicide), methyl parathion (insecticide), and cyfluthrin (insecticide)). Verification samples from the three wells with exceedances were found to be non-detect for those compounds. Sampling performed in 2019- spring 2021 showed that 62 wells exceeded a drinking water reference value (primarily for total cyanazine (herbicide)). Verification sampling results also indicated that most of the water samples at these locations were above the reference value for total cyanazine.

Table 1: Total Pesticide Detections by County for PWPS Project Phase 1 Sampling
 

*Based on 2016 sampling, follow-up samples were non-detect. Initial detection was likely due to cross contamination.
**Based on 2017 sampling, follow-up samples were non-detect. Initial detection was likely due to cross contamination.

^The majority of sampling for this county occurred prior to 2019, before cyanazine degradates were analyzed.
+ Please note the year sampled for each county. Laboratory techniques changed in 2016 to include the analysis of additional compounds and lower detection levels. Additional compounds were added to the analytical list in 2019.

Table 2: Concentration Statistics and Health Reference Values (pesticide chemicals detected in >14% of samples analyzed), as based on PWPS Project Phase 1 sampling

*<MRL = concentration was below the method reporting limit from the laboratory;
**HRL = Health Risk Limit, a promulgated drinking water standard established by the Minnesota Department of Health;
***RAA = Risk Assessment Advice, a non-promulgated drinking water advice level established by the Minnesota Department of Health;
****RA = Rapid Assessment, a non-promulgated value established by the Minnesota Department of Health.

†DEDI Atrazine, unlike the other pesticides in this table, was not tested for in 2016.

Reports & Work Plans

The links below provide access to some of the more recent reports and work plans prepared by the MDA. They are located in the Minnesota Water Research Digital Library. Many older reports and work plans are also available in the digital library or by contacting the email under Contact Us.

Current Work Plan (past work plans are located in the Minnesota Water Research Digital Library)

• Private Well Pesticide Sampling Project Phase 2 Monitoring Plan, 2024*

2022

• Private Well Pesticide Sampling Project Phase 1 Summary Report* 

*Reports are located in the Minnesota Digital Water Research Library (MnWRL)