Neonicotinoids Are in More Than One in Four Groundwater Samples

Below the streams

Our stream analyses, from the monitoring audit to the benchmark comparison, covered surface water. Groundwater is a separate question: it supplies wells, including drinking-water wells, and it moves slowly, so what reaches it can stay for years.

The Water Quality Portal holds results from monitoring wells as well as streams. We downloaded every well result since 2015 for twelve insecticides and breakdown products used in pest control and agriculture, and applied the same correction for hidden non-detects as in our stream work.

What is found in wells

Source: Water Quality Portal, siteType=Well, results from 1 January 2015, retrieved 5 October 2026, hidden non-detects corrected. ng/L is nanograms per litre. Author calculation.

What it means. Three neonicotinoids dominate. Clothianidin was found in more than a quarter of well samples, at a median detected concentration of 123 nanograms per litre, higher than in streams. The insecticides most associated with structural pest control are rare in groundwater: fipronil in 1.3 percent of samples, its breakdown products in 0.8 to 2.3 percent, bifenthrin in five of 6,485 samples, permethrin in none. Dinotefuran, the neonicotinoid most used in California pest control, was found in 0.4 percent, and acetamiprid in none of 6,147.

What limits it. Two state agricultural programmes, Wisconsin’s and Minnesota’s, supply most of the neonicotinoid well samples, so these rates describe the wells those programmes chose to sample, largely in farming regions. “Well” in the portal includes monitoring wells, not only drinking-water wells.

Against drinking-water levels

What it means. Even the highest single results were well below the levels California’s Department of Pesticide Regulation uses to screen groundwater for human health: 120 micrograms per litre for thiamethoxam, 283 for imidacloprid and 980 for clothianidin. The highest thiamethoxam result was 12.3 percent of its level; the highest imidacloprid and clothianidin results were 2.4 and 1.4 percent. EPA’s chronic human health benchmark for imidacloprid is 360 micrograms per litre.

But not against aquatic benchmarks. Groundwater feeds streams. 93.8 percent of imidacloprid detections in wells, and 69.9 percent of clothianidin detections, were above EPA’s chronic benchmarks for freshwater invertebrates, which are thousands of times lower than the drinking-water levels.

What limits it. Drinking-water reference levels and aquatic benchmarks answer different questions; neither says a given well is unsafe or safe in practice.

By state

Clothianidin was found in 32.3 percent of 3,408 Wisconsin well samples and 24.2 percent of 2,517 Minnesota samples; imidacloprid in 19.8 and 9.9 percent. Outside those two states, sample numbers are small. Results for every state with at least 20 samples are in the CSV.

Checks

1. Complete downloads

Result. Every download matched the portal’s reported result count, for example 10,073 imidacloprid results.

2. Hidden non-detects

Method. Detection limits stored as values were treated as non-detects, as in our monitoring audit; no programme met the exclusion test for these well data.

Download the data

The tables in this report are published as CSV files under a Creative Commons Attribution 4.0 licence. You may republish, remix and build on them, including commercially, with attribution to The Pest Control Brief and a link to this page.

• Detection rates and concentrations for 12 insecticides and breakdown products in US well samples (CSV, 12 rows)
• Groundwater detection rates by state for clothianidin, thiamethoxam, imidacloprid and fipronil (CSV, 40 rows)

How to cite this report

The Pest Control Brief, “Neonicotinoids Are in More Than One in Four US Groundwater Samples. Pest Control Insecticides Mostly Are Not”, 2 October 2026. https://thepestcontrolbrief.com/pesticides-in-groundwater-data/

References

1. Water Quality Portal. Result and Station searches, siteType=Well, from 1 January 2015. waterqualitydata.us. Retrieved 5 October 2026.
2. California Department of Pesticide Regulation. Evaluation of the potential human health effects from imidacloprid in ground water (memorandum). cdpr.ca.gov (PDF). Accessed 5 October 2026. Source of the 283 ppb reference level and the US EPA human health benchmarks.
3. California Department of Pesticide Regulation. Human health reference levels for clothianidin and thiamethoxam (memorandum). cdpr.ca.gov (PDF). Accessed 5 October 2026. Source of the groundwater levels of 980 and 120 ppb.
4. US Environmental Protection Agency. Aquatic Life Benchmarks. Last updated 2 February 2026. epa.gov. Accessed 2 October 2026.

How the analysis was done

Data. Water Quality Portal results with monitoring location type Well, from 1 January 2015, for imidacloprid, clothianidin, thiamethoxam, dinotefuran, acetamiprid, fipronil and three of its breakdown products, bifenthrin, permethrin and chlorpyrifos.

Rule. The corrected detection rule from our monitoring audit.

Limitations. Purposive sampling concentrated in two states; monitoring and drinking-water wells are not distinguished; results are not finished drinking water.

Last reviewed: 5 October 2026.

For reporters

Most quotable statistics

1. Clothianidin was detected in 28.6 percent of US groundwater well samples tested since 2015.
2. Imidacloprid was detected in 12.9 percent and thiamethoxam in 14.9 percent.
3. Fipronil was found in 1.3 percent of well samples and bifenthrin in 0.1 percent.
4. The highest neonicotinoid concentration was 12.3 percent of California’s drinking-water reference level.

Press summary

Neonicotinoid insecticides are common in US groundwater but at levels far below drinking-water health thresholds, according to an analysis of the national water-quality record by The Pest Control Brief. Clothianidin was detected in 28.6 percent of well samples since 2015, thiamethoxam in 14.9 percent and imidacloprid in 12.9 percent, mostly in Wisconsin and Minnesota monitoring. The highest concentrations were at most 12 percent of California’s reference levels. Insecticides used mainly in structural pest control, such as fipronil and bifenthrin, were rarely found.

Headlines editors could use

1. One in four groundwater samples contains clothianidin
2. Neonicotinoids in wells: common, but far below health limits
3. Pest control’s insecticides rarely reach groundwater

Questions

Are neonicotinoids in groundwater?

Yes. Since 2015, clothianidin was detected in 28.6 percent of US well samples tested, thiamethoxam in 14.9 percent and imidacloprid in 12.9 percent.

Is it safe to drink?

The highest concentrations measured were well below California’s human health reference levels for groundwater, at most 12.3 percent of the level. These are monitoring results, not finished drinking water.

Does fipronil reach groundwater?

Rarely: 1.3 percent of 5,906 well samples, at a median detected concentration of 3.9 ng/L.

Which states have neonicotinoids in groundwater?

Most data come from Wisconsin and Minnesota, where clothianidin was found in 32.3 and 24.2 percent of well samples.

Where can I get the data?

Both tables are in the CSV files above.