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Water Quality and Pesticides

Pesticides likely to get into Groundwater

Michael Laurie and Diane Emerson, August 17, 2016


Pesticide residues are found far and wide in water samples, but thankfully in most cases they are found at very low levels.

Greater than 36 percent of the samples tested by the United States Geological Survey, USGS, from major aquifers used for drinking water supplies contained pesticides. This indicates the vulnerability of aquifers to contamination from human activities at the land surface and the importance of wellhead protection.

There is still a lot we don't know about the impact of pesticide active ingredients, especially in terms of their interactions with each other and other chemicals and their breakdown products.

The only pesticide found in 2 percent or more of the 1,271 wells sampled by USGS in their latest study, published in 2014, commonly for sale to consumers is 2,4-D which is in Weed and Feed. 2,4-D has also been found harmful to salmon by National Marine Fisheries Service (NMFS) studies in Puget Sound.

​In their previous study, published in 2006, the pesticides commonly for sale to consumers that the USGS found most often were 2,4-D and carbaryl.

Summary of research

1. Summary of findings from 'Pesticides in the Nation's Streams and Groundwater, 1992 - 2001 - A Summary, USGS'

The pesticides most commonly found in groundwater were atrazine, metolachlor, deethylatrazine, alachlor, acetochlor, simazine, prometon, tebuthiuron, cyanazine, diuron, diazinon, chlorpyrifos, carbaryl, and 2, 4-D. 2,4-D was found overwhelmingly in urban areas because it is in lawn care products. 

2. Summary of findings in 'Quality of Streamwater in the Puget Sound Basin - A Decade of Study and Beyond'

The 3 most commonly found pesticides in Thornton Creek in Seattle were the herbicide prometon, the insecticide carbaryl, and the glyphosate degradation compound aminomethylphosphonic acid, AMPA.

Carbaryl has been found by NMFS to be very harmful to salmon. Roundup® products which contain glyphosate continue to be some of the top sellers in retail stores.

3. Summary of findings in the USGS study 'Pesticides in Groundwater of the United States; Decadal-Scale Changes, 1993 - 2011'

Overall one or more pesticides were found in 51 percent of the samples they collected in decade 1, and in 54 percent in decade 2. The highest frequencies of detection were in shallow groundwater beneath agricultural land use areas. Pesticides were detected in about half of the samples taken from shallow groundwater below urban areas and more than 1/3 of samples from deep aquifers. -

Pesticide concentrations seldom exceeded human-health benchmarks in groundwater.

Atrazine, deethylatrazine, metolachlor, simazine, bentazon, metribuzin, norflurazon, 2,6-Diethylaniline, dinoseb, flumenturon, prometon, tebuthiuron, p,p-DDE, dieldrin, carbofuran, aldicarb sulfone, aldicarb sulfoxide, diuron, bromacil, and prometon, 2,4-D were the ones found most frequently in the major nationwide study of pesticides carried out between 1993-2011 by the USGS. 

Three herbicides commonly used in urban areas—simazine, prometon, and tebuthiuron—were detected more often in urban streams than in agricultural streams. Finally, insecticides were found much more frequently in urban streams than in most agricultural streams with p,p-DDE and dieldrin most frequently detected. Overall, results for each individual pesticide reveal a unique pattern of distribution resulting from its primary uses, the distributions of land uses and crops, and the chemical and physical properties of the pesticide. 

Pathways of pesticide movement

See the graph below for some of the many ways that pesticides can move through the hydrological cycle. It is important to note that pesticides can be transported by evaporation, spray drift, wind erosion, runoff, wastewater plant outfalls, seepage, groundwater discharge to streams, and more. For example, glyphosate, the active ingredient in Roundup® products, has been found in rain. And many pesticides can become airborne and travel long distances.

Pesticides in streams vary seasonally, largely based on changes in seasonal use of the pesticide. It is best to try to get an understanding of which pesticides are used and when, and use that knowledge to test for pesticides when use is high and when runoff is greatest. 

Groundwater is more vulnerable to pesticide contamination where the aquifer is shallow and where the soils are highly permeable. Property owners on their own wells with permeable soil should be alerted to this possibility.

Testing for pesticides is expensive. Generally groundwater responds slowly to changes in pesticide use.

Pathways of pesticide movement in the hydrologic cycle.

Modified from Barbash and Resek 1996, USGS

Weaknesses of EPA federal standards on pesticides, reasons for continued caution with pesticides

The USGS has criticized EPA for;

  • not setting adequate water quality standards for pesticides,
  • not setting values for many pesticides
  • not considering mixtures and breakdown products of pesticides
  • not considering seasonal exposures
  • and dragging their feet on setting standards for endocrine disrupters

The EPA did issue a document in 2012 that set guidelines for 350 chemicals but did not set any federal requirements for any of them.

Impacts on wildlife from some pesticides sometimes have been documented at levels determined by the EPA to be safe for humans. According to the Beyond Pesticides publication 'Threatened Waters Turning the Tide on Pesticide Contamination', frogs exhibit hermaphrodism when exposed to legally allowable levels of the herbicide atrazine in waterways. According to USGS work and the publication 'Organic Land Management and the Protection of Water Quality', atrazine, chlorpyrifos, endosulfan, and metolachlor are endocrine disrupters. 


  • Put greater emphasis on encouraging people to stop their use of Weed and Feed
  • Consider periodically testing your local water sources for the presence of commonly used pesticides like 2,4-D and glyphosate. Consult with staff in your local USGS office to determine which products to test for.
  • Remind people that pesticides can travel long distances from where they are applied.
  • Recommend that water utilities and all homeowners and renters not use pesticides close to their water sources, especially in areas of shallow aquifers.


Quality of Streamwater in the Puget Sound Basin - A Decade of Study and Beyond, Sandra Embrey and Patrick Moran, Washington Water Science Center, Tacoma, WA, 2006

Pesticides in Groundwater of the United States: Decadal-Scale Changes, 1993–2011 by Patricia L. Toccalino1, Robert J. Gilliom2, Bruce D. Lindsey3, and Michael G. Rupert4, United States Geological Survey

Pesticides in the Nation's Streams and Groundwater, 1992 - 2001 - A Summary, USGS

May 2016 conversation with John Clemons, USGS Outreach Coordinator, 253-552-1635,

Review of Grow Smart Grow Safe database for presence of pesticides mentioned in USGS study

Review of Garden Green cards database based on Pharos database for presence of pesticides mentioned in USGS study

‘Threatened Waters’ by Nichelle Harriott and Jay Feldman, Beyond Pesticides, 2011

'Pesticides in My Drinking Water', Drew Toher, Beyond Pesticides

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