Prepared in cooperation with the Wyoming Department of Agriculture and the Wyoming Department of Environmental Qual ity, on behalf of the Wyoming Ground-water and Pesticides Strategy Committee


# Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006

By Timothy T. Bartos, Cheryl A. Ed dy-Miller, and Laura L. Hallberg USGS Home Contact USGS Search USGS


## Scientific Investigations Report 2009–5024



## Abstract

Little existing information was available describing pesticide occurrence in ground water of Wyoming, so the U.S. Geological Survey, in cooperation with the Wyoming Department of Agriculture and the Wyoming Department of Environmental Quality on behalf of the Wyoming Ground-water and Pesticides Strategy Committee, collected ground-water samples twice (during late summer/early fall and spring) from 296 wells during 1995–2006 to characterize pesticide occurre nce. Sampling focused on the State’s ground water that was mapp ed as the most vulnerable to pesticide contamination because of either inherent hydrogeologic sensitivity (for example, shallow water table or highly permeable aquifer materials) or a combinatio n of sensitivity and associated land use.

Because of variations in repo rting limits among different compounds and for the same compound during this study, pesticide detections were recens ored to two different assessment levels to facilitate qualitative and quantitative examination of pesticide detection frequencies—a common assessment level (CAL) of 0.07 microgram per liter and an assessment level that  Report PDF (3.5 MB)

Supplemental Data—USGS Fact Sheets published 1998

– 2007

Fact Sheet 2005

- 3087 Fact Sheet 124

- 01 Fact Sheet 2006

- 3123 Fact Sheet 2006

- 3122 Fact Sheet 2005

- 3089 Fact Sheet 070

- 02 Fact Sheet 035

- 00 Fact Sheet 065

- 98 Fact Sheet 2005

- 3088 Fact Sheet 069

- 02 Fact Sheet 034

- 00 Fact Sheet 033

- 00 Fact Sheet 2004

- 3079 Fact Sheet 2007

- 3024 Fact Sheet 097

- 98 Fact Sheet 068

- 02 Fact Sheet 123

- 01 Fact Sheet 2004

- 3078 Fact Sheet 2006

- 3121 Fact Sheet 2004

- 3080 Fact Sheet 2004

- 3093 Posted March 12, 2009

For additional information contact: Director, Wyoming Water Science Center 2617 E. Lincolnway, Suite B 

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5/14/2010 htt p :// p ubs.us g s. g ov/sir/2009/5024 /differed by compound, referred to herein as a compound-specific assessment level (CSAL). Because of severe data censoring (fewer than 50 percent of the data are greater than laboratory reporting limits), categorical st atistical methods were used exclusively for quantitative comp arisons of pesticide detection frequencies between seasons an d among various natural and anthropogenic (human-rel ated) characteristics.

One or more pesticides were dete cted at concentrations greater than the CAL in water from about 23 percent of wells sampled in the fall and from about 22 percent of wells sampled in the spring. Mixtures of two or more pesticid es occurred at concentrations greater than the CAL in about 9 percent of wells sampled in the fall and in about 10 percent of wells sampled in the spring. At least 74 percent of pesticides detected were classified as herbicides. Considering only detections using the CAL, triazine pesticides were detected much more frequently than all other pesticide classes, and the number of different pesticides classified as triazines was the largest of all classes.

More pesticides were detected at concentrations greater than the CSALs in water from wells samp led in the fall (28 different pesticides) than in the spring (21 different pesticides). Many pesticides were detected infreq uently as nearly one-half of pesticides detected in the fall and spring at concentrations greater than the CSALs were detect ed only in one well. Using the CSALs for pesticides analyzed for in 11 or more wells, only five pesticides (atrazine, prometon, tebuthiuron, picloram, and 3,4- dichloroaniline, listed in order of decreasing detection frequency) were each detected in water from more than 5 percent of sampled wells. Atrazine was the pesticide detected most frequently at concentratio ns greater than the CSAL.

Concentrations of detected pestic ides generally were small (less than 1 microgram per liter), althou gh many infrequent detections at larger concentrations were noted. All detected pesticide concentrations were smaller than U.S. Environmental Protection Agency (USEPA) drinking-water st andards or applicable health advisories. Most concentrations were at least an order of magnitude smaller; however, ma ny pesticides did not have standards or advisories.

The largest percentage of pesticide detections and the largest number of different pesticides detected were in samples from wells located in the Bighorn Basi n and High Plains/ Casper Arch geographic areas of north-cent ral and southeastern Wyoming. Prometon was the only pesticide de tected in all eight geographic areas of the State.

Pesticides were detected much mo re frequently in samples from wells located in predominantly urban areas than in samples from wells located in predominantly agricultural or mixed areas. Pesticides were detected distinctly less often in samples from wells located in predominantly ra ngeland/undeveloped areas. The frequency of pesticide detection in samples from wells located in Part or all of this report is presented in Portable Document Format (PDF); the latest version of Adobe Reader or similar software is required to view it. Download the latest version of Adobe Reader, free of charge

. Cheyenne, WY 82001 Phone : (307) 778-2931 http://wy.water.usgs.gov/ 

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5/14/2010 htt p :// p ubs.us g s. g ov/sir/2009/5024 /either predominantly agricultural or mixed land-use areas was intermediate to those samples from urban and rangeland/undeveloped areas. Using the CAL, the proportion of wells with at least one pesticide detected was significantly different among the four land-use categories for both the fall and the spring.

Pesticide detections in ground wate r were examined in relation to hydrogeology (aquifer type, wa ter-level depth, well depth, and well type). The percentage of we lls with at least one pesticide detected was larger for wells completed in unconsolidated-deposit aquifers than for wells completed in bedrock aquifers. Using the CAL, the proportion of wells with at least one pesticide detected was significantly different betw een unconsolidated and bedrock aquifers sampled in the fall but not in the spring. Also using the CAL, the proportion of wells with at least one pesticide detected was significantly different amon g different categories of well depth and well type. The proportion of samples with pesticides detected at concentrations grea ter than their respective CSALs (detection frequency) was sign ificantly different among waterleveldepth categories for one pe sticide—deethylatrazine (spring but not fall samples). In most cases, pesticide detection frequencies decreased as well depth increased, but no clear increase or decrease in pesticid e detection frequencies was noted as water-level-depth increased. The proportion of samples with pesticides detected at concentrations greater than their respective CSALs was significan tly different among different categories of well depth and well type for three pesticides (prometon, tebuthiuron, and bromacil, listed in order of decreasing detection frequency) detected in both the fall and spring, and for one pesticide dete cted (diuron) in the spring.

Pesticide detections in ground wate r were examined in relation to selected soil properties (organic matter content, soil permeability, and soil hydrologic index) from the State Soil Geographic (STATSGO) database mapped wi thin a 500-meter (1,640-foot) radius surrounding each sampled well. The proportion of samples with pesticides detected at co ncentrations greater than theirrespective CSALs was significantly different among organicmatter-content categories for four pesticides (atrazine, tebuthiuron, flumetsulam, and fipronil sulfide, listed in order of decreasing detection frequency) de tected in the fall and in one pesticide (tebuthiuron) detected in the spring. The proportion of samples with pesticides detected at concentrations greater thantheir respective CSALs was significantly different among soilpermeability categories for thr ee pesticides (deethylatrazine, tebuthiuron, and aldicarb sulfone, listed in order of decreasing detection frequency) detected in the fall and for two pesticides (atrazine and tebuthiuron) detected in the spring. The proportion of samples with pesticides detected at concentrations greater than their respective CSALs was significantly different among soil-hydrologic-index categories (derived from several soil properties) for two pesticides (atrazine and deethylatrazine) detected in both fall and spring. For most pesticides, no clear trend of increasing or decreasi ng frequency of detection was 

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## Suggested citation: noted as organic matter content and soil permeability increased, and there was not a clear trend among the soil-hydrologic-index categories.

One water-quality constituent (nit rate) and three different waterquality characteristics (specific conductance, pH, and dissolved oxygen) were examined in relation to the occurrence of pesticides in ground water. Using the CAL, the proportion of wells with at least one pesticide detected was significantly different between the nitrate concentration categories of less than and greater than or equal to 1.1 milligrams per liter for the fall and the spring. The proportion of samples detected at concentrations greater than their respective CSALs was much larger for the “elevated” nitrate concentration category than for the other nitrate concentration category for almost every pestic ide tested, and the difference was significant between the two ni trate concentration categories for three pesticides (atrazine, prometon, and deethylatrazine, listed in order of decreasing dete ction frequency) in both the fall and spring, one pesticide (tebuthiuron) in the fall, and one pesticide (bromacil) in the spring.

The proportion of samples with pesticides detected at concentrations greater than thei r respective CSALs (detection frequency) was much larger for the saline-water category than for the freshwater category (on the basis of specific conductance) for almost every pesticide tested. The proportion of samples with pesticides detected at concen trations greater than their respective CSALs was significantly different between the two specific conductance categories for one pesticide (tebuthiuron) in both the fall and spring and for one pesticide (prometon) only in the fall.

Using the CAL, the proportion of wells with at least one pesticide detected was significantly different among the three pH categories—acidic, circumneutral, and alkaline—for the fall and spring. The proportion of sample s with pesticides detected at concentrations greater than their respective CSALs was significantly different among the three pH categories for three pesticides (prometon, tebuthiuron, and bromacil, listed in order of decreasing detection frequency) detected in both the fall and spring and for one pesticide (diu ron) in the spring. For many pesticides, no clear tr end of increasing or decreasing frequency detection was noted as pH increased.

The proportion of samples with pesticides detected at concentrations greater than their respective CSALs was significantly different betwee n the two dissolved oxygen categories (representative of anox ic and oxic conditions) for one pesticide (tebuthiuron) in the fall. 

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5/14/2010 htt p :// p ubs.us g s. g ov/sir/2009/5024 /Bartos, T.T., Eddy-Miller, C.A., and Hallberg, L.L., 2009, Occurrence of pesticides in ground water of Wyoming, 1995–2006: U.S. Geological Survey Scient ific Investigations Report 2009–5024, 72 p.



## Contents

Abstract Introduction Methods of Investigation Summary of Pesticide Occurrence Relation of Pesticide Occurrence to Selected Factors Summary Selected References

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