Prepared 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 

Occurrence of Pesticides in Ground Water of 

Wyoming, 1995–2006 

By Timothy T. Bartos, Cheryl A. Eddy-Miller, and Laura L. Hallberg 

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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 occurrence. Sampling focused on the 

State’s ground water that was mapped 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 combination of sensitivity and associated 

land use. 

Because of variations in reporting limits among different 

compounds and for the same compound during this study, 

pesticide detections were recensored 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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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 statistical methods were used 

exclusively for quantitative comparisons of pesticide detection 

frequencies between seasons and among various natural and 

anthropogenic (human-related) characteristics. 

One or more pesticides were detected 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 pesticides 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 sampled in the fall (28 different 

pesticides) than in the spring (21 different pesticides). Many 

pesticides were detected infrequently as nearly one-half of 

pesticides detected in the fall and spring at concentrations 

greater than the CSALs were detected 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 concentrations greater than the CSAL. 

Concentrations of detected pesticides generally were small (less 

than 1 microgram per liter), although many infrequent detections 

at larger concentrations were noted. All detected pesticide 

concentrations were smaller than U.S. Environmental Protection 

Agency (USEPA) drinking-water standards or applicable health 

advisories. Most concentrations were at least an order of 

magnitude smaller; however, many 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 Basin and High Plains/ Casper Arch 

geographic areas of north-central and southeastern Wyoming. 

Prometon was the only pesticide detected in all eight geographic 

areas of the State. 

Pesticides were detected much more 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 rangeland/undeveloped areas. The 

frequency of pesticide detection in samples from wells located in 

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Cheyenne, WY 82001

Phone : (307) 778-2931 

http://wy.water.usgs.gov/

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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 water were examined in relation to 

hydrogeology (aquifer type, water-level depth, well depth, and 

well type). The percentage of wells 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 between 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 among different categories of well 

depth and well type. The proportion of samples with pesticides 

detected at concentrations greater than their respective CSALs 

(detection frequency) was significantly different among waterleveldepth categories for one pesticide—deethylatrazine (spring 

but not fall samples). In most cases, pesticide detection 

frequencies decreased as well depth increased, but no clear 

increase or decrease in pesticide detection frequencies was noted 

as water-level-depth increased. The proportion of samples with 

pesticides detected at concentrations greater than their 

respective CSALs was significantly 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 detected (diuron) in the spring. 

Pesticide detections in ground water 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 within a 500-meter (1,640-foot) 

radius surrounding each sampled well. The proportion of samples 

with pesticides detected at concentrations greater than their 

respective CSALs was significantly different among organicmatter-content categories for four pesticides (atrazine, 

tebuthiuron, flumetsulam, and fipronil sulfide, listed in order of 

decreasing detection frequency) detected in the fall and in one 

pesticide (tebuthiuron) detected in the spring. The proportion of 

samples with pesticides detected at concentrations greater than 

their respective CSALs was significantly different among soilpermeability categories for three 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 decreasing 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 (nitrate) 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 pesticide tested, and the difference 

was significant between the two nitrate concentration categories 

for three pesticides (atrazine, prometon, and deethylatrazine, 

listed in order of decreasing detection 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 their 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 concentrations 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 samples 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 (diuron) in the spring. For many 

pesticides, no clear trend 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 between the two dissolved oxygen 

categories (representative of anoxic and oxic conditions) for one 

pesticide (tebuthiuron) in the fall. 

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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 Scientific 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 

Accessibility FOIA Privacy Policies and Notices 

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