U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2009–5024

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
Front cover photograph: Rural well sampled in Platte County, September 14, 2008. Photograph by Seth Davidson, U.S. Geological 

Survey.

Back cover photographs: Upper photographs left to right: (1) Hydrologist processing water sample, August 2003. Photograph by 

Cheryl Miller, U.S. Geological Survey. (2) Hydrologist using an electric tape to measure water level, 

Goshen County, August 1998. Photograph by Laura Gianakos, U.S. Geological Survey. (3) Hydrologist 

beginning sampling of domestic well in Goshen County, August 1998. Photograph by Cheryl Miller, U.S. 

Geological Survey. 

Lower photograph: Rural well sampled in Platte County, September 14, 2008. Photograph by Seth 

Davidson, U.S. Geological Survey.
Occurrence of Pesticides in Ground Water 

of Wyoming, 1995–2006

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

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

Scientific Investigations Report 2009–5024

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

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reproduce any copyrighted materials contained within this report.

Suggested citation:

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.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................2

Purpose and Scope ..............................................................................................................................3

Acknowledgments................................................................................................................................3

Methods of Investigation..............................................................................................................................3

Selection of Sampling Locations........................................................................................................3

Sample Collection and Chemical Analyses......................................................................................4

Data Reporting and Treatment............................................................................................................4

Laboratory Reporting Limits.......................................................................................................4

Recensoring and Assessment Levels ....................................................................................14

Quality-Control Samples and Quality Assurance ..........................................................................15

Blank Samples............................................................................................................................15

Replicate Samples.....................................................................................................................15

Field-Matrix Spike Samples .....................................................................................................15

Statistical Methods.............................................................................................................................17

Summary of Pesticide Occurrence...........................................................................................................18

Detection Frequency by Well Sampled...........................................................................................18

Occurrence of Pesticide Mixtures...................................................................................................18

Pesticide Detections by Type............................................................................................................18

Pesticide Detections by Class ..........................................................................................................19

Individual Pesticide Detections........................................................................................................19

Detected Concentrations and Calculated Detection Frequencies....................................26

Detected Pesticide Concentrations and Comparison to Standards 

or Health Advisories ....................................................................................................32

Detected Pesticides in Relation to Use..................................................................................32

Relation of Pesticide Occurrence to Selected Factors.........................................................................34

Geography............................................................................................................................................34

Land Use...............................................................................................................................................40

Hydrogeology.......................................................................................................................................41

Aquifer Type................................................................................................................................41

Water-Level Depth.....................................................................................................................53

Well Depth...................................................................................................................................53

Well Type.....................................................................................................................................60

Soils ..................................................................................................................................................60

Organic Matter Content............................................................................................................61

Soil Permeability ........................................................................................................................61

Soil Hydrologic Index ................................................................................................................63

Nitrate and Selected Water-Quality Characteristics....................................................................63

Nitrate .........................................................................................................................................63

Specific Conductance...............................................................................................................64

pH .........................................................................................................................................64

Dissolved Oxygen.......................................................................................................................66

Summary........................................................................................................................................................66

Selected References...................................................................................................................................70

..........

.....
iv

Figures

1. Mapped ground-water vulnerability to pesticide contamination and 

sampled wells, Wyoming, 1995–2006 ........................................................................................5

2–7. Graphs showing:

2. Pesticide concentrations and percentage of wells with at least 

one pesticide detected for fall and spring, Wyoming, 1995–2006 .............................19

3. Number of pesticides detected per well in ground-water samples 

collected in Wyoming, 1995–2006 ...................................................................................20

4. Pesticide detections relative to pesticide type in ground-water 

samples collected in Wyoming, 1995–2006 ...................................................................21

5. Pesticide concentrations and detections relative to pesticide 

class in ground-water samples collected in Wyoming, 1995–2006...........................25

6. Concentration and detection frequency of selected pesticides 

in ground-water samples collected in Wyoming, 1995–2006 .....................................30

7. Parent pesticides detected in ground-water samples collected 

during fall and spring, 1995–2006, relative to reported 1994 

agricultural use in Wyoming............................................................................................33

8. Map showing geographic areas and ground-water wells sampled 

in Wyoming during 1995–2006 ..................................................................................................35

9–12. Graphs showing:

9. Pesticide concentrations and percentage of wells with at least 

one pesticide detected relative to land-use category in Wyoming, 

1995–2006 ............................................................................................................................52

10. Selected pesticide concentrations and detections in ground-water 

samples collected relative to land use, Wyoming, 1995–2006 ...................................54

11. Pesticide detections in ground-water samples collected relative 

to aquifer type, Wyoming, 1995–2006 .............................................................................57

12. Pesticide concentrations and percentage of wells with at least 

one pesticide detected relative to aquifer type, Wyoming, 

1995–2006 ............................................................................................................................58

Tables

1. Pesticides analyzed, trade names, pesticide actions, classes, 

laboratory reporting limits, and U.S. Environmental Protection 

Agency standards or health advisories ....................................................................................6

2. Detected pesticides in ground-water samples collected during fall 

and spring using original National Water Quality Laboratory censoring, 

laboratory reporting limit range, and assigned assessment levels, 

Wyoming, 1995–2006 ..................................................................................................................16

3. Pesticide detections relative to pesticide class in ground-water 

samples collected during fall and spring in Wyoming, 1995–2006 .....................................22

4. Summary of pesticide detections and concentrations in ground-water 

samples collected during the fall in Wyoming, 1995–2006 ..................................................27

5. Summary of pesticide detections and concentrations in ground-water 

samples collected during the spring in Wyoming, 1995–2006.............................................29
v

6. Results of contingency-table analysis comparing selected pesticide 

detections in ground-water samples collected during fall and spring 

in Wyoming, 1995–2006..............................................................................................................32

7. Pesticide detections in ground-water samples collected during fall 

and spring relative to eight regional geographic areas in Wyoming, 

1995–2006 .....................................................................................................................................36

8. Results of contingency-table analysis comparing selected pesticide 

detections in ground-water samples collected during fall and spring 

from eight regional geographic areas in Wyoming, 1995–2006 ..........................................40

9. Pesticide detection frequencies and concentrations in ground-water 

samples collected during fall and spring relative to eight regional 

geographic areas in Wyoming, 1995–2006 .............................................................................42

10. Results of contingency-table analysis or Kruskal-Wallis test for 

ordered categorical responses (KWOCR) comparing selected 

pesticide detections in ground-water samples collected during 

fall and spring and land-use categories in Wyoming, 1995–2006.......................................56

11. Results of contingency-table analysis or Kruskal-Wallis test for 

ordered categorical responses comparing selected pesticide 

detections in ground-water samples collected during fall and spring 

and hydrogeologic characteristics in Wyoming, 1995–2006 ...............................................59

12. Results of Kruskal-Wallis test for ordered categorical responses 

comparing selected pesticide detections in ground-water samples 

collected during fall and spring and soil characteristics in Wyoming, 

1995–2006 .....................................................................................................................................62

13. Results of Kruskal-Wallis test for ordered categorical responses 

comparing selected pesticide detections in ground-water samples 

collected during fall and spring and nitrate and selected water-quality 

characteristics in Wyoming, 1995–2006..................................................................................65

Conversion Factors and Datums

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

meter (m) 3.281 foot (ft)

mile (mi) 1.609 kilometer (km)

Volume

ounce, fluid (fl. oz) 0.02957 liter (L) 

Flow rate

inch per hour (in/h) 0.0254 meter per hour (m/h)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25°C).

Horizontal coordinate information is reference to the North American Datum of 1983 (NAD 83).
vi

Abbreviations and Acronyms

Abbreviated Water-Quality Units Used in This Report:

mg/L milligram per liter

μg/L microgram per liter

μS/cm microsiemens per centimeter at 25 degrees Celsius

Abbreviations and Acronyms Used in This Report:

< less than

> greater than

≤ less than or equal to

CAL common assessment level

CSAL compound-specific assessment level

CTA contingency-table analysis

DO dissolved oxygen

GIS geographic information system

GPSC Ground-water and Pesticides Strategy Committee

KWOCR Kruskal-Wallis test for ordered categorical responses

LHA Lifetime Health Advisory

LRL laboratory reporting level
vii

LT-MDL long-term method detection level

MCL Maximum Contaminant Level

MDL method detection limit

MRL minimum reporting level

NAL no assessment level 

NAWQA National Water-Quality Assessment

NLCD National Land Cover Data

NWIS National Water Information System of U.S. Geological Survey

NWQL National Water Quality Laboratory of U.S. Geological Survey

QA quality assurance

QC quality control

RPD relative percentage difference

RSD Risk-Specific Dose

RSD4 Risk-Specific Dose at 10-4 Cancer Risk

SMP State of Wyoming Generic Management Plan for Pesticides in Ground Water

SSURGO Soil Survey Geographic database

STATSGO State Soil Geographic database

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

WDEQ Wyoming Department of Environmental Quality

Occurrence of Pesticides in Ground Water of Wyoming, 

1995–2006

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

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

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 water-leveldepth 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 organic-matter-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 soil-permeability 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 soilhydrologic-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 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 

water-quality 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. 

Introduction

Since 1986, the U.S. Environmental Protection Agency 

(USEPA) has been developing a strategy to address groundwater contamination by pesticides and other agricultural 

chemicals. In October 1991, the USEPA completed the “Pesticides and Ground-Water Strategy” that describes, in part, a 

new Federal-State partnership approach to address potential 

risks posed to ground water by the use of pesticides (Wyoming 

Ground-water and Pesticides Strategy Committee, 1999). In 

response, the State of Wyoming created the Ground-water and 

Pesticide Strategy Committee (GPSC) consisting of members 

of local, State, and Federal government, as well as industry 

and interest groups, to prepare the State of Wyoming Generic 

Management Plan for Pesticides in Ground Water (SMP; 

Wyoming Ground-water and Pesticides Strategy Committee, 

1999). The SMP includes information describing individuals and organizations involved with implementation of the 
Methods of Investigation 3

SMP, ground-water contamination prevention, ground-water 

monitoring, and required responses if pesticides are detected 

in ground water.

 In Wyoming, little existing information was available 

describing pesticide occurrence in ground water. In accordance 

with the SMP, the GPSC began a program to conduct “baseline ground-water sampling” to characterize current pesticide 

occurrence in Wyoming’s ground water. In addition, the GPSC 

identified 20 pesticides (18 parent pesticides and 2 degradates) to be of greatest interest during baseline ground-water 

sampling (defined as “focal compounds” and referred to herein 

as “focal pesticides”). 

The U.S. Geological Survey (USGS), in cooperation with 

the Wyoming Department of Agriculture and the Wyoming 

Department of Environmental Quality (WDEQ) and acting on 

behalf of the GPSC, began statewide implementation of this 

baseline ground-water sampling in 1995. The objectives of the 

baseline sampling were to:

1. Summarize the occurrence of pesticides in ground water 

in the State of Wyoming, and

2. Evaluate the occurrence of pesticides in relation to 

selected natural and anthropogenic (human-related) 

characteristics such as geography, land use, hydrogeology, 

soils, and selected water-quality constituents and characteristics.

As part of this baseline sampling, water samples were 

collected during fall and spring from 296 wells in Wyoming 

during 1995–2006 and analyzed for pesticides. To date, data 

from these ground-water samples have been published in 

the USGS Annual Water Data Reports for Wyoming (Water 

Years 1995–1999 are available as hardcopy only and are listed 

at http://wy.water.usgs.gov/pubs/statebiblio/data.htm#wdrn

and Water Years 2000–2006 are available online at http://

wy.water.usgs.gov/pubs/adr/index.htm) and summarized by 

county in a series of 22 USGS Fact Sheets (listed individually in the “Selected References” section at the end of this 

report and provided on a CD-ROM at the back of the report). 

Ground-water-quality data for these and all USGS waterquality samples can be obtained on the Worldwide Web from 

the National Water Information System (NWISWeb at http://

waterdata.usgs.gov/wy/nwis/). Descriptions of wells sampled 

for this study are available in NWISWeb and USGS Annual 

Data Reports for Wyoming.

Purpose and Scope

The purposes of this report are to (1) present a summary 

on the occurrence of pesticides in ground water in the State 

of Wyoming during 1995–2006 and to (2) describe relations between pesticide occurrence and selected natural and 

anthropogenic factors. The summary of pesticide occurrence 

includes detection frequencies; occurrence of mixtures; detections by type, class, and individual pesticides; concentrations 

and comparisons to USEPA standards and health advisories; 

and detections in relation to pesticide use. Natural and anthropogenic factors that were evaluated for relations with pesticide 

occurrence include geography, land use, hydrogeology, soils, 

nitrate, and other water-quality characteristics. 

Acknowledgments

The authors gratefully acknowledge the generous 

assistance of landowners throughout Wyoming who provided 

access to their property and wells for the baseline sampling. 

The authors also acknowledge assistance with site selection from the many Conservation Districts and County Weed 

and Pest Districts in Wyoming. David L. Lorenz (USGS) is 

thanked for his invaluable assistance and guidance with the 

categorical statistical methods used in this report.

Methods of Investigation

This section describes the methods used for the selection 

of sampling locations, collection of ground-water samples 

and analysis for pesticides using USGS laboratory analytical methods and reporting conventions, and the collection 

and analysis of quality-control samples as part of a qualityassurance program. Finally, the approach used to censor data, 

assemble final data sets, and analyze data using statistical 

methods is described.

Selection of Sampling Locations

Baseline ground-water sampling to describe the occurrence and distribution of pesticides in ground water of 

Wyoming began in 1995 and was completed in 2006. During 

this time, water samples were collected twice—once each 

during late summer/fall (August through November and 

referred to herein as “fall”) and spring (March through 

May)—from 296 wells (except for one well that was only 

sampled in the fall). The wells were located in areas of “high” 

or “medium-high” ground-water vulnerability to pesticide 

contamination that were mapped for the entire State of 

Wyoming (Hamerlinck and Arneson, 1998) as part of the 

SMP. The most vulnerable ground water was determined for 

the uppermost or shallowest aquifers in Wyoming by using a 

geographic information system (GIS) to overlay and combine 

digital information describing hydrogeologic and land-use 

characteristics (Hamerlinck and Arneson, 1998). Using this 

procedure, ground water was identified and mapped as vulnerable due to inherent “sensitive” hydrogeologic characteristics 

(for example, shallow water table or highly permeable aquifer 

materials) and overlying land use involving the application of 

pesticides. 

The ground-water vulnerability map (fig. 1) then was 

used in combination with cropland extent, percentage of 

urban area, and 1991 pesticide sales in each county (Wyoming 
4 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006

Ground-water and Pesticides Strategy Committee, 1999) to 

evaluate and rank each county in order of highest to lowest 

ground-water vulnerability to pesticide contamination. Each 

Wyoming county then was sampled in order of this predetermined rank (fig. 1). Some county ranks were elevated 

above others due to previous pesticide detections in historical ground-water-quality samples. All 23 Wyoming counties 

were sampled on the basis of ranked order with the exception of Goshen County (southeastern Wyoming), which was 

selected as a pilot study, and Niobrara and Weston Counties 

(east-central Wyoming), which were combined due to minimal 

mapped vulnerability. The majority of the wells sampled 

during the study were located in the most vulnerable areas 

mapped in each county (shown “high” vulnerability in fig. 1), 

with a few sampled wells located in the second-most vulnerable areas (“medium-high” vulnerability in fig. 1). 

Sample Collection and Chemical Analyses

Ground-water samples were collected and processed 

in a mobile water-quality laboratory using USGS National 

Water-Quality Assessment (NAWQA) procedures (Koterba 

and others, 1995) and the USGS National Field Manual for 

the Collection of Water-Quality Data (U.S. Geological Survey, 

1997–2006). Samples were sent to the USGS National Water 

Quality Laboratory (NWQL) in Denver, Colo., for analysis using different methods selected to detect 20 pesticides 

(18 parent pesticides and 2 pesticide degradates) identified 

in the SMP as focal pesticides of greatest concern (table 1). 

The USGS NWQL analytical methods selected to detect 

focal pesticides in ground-water samples also could detect 

many additional nonfocal (as many as 136) pesticides (table 

1). Ground-water samples were analyzed for all pesticides 

listed in table 1, with the exception of difenzoquat (not listed 

in table 1), for which analytical methods were not available. 

However, some of these pesticides listed in table 1 were 

analyzed for in relatively few samples because of changes 

to laboratory analytical schedules during the baseline study. 

Samples collected to detect the semivolatile pesticide cis- and 

trans-1,3–dichloropropene (trade name telone) were sent to 

TestAmerica Laboratory (formerly known as Quanterra and 

Severn Trent) in Denver, Colo., for analysis using USEPA 

method 8260B (U.S. Environmental Protection Agency, 1986).

Data Reporting and Treatment

Ground-water samples were analyzed using USGS 

analytical methods with laboratory reporting limits much 

smaller than typically used in routine pesticide monitoring 

of public drinking-water supplies. Consequently, it is likely 

that much more frequent rates of detection were obtained 

than would have been possible with less sensitive analytical methods. Reporting limits varied by pesticide, and many 

reporting limits changed during the course of the study, so 

pesticide detections were recensored to account for variations 

in reporting limits among the different compounds and for the 

same compound before data were summarized and analyzed. 

Detections were recensored to two different assessment levels 

to facilitate qualitative and quantitative examination of pesticide detection frequencies. The types of reporting limits used 

and the process used to recensor data and assemble final data 

sets for analysis are discussed in this section of the report.

Laboratory Reporting Limits

During this study, the USGS NWQL reported analytical 

results for pesticides relative to three types of reporting limits, 

referred to as “original NWQL censoring.” Very small concentrations are censored and reported as “less than” values by the 

NWQL to avoid false-positive detections (reporting detections 

when the analytes are not actually present in the sample). 

Censoring levels, generally known as “reporting limits,” are 

specific to analytical methods and can change over time as 

methods change.

The oldest and most basic of three types of reporting 

limits used by the NWQL during this study was the minimum 

reporting level (MRL), which is defined as the minimum 

concentration of a constituent that can be reliably measured 

using a given analytical method (Timme, 1995). This is the 

least “robust” of three reporting limits used during this study 

for pesticide analyses as “establishment of the MRL has been 

inconsistent across methods and typically inadequately defined 

and often undocumented” (Childress and others, 1999, p. 2). 

Use of the MRL was infrequent during this study as it was 

applied only to one laboratory method (NWQL laboratory 

schedule 1379) that was used for selected pesticide analyses in 

25 ground-water samples collected during fall and spring 1995 

in Goshen County (southeastern Wyoming) and for selected 

pesticide analyses in 25 ground-water samples collected 

during spring 1997 in Park County (northwestern Wyoming). 

The second of three types of reporting limits used by 

the NWQL during this study was the method detection limit 

(MDL), which is defined as the minimum concentration of 

a constituent that can be identified, measured, and reported 

with 99-percent confidence to be significantly greater than 

zero (Childress and others, 1999). The NWQL determined the 

MDL using the “USEPA MDL procedure” (U.S. Environmental Protection Agency, 1997). At the MDL concentration, the 

risk of false-positive detection is no more than 1 percent. The 

MDL generally is less than and more statistically defined than 

the MRL. Some detections greater than or less than the MDL 

were qualified using an “E” remark code because of measurement uncertainty (U.S. Geological Survey Water Quality 

Laboratory Technical Memorandum 94.12, 1994). Values 

given an “E” remark code are considered semiquantitative. 

The NWQL set the reporting limit equivalent to the MDL for 

many pesticide analyses during this study including NWQL 

analytical schedules 2001 (Zaugg and others, 1995) and 2060 

(Furlong and others, 2001) and for many of the analytes in 

schedule 2050 (U.S. Geological Survey, 1997). 
Alcova

Reservoir

L

Cody

Basin

Casper

Lander

Laramie

Rawlins

Douglas

Jackson

Worland

Buffalo

Cheyenne

Evanston

Kemmerer

Pinedale

Gillette

Sun

Sheridan

Wheatland

New

GreenRiver

Thermopolis

YELLOWSTONE

NATIONAL

PARK 90

25

25

80

90

111°

110°

109° 108° 107° 106° 105°

41°

42°

43°

44°

45°

00

50

50

100 MILES

100 KILOMETERS

Base from U.S. Geological Survey digital data, 2003, 1:2,000,000Albers Equal-Area Conic projectionStandard parallels 29°30′N and 45°30′N, central meridian 107°30′W

Ground-water vulnerability mHamerlinck and Arneson (199

River

Sweetwater

PathfinderReservoir

Seminoe Reservoir

BoysenReservoir

 KeyholeReservoir

GlendoReservoir

Guernsey Reservoir

Fontenelle Reservoir

WheatlandReservoir

River

River

Platte North

Laramie

Cheyenne

Rive

Belle

Little Missouri

River

Fourche

River

Tongue

Powder

R

Lake DeSmet

Bighorn  Lake

Bighorn Shoshone

R Buffalo Bill

Reservoir

River

South

Fork

Greybull

River

River

Wind

River

Snake

River

Greys

River

Fork New

Green

Fork

River

Hams

River

Henrys Fork

River

River

Big Sandy

North Platte

River

River

Popo Agie

Medicine Bow

Fork

B

Blacks

ear R Smiths Fork

River Salt Afton

Saratoga

Glenrock

Riverton

Rock Springs

Lovell

Powell

Yellowstone

Lake

Jackson

Lake

Flaming GorgeReservoir

Gros Ventre River

PARK

(2)

BIG HORN

(7) 

SHERIDAN

(8)

WASHAKIE

(1)

JOHNSON

(10)

CAMPBELL

(21)

CROOK

(11)

WESTON

(13)

TETON

(15)

HOT SPRINGS

(19)

LINCOLN

(5)

UINTA

(16) SWEETWATER

(14)

NATRONA

(12)

CONVERSE

(18)

ALBANY

(17)

CARBON

(20)

LARAM

(6)

GO

PLATTE

(9)

NIOBRA

(23)

FREMONT

(4)

SUBLETTE

(22)

Figure 1. Mapped ground-water vulnerability to pesticide contamination (from Hamerlinck and Arneson, 1998) and sampled wells, Wyoming, 1995–2
Table 1. Pesticides analyzed, trade names, pesticide actions, classes, laboratory reporting limits, and U.S. Environmental Protection Agency standa

—Continued[Compounds detected with original censoring during study are in bold type; USGS, U.S. Geological Survey; μg/L, micrograms per liter; USEPA, U.S. Environmental ProtectioU, chemical was analyzed for, but not detected; analytical method type: 1 = USGS laboratory schedule 2001, 2 = USGS laboratory schedule 2050, 3 = USGS laboratory schedulschedule 2060, 5 = USGS laboratory schedule 2033; 6 = Severn Trent Laboratories, USEPA method 8260B; USEPA standard or health advisory: RSD4 = U.S. Environmental PDose at 10-4 Cancer Risk (U.S. Environmental Protection Agency, 2006); MCL = U.S. Environmental Protection Agency Maximum Contaminant Level (U.S. Environmental ProLHA = U.S. Environmental Protection Agency Lifetime Health Advisory (U.S. Environmental Protection Agency, 2006)]

Pesticide Trade name Pesticide action

Pesticide class 

(or parent compound 

class)

USGS 

laboratory parameter 

code

Analytical 

method1

(see headnote)

Minimum 

laboratory 

reporting 

limit 

(μg/L)

M

l

Acetochlor Guardian, Harness, Relay Preplant herbicide Amide/chloroacetamide 49260 1, 3, 5 0.002 Acifluorfen Blazer, Tackle 2S, Astic Herbicide Miscellaneous acid 49315 2, 4 .007 Alachlor2 Alanex, Lasso, Shroud Herbicide Amide/acetanilide 46342 1, 3, 5 .002 Aldicarb2 Temik Insecticide, nematicide, acaricide Carbamate 49312 2, 4 .02 Aldicarb sulfone2 Aldicarb degradate, Standak, 

Aldoxycarb

-- Carbamate 49313 2, 4 .02 

Aldicarb sulfoxide2 Aldicarb degradate -- Carbamate 49314 2, 4 .008 Ametryn Evik Herbicide Triazine 38401 3 .05 Atrazine2 Aatrex, Atranex Herbicide Triazine 39632 1, 3, 4, 5 .001 Azinphos-methyl Guthion, Crysthyon Insecticide Organophosphate 82686 1, 5 .001 Bendiocarb Ficam, Garrox, Turcam Insecticide Carbamate 50299 4 .02 Benfluralin Balan, Benefin Herbicide Dinitroaniline 82673 1, 5 .002 Benomyl Benlate, Benex Fungicide Carbamate 50300 4 .004 Bensulfuron, methyl Escuri, Londax Herbicide Urea/sulfonylurea 61693 4 .02 Bentazon Basagram, Bentazone Herbicide Miscellaneous 38711 2, 4 .01 Bromacil2 Hyvar X Herbicide Miscellaneous 04029 2, 3, 4 .01 Bromoxynil Buctril, Brominal, Agristar Herbicide Miscellaneous 49311 2, 4 .01 Butachlor Butanex, Lambast, Machete Herbicide Amide 04026 3 .05 Butylate Sutan+, Genate Plus Herbicide Carbamate 04028 1, 3 .002 Carbaryl Carbatox, Sevin Insecticide Carbamate 49310 2, 4 .008 Carbaryl Carbatox, Sevin Insecticide Carbamate 82680 1, 5 .003 Carbofuran Furadan, Futura Insecticide Carbamate 49309 2, 4 .006 Carbofuran Furadan, Futura Insecticide Carbamate 82674 1, 5 .003 Carboxin Kisvax, Oxatin, Vitavax Fungicide Miscellaneous 04027 3 .05 
Table 1. Pesticides analyzed, trade names, pesticide actions, classes, laboratory reporting limits, and U.S. Environmental Protection Agency standa

—Continued[Compounds detected with original censoring during study are in bold type; USGS, U.S. Geological Survey; μg/L, micrograms per liter; USEPA, U.S. Environmental ProtectioU, chemical was analyzed for, but not detected; analytical method type: 1 = USGS laboratory schedule 2001, 2 = USGS laboratory schedule 2050, 3 = USGS laboratory schedulschedule 2060, 5 = USGS laboratory schedule 2033; 6 = Severn Trent Laboratories, USEPA method 8260B; USEPA standard or health advisory: RSD4 = U.S. Environmental PDose at 10-4 Cancer Risk (U.S. Environmental Protection Agency, 2006); MCL = U.S. Environmental Protection Agency Maximum Contaminant Level (U.S. Environmental ProLHA = U.S. Environmental Protection Agency Lifetime Health Advisory (U.S. Environmental Protection Agency, 2006)]

Pesticide Trade name Pesticide action

Pesticide class 

(or parent compound 

class)

USGS 

laboratory parameter 

code

Analytical 

method1

(see headnote)

Minimum 

laboratory 

reporting 

limit 

(μg/L)

M

l

Chloramben, methyl 

ester

Chloramben Herbicide Miscellaneous acid 61188 2, 4 0.01 

Chlorimuron Classic, Darban, Lory Herbicide Urea/sulfonylurea 50306 4 .01 2-Chloro-2’,6’-diethylacetanilide Butenachlor degradate -- Amide 61618 5 .005 4-Chloro-2-methylphenol MCPA degradate -- Urea 61633 5 .005 Chlorothalonil Bravo Fungicide Organochlorine 49306 2, 4 .01 Chlorpyrifos Dursban, Lorsban Insecticide Organophosphate 38933 1, 5 .004 Clopyralid2 Stinger, Lontrel Herbicide Pyridinecarboxylic 

acid

49305 2, 4 .01 

Cyanazine2 Bladex Selective herbicide Triazine 04041 1, 3, 5 .004 Cycloate Ro-Neet Selective herbicide Carbamate/thiocarbamate 04031 3, 4 .01 lambda-Cyhalothrin Commodore, Icon Insecticide Pyrethroid 61595 5 .009 Cyfluthrin Aztec, Bug-b-gon, Laser, 

Raid, Tempo

Insecticide Pyrethroid 61585 5 .027 

Cypermethrin Barricade, Cymbush Insecticide Pyrethroid 61586 5 .009 2,4–D2 Dacamine, Weed-B-Gon Herbicide Chlorophenoxy 39732 2, 4 .01 2,4–DB Butoxone, Butyrac Selective herbicide Chlorophenoxy 38746 2, 4 .01 2,4–D methyl ester -- Herbicide Chlorophenoxy 50470 4 0.009 Dacthal mono-acid Dacthal degradate -- Organochlorine 49304 2, 4 .01 DCPA2 Dacthal Herbicide Organochlorine 82682 1, 5 .002 p,p’–DDE DDT degradate -- Organochlorine 34653 1 .003 Deethylatrazine 

(CIAT)

Atrazine degradate -- Triazine 04040 1, 3, 4, 5 .002 
Table 1. Pesticides analyzed, trade names, pesticide actions, classes, laboratory reporting limits, and U.S. Environmental Protection Agency standa

—Continued[Compounds detected with original censoring during study are in bold type; USGS, U.S. Geological Survey; μg/L, micrograms per liter; USEPA, U.S. Environmental ProtectioU, chemical was analyzed for, but not detected; analytical method type: 1 = USGS laboratory schedule 2001, 2 = USGS laboratory schedule 2050, 3 = USGS laboratory schedulschedule 2060, 5 = USGS laboratory schedule 2033; 6 = Severn Trent Laboratories, USEPA method 8260B; USEPA standard or health advisory: RSD4 = U.S. Environmental PDose at 10-4 Cancer Risk (U.S. Environmental Protection Agency, 2006); MCL = U.S. Environmental Protection Agency Maximum Contaminant Level (U.S. Environmental ProLHA = U.S. Environmental Protection Agency Lifetime Health Advisory (U.S. Environmental Protection Agency, 2006)]

Pesticide Trade name Pesticide action

Pesticide class 

(or parent compound 

class)

USGS 

laboratory parameter 

code

Analytical 

method1

(see headnote)

Minimum 

laboratory 

reporting 

limit 

(μg/L)

M

l

Deethyl, deisopropyl-atrazine 

(CAAT)

Atrazine degradate -- Triazine 04039 4 0.04 

Deisopropylatrazine 

(CEAT)

Atrazine/cyanazine/simazine 

degradate

-- Triazine 04038 3, 4 .01 

Desulfinylfipronil Fipronil degradate -- Pyrazole 62170 1, 5 .004 Desulfinylfipronil 

amide

Fipronil degradate -- Pyrazole 62169 1, 5 .009 

Diazinon Basudin, Spectracide, 

Knoxout

Insecticide, nematicide Organophosphate 39572 1, 5 .002 

Dicamba2 Banvel, Banex Herbicide Miscellaneous acid 38442 2, 4 .01 Dichlobenil Barrier, Casoron, Rootx Herbicide Organochlorine 49303 2 .02 3,4-Dichloroaniline Propanil degradate -- Urea 61625 5 .004 3,5-Dichloroaniline Iprodione degradate -- Miscellaneous 61627 5 .004 Dichlorprop Weedone, Polymone Herbicide Chlorophenoxy 49302 2, 4 .01 cis-1,3-Dichloropropene2 Telone Nematicide Miscellaneous 34704 6 .09 trans-1,3-Dichloropropene2 Telone Nematicide Miscellaneous 34699 6 .09 Dicrotophos Bidrin, Penetrex Insecticide Organophosphate 38454 5 .08 Dieldrin Panoram D-31, Octalox Insecticide Organochlorine 39381 1, 5 .001 2,6-Diethylaniline Alachlor degradate -- Amide/acetanilide 82660 1, 5 .002 Dimethoate Cygon, Defend, Rogor Insecticide Organophosphate 82662 1, 5 .006 Dinoseb Premerge Herbicide Miscellaneous 49301 2, 4 .01 Diphenamid Dymid, Enide Selective herbicide Amide 04033 3, 4 .01 Disulfoton Di-Syston Insecticide, acaricide Organophosphate 82677 1, 5 .02 Disulfoton sulfone Disyston sulfone Insecticide Organophosphate 61640 5 .01 
Table 1. Pesticides analyzed, trade names, pesticide actions, classes, laboratory reporting limits, and U.S. Environmental Protection Agency standa

—Continued[Compounds detected with original censoring during study are in bold type; USGS, U.S. Geological Survey; μg/L, micrograms per liter; USEPA, U.S. Environmental ProtectioU, chemical was analyzed for, but not detected; analytical method type: 1 = USGS laboratory schedule 2001, 2 = USGS laboratory schedule 2050, 3 = USGS laboratory schedulschedule 2060, 5 = USGS laboratory schedule 2033; 6 = Severn Trent Laboratories, USEPA method 8260B; USEPA standard or health advisory: RSD4 = U.S. Environmental PDose at 10-4 Cancer Risk (U.S. Environmental Protection Agency, 2006); MCL = U.S. Environmental Protection Agency Maximum Contaminant Level (U.S. Environmental ProLHA = U.S. Environmental Protection Agency Lifetime Health Advisory (U.S. Environmental Protection Agency, 2006)]

Pesticide Trade name Pesticide action

Pesticide class 

(or parent compound 

class)

USGS 

laboratory parameter 

code

Analytical 

method1

(see headnote)

Minimum 

laboratory 

reporting 

limit 

(μg/L)

M

l

Diuron Durashield, Karmex Herbicide Urea 49300 2, 4 0.01 DNOC (2-Methyl4,6-dinitrophenol) Dinitro-o-cresol, Elgetol Herbicide Miscellaneous 49299 2 .01 alpha-Endosulfan -- Insecticide Organochlorine 34362 5 .005 Endosulfan sulfate Endosulfan degradate -- Organochlorine 61590 5 .014 EPTC Eptam, Eradicane Herbicide Carbamate 82668 1, 5 .002 Ethalfluralin Eptam, Eradicane Herbicide Dinitroaniline 82663 1 .004 Ethion Klear-all, Rhodocide Insecticide Organophosphate 82346 5 .004 Ethion monoxon Ethion degradate -- Organophosphate 61644 5 .002 Ethoprop (Ethoprophos) Mocap, Prophos Insecticide, nemati- cide Organophosphate 82672 1, 5 .003 2-Ethyl-6-methylaniline Metolachlor degradate -- Amide 61620 5 .004 Fenamiphos Nemacur Insecticide Organophosphate 61591 5 .03 Fenamiphos sulfone Fenamiphos degradate -- Organophosphate 61645 5 .049 Fenamiphos sulfoxide Fenamiphos degradate -- Organophosphate 61646 5 .04 Fenuron Fenuron Herbicide Urea 49297 2, 4 .01 Fipronil Combat, Frontline, Maxforce, 

Regent

Insecticide Pyrazole 62166 1, 5 .007 

Fipronil sulfide Fipronil degradate -- Pyrazole 62167 1, 5 .005 Fipronil sulfone Fipronil degradate -- Pyrazole 62168 1, 5 .005 Flumetsulam Broadstrike, Python Herbicide Miscellaneous 61694 4 .01 Fluometuron Cotoran Herbicide Urea 38811 2, 4 .01 Fonofos Dyfonate Insecticide Organophosphate 04095 1, 5 .003 alpha-HCH -- Insecticide Organochlorine 34253 1, 3, 5 .002 
Table 1. Pesticides analyzed, trade names, pesticide actions, classes, laboratory reporting limits, and U.S. Environmental Protection Agency standa

—Continued[Compounds detected with original censoring during study are in bold type; USGS, U.S. Geological Survey; μg/L, micrograms per liter; USEPA, U.S. Environmental ProtectioU, chemical was analyzed for, but not detected; analytical method type: 1 = USGS laboratory schedule 2001, 2 = USGS laboratory schedule 2050, 3 = USGS laboratory schedulschedule 2060, 5 = USGS laboratory schedule 2033; 6 = Severn Trent Laboratories, USEPA method 8260B; USEPA standard or health advisory: RSD4 = U.S. Environmental PDose at 10-4 Cancer Risk (U.S. Environmental Protection Agency, 2006); MCL = U.S. Environmental Protection Agency Maximum Contaminant Level (U.S. Environmental ProLHA = U.S. Environmental Protection Agency Lifetime Health Advisory (U.S. Environmental Protection Agency, 2006)]

Pesticide Trade name Pesticide action

Pesticide class 

(or parent compound 

class)

USGS 

laboratory parameter 

code

Analytical 

method1

(see headnote)

Minimum 

laboratory 

reporting 

limit 

(μg/L)

M

l

gamma-HCH Lindane, Isotoz Insecticide Organochlorine 39341 1 0.004 Hexazinone2 Buckshot, Pronone, Velpar Herbicide Triazine 04025 3, 5 .013 Hydroxyatrazine 

(OIET)

Atrazine degradate -- Triazine 50355 4 .008 

3-Hydroxycarbofuran Carbofuran degradate -- Carbamate 49308 2, 4 .006 Imazaquin Scepter Herbicide Miscellaneous 50356 4 .02 Imazethapyr New Path, Pursuit Herbicide Miscellaneous 50407 4 .02 Imidacloprid Admire, Provado Insecticide Miscellaneous 61695 4 .007 Iprodione Chipco, Rovral Fungicide Dicarboximide 61593 5 .026 Isofenphos Amaze, Pryfon Insecticide Organophosphate 61594 5 .003 3-keto Carbofuran Carbofuran degradate -- Carbamate 50295 4 .01 Linuron Linurex, Lorox Herbicide Urea 38478 2, 4 .01 Linuron Linurex, Lorox Herbicide Urea 82666 1 .002 Malaoxon Malathion degradate -- Organophosphate 61652 5 .03 Malathion Cythion, Malaspray Insecticide Organophosphate 39532 1, 5 .005 MCPA Solve, MCP Herbicide Chlorophenoxy 38482 2, 4 .01 MCPB Butoxone M40, Thistrol Herbicide Chlorophenoxy 38487 2, 4 .01 Metalaxyl Apron, Ridamil, Subdue Fungicide Miscellaneous 50359 4 .01 Metalaxyl Apron, Ridamil, Subdue Fungicide Miscellaneous 61596 5 .005 Methidathion Somanil, Supracide Insecticide Organophosphate 61598 5 .006 Methiocarb Mesurol Insecticide Carbamate 38501 2, 4 .008 Methomyl Lannate, Nudrin Insecticide Carbamate 49296 2, 4 .004 Methyl paraoxon Methyl parathion degradate -- Organophosphate 61664 5 .02 Methyl parathion Penncap-M, Paratox Insecticide Organophosphate 82667 1, 5 .006 Metolachlor2 Bicep, Dual Herbicide Amide 39415 1, 3, 5 .002 
Table 1. Pesticides analyzed, trade names, pesticide actions, classes, laboratory reporting limits, and U.S. Environmental Protection Agency standa

—Continued[Compounds detected with original censoring during study are in bold type; USGS, U.S. Geological Survey; μg/L, micrograms per liter; USEPA, U.S. Environmental ProtectioU, chemical was analyzed for, but not detected; analytical method type: 1 = USGS laboratory schedule 2001, 2 = USGS laboratory schedule 2050, 3 = USGS laboratory schedulschedule 2060, 5 = USGS laboratory schedule 2033; 6 = Severn Trent Laboratories, USEPA method 8260B; USEPA standard or health advisory: RSD4 = U.S. Environmental PDose at 10-4 Cancer Risk (U.S. Environmental Protection Agency, 2006); MCL = U.S. Environmental Protection Agency Maximum Contaminant Level (U.S. Environmental ProLHA = U.S. Environmental Protection Agency Lifetime Health Advisory (U.S. Environmental Protection Agency, 2006)]

Pesticide Trade name Pesticide action

Pesticide class 

(or parent compound 

class)

USGS 

laboratory parameter 

code

Analytical 

method1

(see headnote)

Minimum 

laboratory 

reporting 

limit 

(μg/L)

M

l

Metribuzin2 Lexone, Sencor Herbicide Triazine 82630 1, 3, 5 0.004 Metsulfuron2 Ally, Escort Herbicide Urea/sulfonylurea 61697 4 .03 Molinate Hydram, Ordram Herbicide Carbamate 82671 1, 5 .002 Myclobutanil Rally, Systhane Fungicide Miscellaneous 61599 5 .008 Napropamide Devrinol Herbicide Amide 82684 1 .003 1-Naphthol Fourrine Insecticide Carbamate 49295 2, 5 .09 Neburon Granurex, Propuron Herbicide Urea 49294 2, 4 .01 Nicosulfuron Accent, OneHope Herbicide Urea/sulfonylurea 50364 4 .01 Norflurazon Zorial, Solicam Herbicide Miscellaneous 49293 2, 4 .02 Oryzalin Surflan Herbicide Dinitroaniline 49292 2, 4 .01 Oxamyl Vydate Insecticide, acaricide, 

nematicide

Carbamate 38866 2, 4 .01 

Oxyfluorfen Goal Herbicide Miscellaneous 61600 3 .007 Parathion Alkron, Bladan, Fighter Insecticide Organophosphate 39542 1 .004 Pebulate Tillam Herbicide Carbamate 82669 1 .002 Pendimethalin Prowl, Stomp Herbicide Dinitroaniline 82683 1, 5 .004 cis-Permethrin Ambush, Pounce Insecticide Pyrethroid 82687 1, 5 .005 Phorate Thimet, Rampart Insecticide Organophosphate 82664 1, 5 .03 Phorate oxon Phorate degradate -- Organophosphate 61666 5 .03 Phosmet Imidan, Percolate, Prolate Insecticide Organophosphate 61601 5 .008 Phosmet oxon Phosmet degradate -- Organophosphate 61668 5 .05 Picloram2 Tordon Herbicide Pyridinecarboxylic 

acid

49291 2, 4 .02 

Prometon Pramitol, Gesafram Herbicide Triazine 04037 1, 3, 5 .01 Prometryn Caparol, Selectin Herbicide Triazine 04036 3, 5 .005 
Table 1. Pesticides analyzed, trade names, pesticide actions, classes, laboratory reporting limits, and U.S. Environmental Protection Agency standa

—Continued[Compounds detected with original censoring during study are in bold type; USGS, U.S. Geological Survey; μg/L, micrograms per liter; USEPA, U.S. Environmental ProtectioU, chemical was analyzed for, but not detected; analytical method type: 1 = USGS laboratory schedule 2001, 2 = USGS laboratory schedule 2050, 3 = USGS laboratory schedulschedule 2060, 5 = USGS laboratory schedule 2033; 6 = Severn Trent Laboratories, USEPA method 8260B; USEPA standard or health advisory: RSD4 = U.S. Environmental PDose at 10-4 Cancer Risk (U.S. Environmental Protection Agency, 2006); MCL = U.S. Environmental Protection Agency Maximum Contaminant Level (U.S. Environmental ProLHA = U.S. Environmental Protection Agency Lifetime Health Advisory (U.S. Environmental Protection Agency, 2006)]

Pesticide Trade name Pesticide action

Pesticide class 

(or parent compound 

class)

USGS 

laboratory parameter 

code

Analytical 

method1

(see headnote)

Minimum 

laboratory 

reporting 

limit 

(μg/L)

M

l

Pronamide (Propyzamide) Kerb Herbicide Amide 82676 1, 5 0.003 Propachlor Ramrod, Prolex Herbicide Amide 04024 1, 3 .007 Propanil Stamp Herbicide Amide 82679 1, 5 .004 Propargite Comite, Omite Insecticide, acaricide Miscellaneous 82685 1, 5 .01 Propazine Milogard, Tritol Herbicide Triazine 38535 3 .05 Propham Chem Hoe Herbicide Carbamate 49236 2, 4 .01 Propiconazole Banner Fungicide Miscellaneous 50471 4 .01 cis-Propiconazole -- Miscellaneous 79846 5 .008 trans-Propiconazole -- Miscellaneous 79847 5 .01 Propoxur Baygone, Suncide Fungicide Carbamate 38538 2, 4 .008 Siduron Tupersan Herbicide Urea 38548 4 .02 Simazine2 Aquazine, Primatol, Princep Herbicide Triazine 04035 1, 3, 5 .005 Simetryn Cymetrin, Gy-bon Herbicide Triazine 04030 3 .05 Sulfometuron, 

methyl

Oust Herbicide Urea/sulfonylurea 50337 4 .009 

2,4,5–T Dacamine, Emulsavert, Line 

Rider

Herbicide Chlorophenoxy 39742 2, 4 .01 

2,4,5–TP Silvex, Weed-B-Gon Herbicide Chlorophenoxy 39762 2 .02 Tebuconazole Elite, Folicur, Raxil Fungicide Azole 62852 5 -- Tebuthiuron2 Graslan, Spike Herbicide Urea 82670 1, 4, 5 .01 Tefluthrin Demand, Force, Karate Insecticide Pyrethroid 61606 5 .003 Terbacil Sinbar, Herbicide 732 Herbicide Miscellaneous 04032 3, 4 .01 Terbacil Sinbar, Herbicide 732 Herbicide Miscellaneous 82665 1 .007 Terbufos Counter, Contraven Insecticide Organophosphate 82675 1, 5 .01 Terbuthylazine Gardoprim Herbicide Triazine 04022 1, 3, 5 U 
Table 1. Pesticides analyzed, trade names, pesticide actions, classes, laboratory reporting limits, and U.S. Environmental Protection Agency standa

—Continued[Compounds detected with original censoring during study are in bold type; USGS, U.S. Geological Survey; μg/L, micrograms per liter; USEPA, U.S. Environmental ProtectioU, chemical was analyzed for, but not detected; analytical method type: 1 = USGS laboratory schedule 2001, 2 = USGS laboratory schedule 2050, 3 = USGS laboratory schedulschedule 2060, 5 = USGS laboratory schedule 2033; 6 = Severn Trent Laboratories, USEPA method 8260B; USEPA standard or health advisory: RSD4 = U.S. Environmental PDose at 10-4 Cancer Risk (U.S. Environmental Protection Agency, 2006); MCL = U.S. Environmental Protection Agency Maximum Contaminant Level (U.S. Environmental ProLHA = U.S. Environmental Protection Agency Lifetime Health Advisory (U.S. Environmental Protection Agency, 2006)]

Pesticide Trade name Pesticide action

Pesticide class 

(or parent compound 

class)

USGS 

laboratory parameter 

code

Analytical 

method1

(see headnote)

Minimum 

laboratory 

reporting 

limit 

(μg/L)

M

l

Thiobencarb Bolero, Saturn Herbicide Carbamate 82681 1, 5 0.002 Triallate Far-Go, Avadex BW Herbicide Carbamate 82678 1 .001 Tribenuron Express, Urgent Herbicide Urea/sulfonylurea 61159 4 .009 Tribuphos Def, Easy off-D, Folex Herbicide Organophosphate 61591 5 .004 Triclopyr Garlon Herbicide Chlorophenoxy 49235 2, 4 .02 Trifluralin Treflan, Trim Herbicide Dinitroaniline 04023 3 .05 Trifluralin Treflan, Trim Herbicide Dinitroaniline 82661 1, 5 .002 Urea, 3,4-Chlorophenyl, methyl Diuron degradate -- Urea 61692 4 .02 Vernolate Surpass, Vernam Herbicide Carbamate 04034 2 .05 

1http://nwql.cr.usgs.gov/USGS/catalog/index.cfm2Identified as a focal pesticide in the State of Wyoming’s Generic Management Plan for Pesticides in Ground Water (SMP; Wyoming Ground-Water and Pesticides Strategy C3The MCL is under administrative stay pending review by U.S. Environmental Protection Agency. The MCL for any combination of 2 or more of these 3 chemicals (aldicarb, 

sulfone) should not exceed 7 μg/L because of similar mode of action.
14 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006

The LRL is the third type of reporting limit used by the 

NWQL during this study. In 1998, the NWQL began to change 

from censoring at the MDL to censoring at a laboratory reporting level (LRL) to minimize both false-positive and falsenegative detections (reporting a nondetection when the actual 

concentration in the sample is greater than the MDL; Childress 

and others, 1999). During this transition, the NWQL began 

to set the reporting limit equivalent to the LRL rather than 

the MDL for as many pesticide analyses as possible in most 

NWQL analytical schedules used during this study. Every 

individual pesticide analysis was reviewed using information 

provided by the NWQL to determine the type of reporting 

limit used (MRL, MDL, or LRL).

The LRL is statistically calculated on an annual basis 

using quality-control data for each analyte to determine the 

long-term method detection level (LT-MDL). The LT-MDL 

is determined from the standard deviation of long-term 

laboratory spike samples and is set to a concentration to 

minimize false-positive detections to no more than 1 percent; 

however, the risk of false-negative detections is greater than 

1 percent. The LRL is set to a concentration to minimize these 

false-negative detections to no more than 1 percent, equal 

to twice the LT-MDL. Using this NWQL reporting convention, reported concentrations greater than the LRL are not 

censored or qualified. Concentrations measured as less than 

the LT-MDL are reported as “less than” (<) the LRL. Concentrations measured between the LT-MDL and the LRL are quantified but qualified with an “E” remark code.

Concentrations smaller than the LRL, but greater than 

the LT-MDL, are qualified as estimated using an “E” remark 

code. In some cases, concentrations smaller than the LT-MDL 

are quantified but are qualified with an “E” remark code, 

indicating that the analyte was “positively identified” and that 

laboratory quality-control criteria were met, although there 

is an increased risk (greater than 1 percent) of false-positive 

detections (Childress and others, 1999). An “E” remark code 

also can be used if the analyte did not meet method-specific 

laboratory quality-control criteria. 

For this study, detections qualified with an “E” remark 

code were treated as quantified values regardless of whether 

the value was associated with an MRL, MDL, or LRL. This 

approach preserves all of the information provided by the 

laboratory and acknowledges that values with an “E” remark 

code have concentrations greater than true nondetections. The 

resulting data set was appropriate for analysis using categorical statistical methods that classified pesticide analyses as 

either “detections” or “nondetections” greater than a selected 

assessment (censoring) level.

Recensoring and Assessment Levels

Because of the use of three different types of “reporting limits” and variability in reporting-limit values during 

this study (1995–2006), analytical results received from the 

NWQL with original censoring were recensored. Recensoring 

was necessary to account for different types of reporting limits 

(MRL, MDL, or LRL) and (or) variable laboratory reporting limits either for a specific pesticide or between individual 

pesticides. Laboratory reporting limits typically vary over time 

due to changes in analytical methods, differences in laboratory equipment, equipment sensitivity, experience and skill of 

equipment operators, and (or) laboratory conditions. Recensoring to a common value (referred to herein as the assessment level) allows for accurate calculation and comparison of 

detection frequencies and concentrations between individual 

pesticides or groups of pesticides with different types and 

values of reporting limits, as well as the use of quantitative 

statistical methods to examine pesticide occurrence. 

Typically, recensoring is conducted by comparing 

concentrations of reported detections to all laboratory reporting limits; all detections less than the largest laboratory 

reporting limit are recoded as nondetections, and all detections greater than the largest laboratory reporting limit are 

retained as detections at the originally reported laboratory 

concentrations. Unfortunately, pesticide reporting limits varied 

widely during this study (table 1), and selection of the largest 

laboratory reporting limit for recensoring would result in 

many, if not most, detections being recoded as nondetections. 

Consequently, an approach was used to recensor the data in 

an attempt to retain as many pesticide detections as possible 

while still adhering to a rigorous and defensible procedure for 

assembly of the final data set. 

 Recensoring and assembly of the final data set began by 

obtaining information from the NWQL to determine the type 

and value of reporting limits associated with all individual 

pesticides detected during this study. The reporting limit can 

affect the detection frequency of pesticide compounds—

pesticides with smaller reporting limits are likely to have 

greater detection frequencies (Burkart and Kolpin, 1993; 

Barbash and others, 1999; Kolpin and others, 2000; Gilliom 

and others, 2006). For all detected pesticides, every individual 

pesticide analysis was reviewed using information provided 

by the NWQL (http://nwql.cr.usgs.gov/usgs/ltmdl/ltmdl.cfm)

to determine the type of reporting limit used (MRL, MDL, or 

LRL) as well as the value at the time of analysis for the entire 

study period (1995–2006). 

Initially, pesticide analyses reported in relation to the 

MRL or MDL were censored using the laboratory-provided 

values, whereas analyses reported in relation to an LRL generally were recensored to the LT-MDL concentration. Because 

this was an analysis of a large sampling network, a greater 

level of uncertainty (increased risk of false-negative error) 

for any individual value as a result of recensoring data at the 

LRL to the LT-MDL was considered acceptable (Childress and 

others, 1999). Individual analyses with reporting limits much 

larger than reported by the NWQL for the method (“elevated 

reporting limits”) indicated likely problems at the time of analysis due to sample characteristics (for example, matrix effects) 

and (or) laboratory procedures/equipment. These analyses 

with elevated reporting limits generally were removed from 

the data set if the reporting limits for an individual pesticide 

were two times or more greater than reported by the NWQL at 
Methods of Investigation 15

the time of analysis using the same analytical method for the 

same pesticide. Detections then were recensored to two different assessment levels to facilitate qualitative and quantitative 

examination of pesticide detection frequencies—an assessment level that differed by compound, referred to herein as a 

compound-specific assessment level (CSAL), and a common 

assessment level (CAL) of 0.07 microgram per liter (μg/L) 

(table 2). 

To remove the effect of variable reporting limits, detection frequencies of pesticides were calculated using the assessment levels. The assessment level is a fixed concentration that 

was applied to pesticide analyses received from the laboratory and was applied subsequent to the laboratory reporting 

limits (table 2). Concentrations reported by the laboratory as 

detections, but less than the selected assessment level, were 

considered as nondetections in the calculation of detection 

frequencies and in statistical analyses. For this study, the 

CAL of 0.07 μg/L was selected as a compromise between the 

need to facilitate comparisons with compounds with larger 

reporting limits and the need to minimize the loss of pesticide 

detections for compounds detected only at small concentrations. Recensoring to the CAL allowed for qualitative and 

quantitative comparisons among pesticides but resulted in the 

loss of many detections of the same pesticide, primarily those 

with the smallest reporting limits. Consequently, a process 

similar to that used to select the CAL was applied to individual 

pesticides to determine values for the CSALs that would be 

a compromise between the need to censor to larger reporting 

limits and the need to minimize the loss of detections at small 

concentrations with much smaller reporting limits (table 2). 

Quality-Control Samples and Quality Assurance

In addition to collection of environmental water-quality 

samples, three types of quality-control (QC) samples were 

collected as part of the overall quality-assurance (QA) 

program—field-blank samples, replicate samples, and fieldmatrix spike samples. The QC samples were collected, 

preserved, and analyzed using the same methods and equipment as for environmental samples. Collection and evaluation 

of QC samples, in addition to strict sample collection, processing, and analysis procedures, composed the field QA program.

Blank Samples

Field-blank samples were collected to evaluate bias from 

the potential introduction of contamination to environmental 

samples during sample collection, sampling equipment cleaning, and laboratory analytical procedures. Specially prepared 

water that is certified to be free of organic constituents was 

used as the source water for all blank samples. Samples were 

collected at a rate of one field-blank sample for about every 

12 environmental samples collected (8.3 percent).

A total of 48 field-blank samples were collected for 

analysis resulting in 3,953 individual analyses. When 

evaluating uncensored data with original NWQL censoring 

from these 3,953 analyses, 9 pesticide detections in field-blank 

samples were seen. However, when these field-blank detections are evaluated in relation to applicable censoring levels, 

all pesticide detections in field-blank samples were less than 

all recensored detections for a given pesticide compound. 

Results of the field-blank samples indicated that decontamination procedures were adequate and that field and laboratory 

contamination of environmental samples by pesticides was 

minimal.

Replicate Samples

A replicate sample (duplicate) is a sample collected 

immediately after the primary environmental sample to assess 

combined effects of sample-collection and laboratory procedures on measurement variability (precision). Forty-three 

samples were collected for replicate analysis, a frequency 

of about 1 replicate for every 14 environmental samples 

(7.1 percent). These replicate and environmental samples had 

a total of 2,050 pairs of individual analyses of which 71 pairs 

had detections of a pesticide in both samples (4 sample pairs 

had detections in only one set of samples). All reported detections were used in analysis of the replicate data including 

the values assigned an “E” remark code, which accounted 

for about one-half of reported detections. All data were used 

without recensoring.

The relative percentage difference (RPD) was calculated 

to compare the constituent concentrations measured in both 

the environmental (sample1) and replicate (sample2) samples 

using the following equation:

RPD sample sample

sample sample = −

 +



 









absolute value 1 2

1 2

2





















x 100. (1)

RPDs were not calculated for pairs where one value was 

reported as less than the laboratory reporting limit.

For the 71 pairs of individual results with detections, 

56 pairs had a RPD of less than 20 percent. Of those 15 pairs 

with a RPD of greater than 20 percent, the difference generally was attributable to small concentration differences that 

resulted in relatively large RPDs because both environmental 

and replicate sample concentrations were small. These results 

were considered acceptable for this baseline study, and consequently, no detections were qualified or deleted on the basis of 

calculated RPDs.

Field-Matrix Spike Samples

Field-matrix spike samples were analyzed to evaluate 

bias and variability from the environmental ground-water 

matrix or potential degradation of the constituent during 

sample processing, storage, and analysis. Field-matrix spike 
Table 2. Detected pesticides in ground-water samples collected during fall and spring using original National Water Quality 

Laboratory censoring, laboratory reporting limit range, and assigned assessment levels, Wyoming, 1995–2006.—Continued

[Pesticides are listed in order of decreasing detection frequency by jointly ranking fall and spring detections with original censoring (no assessment level 

applied) and grouped by four types. NWQL, National Water Quality Laboratory; NAL, no assessment level applied; μg/L, micrograms per liter. Assessment 

level: CSAL, compound-specific assessment level; CAL, common assessment level. F, fall; S, spring]

Detected pesticide

Detection with original 

NWQL censoring 

(NAL)

Laboratory reporting 

limit range 

(μg/L)

Assigned 

CSAL 

(μg/L)

Assigned 

CAL 

(μg/L)

Herbicides

Atrazine F, S 0.001–0.05 0.004 0.07

Prometon F, S 0.01–0.05 .05 .07

Tebuthiuron F, S 0.01–0.026 .01 .07

Simazine F, S 0.005–0.05 .05 .07

Picloram F, S 0.02–0.26 .06 .07

Oxyfluorfen F 0.007–0.017 .017 .07

Bromacil F, S 0.01–1.1 .06 .07

Diuron F, S 0.01–0.42 .06 .07

Metolachlor F, S 0.002–0.05 .009 .07

Hexazinone S 0.013–0.05 .05 .07

Sulfometuron, methyl F 0.009–0.091 .09 .07

Clopyralid F, S 0.01–1.82 .26 .07

Flumetsulam F 0.01–0.06 .06 .07

DCPA F, S 0.002–0.004 .004 .07

2,4–D methyl ester F 0.009–0.016 .009 .07

Chlorimuron F 0.01–0.046 .046 .07

Metsulfuron F 0.03–0.07 .07 .07

Bentazon F, S 0.01–0.06 .06 .07

Cyanazine F, S 0.004–0.2 .013 .07

2,4–D F, S 0.01–0.73 .15 .07

Metribuzin F, S 0.004–0.05 .05 .07

Triallate F, S 0.001–0.008 .003 .07

Dichlorprop F, S 0.01–0.13 .06 .07

Triclopyr F, S 0.02–0.36 .36 .07

Norflurazon F, S 0.02–0.32 .06 .07

Trifluralin F 0.002–0.012 .012 .07

Bromoxynil F 0.01–1.6 .07 .07

Oryzalin F 0.01–1.25 .31 .07

Benfluralin F 0.002–0.013 .013 .07

Alachlor S 0.002–0.05 .05 .07

Dicamba S 0.01–0.35 .13 .07

Herbicide degradates

Deethylatrazine F, S 0.002–0.05 0.05 0.07

Deisopropylatrazine F, S 0.01–0.08 .05 .07

3,4-Dichloroaniline F 0.004 .004 .07

Hydroxyatrazine F 0.008–0.032 .016 .07

16 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006
Table 2. Detected pesticides in ground-water samples collected during fall and spring using original National Water Quality 

Laboratory censoring, laboratory reporting limit range, and assigned assessment levels, Wyoming, 1995–2006.—Continued

[Pesticides are listed in order of decreasing detection frequency by jointly ranking fall and spring detections with original censoring (no assessment level 

applied) and grouped by four types. NWQL, National Water Quality Laboratory; NAL, no assessment level applied; μg/L, micrograms per liter. Assessment 

level: CSAL, compound-specific assessment level; CAL, common assessment level. F, fall; S, spring]

Detected pesticide

Detection with original 

NWQL censoring 

(NAL)

Laboratory reporting 

limit range 

(μg/L)

Assigned 

CSAL 

(μg/L)

Assigned 

CAL 

(μg/L)

Insecticides

alpha-Endosulfan F 0.005–0.011 0.006 0.07

Imidacloprid F, S 0.007–0.241 .02 .07

Carbaryl F 0.003–0.046 .05 .07

Carbofuran F 0.003–0.02 .02 .07

Dieldrin F 0.001–0.009 .008 .07

Malathion F 0.005–0.031 .014 .07

Diazinon S 0.002–0.008 .008 .07

cis-Permethrin F 0.005–0.016 .016 .07

Insecticide degradates

Aldicarb sulfoxide F, S 0.008–0.27 0.14 0.07

Aldicarb sulfone F, S 0.02–1.31 .11 .07

Fipronil sulfide F 0.005–0.013 .006 .07

Fipronil sulfone F 0.005–0.024 .012 .07

3-Hydroxycarbofuran F 0.007–0.57 .05 .07

Methods of Investigation 17

samples were collected in the same manner as the environmental samples, immediately following collection of the 

environmental samples. The samples then were injected with a 

known concentration of selected pesticides. Forty-five samples 

were field spiked, a frequency of about 1 spike for every 

13 environmental samples (7.7 percent). These spike and environmental samples resulted in the calculation of 3,683 spike 

recoveries.

Laboratory recoveries of most field-matrix spike samples 

ranged from 60 to 120 percent of the pesticide or degradate 

known to be in the sample. Overall recoveries of pesticides 

and degradates during the study averaged about 80 percent, 

which indicates a possible low bias in reported pesticide 

concentrations. Analysis of a subset of field-matrix spike 

data that included only pesticides and degradates detected 

in environmental samples showed about the same average 

(79 percent). Eight pesticides and degradates detected in environmental samples (all aldicarb compounds, 2,4–DB, 2,4–D 

methyl ester, clopyralid, dicamba, and picloram), however, 

had field-matrix spike recoveries indicating a general low 

bias (average recoveries less than 60 percent). Tebuthiuron 

concentrations may have been biased slightly high as the 

average calculated recovery was 121 percent. These results 

were considered acceptable for this study, and consequently, 

no detections were qualified or deleted on the basis of poor 

recoveries.

Statistical Methods

Because of severe data censoring (fewer than 50 percent 

of the data were greater than laboratory reporting limits or 

assessment levels), two categorical statistical methods were 

used exclusively for quantitative comparisons of pesticide 

detection frequencies between seasons and among various 

groups of natural and anthropogenic characteristics. These 

tests were contingency-table analysis (CTA) and the KruskalWallis test for ordered categorical responses (KWOCR) 

(Helsel and Hirsch, 1992; Helsel, 2005).

Pesticide data were tabulated by row and column according to the grouped characteristics selected for analysis and 

whether or not the pesticide was detected at concentrations 

greater than or less than the CAL or CSAL. The null hypothesis tested was that detection or nondetection of any pesticide 

was independent of the grouped characteristic selected for 

examination. Rejection of the null hypothesis at a probability (p-value) of 0.05 (alpha level) was considered evidence 

supporting the alternative hypothesis that there was a relation between the variable (pesticide) and the factor (grouped 

characteristic) tested. Results of statistical tests indicating 

computed probabilities (p-values) close to and slightly larger 

than the alpha level of 0.05 (0.05<p<0.09) often are reported 

in the text as “close to statistical significance.” The CTA was 

used when the response variable was nominal (no ordering 
18 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006

of categories), and the KWOCR was used when the response 

variable was ordinal (values or a category could be ordered 

into a sequence from smallest to largest). 

An algorithm within ArcGIS 9.2 (Environmental Systems 

Research Inc., 1999–2007) was used to classify various 

continuous ordinal variables into categorical ordinal variables 

for statistical analysis using the KWOCR. This algorithm 

(referred to as “Jenks” or “natural breaks”) was used to group 

continuous data into classes “based on natural groupings 

inherent in the data” and “identifies breakpoints by picking 

the class breaks that best groups similar values and maximizes 

the differences between classes” (Environmental Systems 

Research Inc., 2008). Median values presented herein were 

calculated using standard methods (Helsel and Hirsch, 1992).

Summary of Pesticide Occurrence

The general occurrence of pesticides is described in this 

section of the report. The overall frequency of detection of 

pesticides in wells sampled during this study is examined. 

Pesticides are classified by type and class, and the occurrence 

of pesticides as mixtures is examined. Detection frequencies for individual pesticides are summarized, discussed, and 

examined in relation to time of sampling (season). Finally, 

detected pesticides were compared to reported agricultural use 

in Wyoming. In this report, pesticide detection frequencies and 

concentrations (“pesticide occurrence”) are presented in parts 

of the report with original censoring from the NWQL [referred 

to as no assessment level (NAL)] and with recensoring to two 

different assessment levels (the CSAL and CAL). Pesticide 

occurrence with original censoring (NAL) is presented in 

tables and figures to summarize data previously published in 

22 USGS county-specific pesticide fact sheets (many readers 

are familiar with the data uncensored in this manner) and 

to show the effects of recensoring to two assessment levels. 

However, for reasons described previously in the “Reporting 

and Data Treatment” section of the report, quantitative (statistical) analysis of pesticide occurrence without recensoring is 

not “robust,” and pesticide occurrence without recensoring is 

limited to qualitative descriptions in the text. Consequently, 

pesticide occurrence in the text generally is described and 

statistically examined using data recensored to the CSAL and 

(or) CAL. 

Although detection frequencies and measured concentrations are presented in tables and figures in relation to both 

assessment levels, the determination of whether pesticide 

occurrence is described in relation to the CSAL or CAL 

depends upon the type of analysis conducted. In the text, 

individual pesticide occurrence generally is described and 

statistically examined in relation to various natural and anthropogenic characteristics using the CSAL (which is typically but 

not always a concentration smaller than the CAL), whereas 

the detection of one or more pesticides in the same well and 

comparison between different pesticides generally is described 

and statistically examined in relation to various natural and 

anthropogenic characteristics using the CAL. However, there 

are some cases in the text where qualitative comparison using 

different assessment levels was warranted. 

Detection Frequency by Well Sampled

The overall detection frequency of pesticides in ground 

water was examined by calculating the percentage of wells 

with at least one pesticide detected at concentrations greater 

than the CAL. One or more pesticides were detected at 

concentrations greater than the CAL in water from 68 of 

296 wells sampled (about 23 percent) in the fall and in water 

from 66 of 295 wells sampled (about 22 percent) in the spring 

(fig. 2). The proportion of wells with at least one pesticide 

detected at concentrations greater than the CAL was not significantly different between fall and spring (p-value = 0.9394, 

CTA). Median concentrations calculated for both fall and 

spring pesticide detections greater than the CAL were similar 

(fig. 2).

Occurrence of Pesticide Mixtures

Pesticides detected in ground water commonly occur 

as mixtures of two or more different compounds (Barbash 

and Resek, 1996, and references therein; Gilliom and others, 

2006). The mixtures can include both parent pesticides and 

degradates that result from the use of different pesticides for 

multiple purposes within the same geographic area.

 Mixtures of two or more pesticides at concentrations 

greater than the CAL occurred in about 9 percent of wells 

sampled in the fall and in about 10 percent of wells sampled 

in the spring (fig. 3). Most mixtures were composed of two to 

four different pesticides; more than five different pesticides 

with concentrations greater than the CAL were identified only 

in one well sampled in the fall (fig. 3A). The health effects of 

water containing pesticide mixtures are unknown (Gilliom, 

2001; Gilliom and others, 2006). In ground water, pesticides 

also are found frequently in samples containing other contaminants such as nitrate and volatile organic compounds (Squillace and others, 2002).

Pesticide Detections by Type

Detected pesticides were classified into one of four types 

(table 2; fig. 4). At least 74 percent of detected pesticides 

were classified as herbicides regardless of whether data were 

examined with original censoring (NAL) or with either assessment level (CSAL and CAL). Pesticides classified as herbicide degradates, insecticides, or insecticide degradates were 

detected much less frequently. The percentages of pesticides 

classified into the four groups were nearly identical for fall and 

spring. The prevalence of pesticides classified as herbicides 

was expected because herbicide use is more common than 
Figure 2. Pesticide concentrations and percentage of wells with at least one pesticide detected for fall and spring, Wyoming, 1995–2006.

NAL

CSAL

CAL

PERCENTAGE OF WELLS WITH AT LEAST ONE PESTICIDE DETECTED

 Spring

 Fall

 Spring

 Fall

Concentration of any pesticide detected

EXPLANATION

Pesticide detections with no assessment level applied

Pesticide detections greater than common assessment level

Pesticide detections greater than compound-specific assessment levels

NAL

CSAL

CAL

66/295 Number of wells with at least one pesticide detected/number of wells sampled

0 10 20 30 40 50 60

160/296

154/295

122/296

115/295

68/296

66/295

0.0001 0.001 0.01 0.1 1 10 100

CONCENTRATION, IN MICROGRAMS PER LITER

Percentage of wells with at 

 least one pesticide detected

Median concentration of detections

Lowest compound-specific assessment level

Common assessment level

Summary of Pesticide Occurrence 19

insecticide use in Wyoming (Ferrell and others, 1996), and the 

ground-water samples were analyzed with laboratory schedules composed of relatively few insecticides and degradates 

(both herbicide and insecticide) compared to herbicides. 

Pesticide Detections by Class

Detected pesticides also were classified by their chemical structure into 1 of 10 classes or 2 miscellaneous (herbicide 

or insecticide) classes for pesticides not easily classifiable 

(table 3; fig. 5). Regardless of assessment level, pesticides 

classified into one of five classes (triazine, urea, pyridinecarboxylic acid, miscellaneous herbicide, and carbamate) were 

most commonly detected. Pesticides classified into the remaining seven classes were rarely detected. 

Triazine pesticides were detected at a frequency much 

greater than all other pesticide classes. In fact, pesticides 

classified as triazines comprised about 48 percent (fall) and 

45 percent (spring) of all detections at concentrations greater 

than the CAL. Detected triazines were either herbicides or 

herbicide degradates. In addition, the number of detected 

pesticides classified as triazines was the largest of all classes 

regardless of assessment level (table 3).

Individual Pesticide Detections

The general occurrence of individual pesticide 

compounds is described in this section of the report. Reported 

pesticide concentrations and calculated detection frequencies are summarized in both tabular and graphical form for 

both fall and spring. All data are presented and summarized 

with original censoring (NAL) and in comparison with both 

assessment levels (CSAL and CAL). Quantitative (statistical) 

examination of individually detected pesticides was made 

using the CSALs.
Figure 3. Number of pesticides detected per well in ground-water samples collected in Wyoming, 1995–2006. 

A, Fall; B, Spring.

PERCENTAGE OF WELLS

Assessment level

EXPLANATION

 Pesticide detections with no assessment level applied

 Pesticide detections greater than common assessment level

 Pesticide detections greater than compound-specific assessment levels

0 10 20 30 40 50 60 70 80

A Fall

B Spring

0

1

2

3

4

5

Greater than 5

NUMBER OF PESTICIDES DETECTED PER WELL

0

1

2

3

4

5

Greater than 5

NUMBER OF PESTICIDES DETECTED PER WELL

20 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006
Figure 4. Pesticide detections relative to pesticide type in ground-water samples collected in Wyoming, 1995–2006. A, Fall; B, Spring.

PERCENTAGE OF DETECTIONS BY PESTICIDE TYPE AND ASSESSMENT LEVEL

 Herbicide

Pesticide type

 Herbicide degradate

 Insecticide

 Insecticide degradate

EXPLANATION

Pesticide detections with no assessment level applied

Pesticide detections greater than common assessment level

Pesticide detections greater than compound-specific assessment levels

NAL

CSAL

CAL

0 10 20 30 40 50 60 70 80 90 100

A Fall

B Spring

NAL

CSAL

CAL

NAL

CSAL

CAL

Summary of Pesticide Occurrence 21
Table 3. Pesticide detections relative to pesticide class in ground-water samples collected during fall and spring in Wyoming, 1995–2006.—Continu

[Pesticide classes listed in order of decreasing detection frequency by jointly ranking fall and spring detections by class with no assessment level. Assessment level: NAL, no ascompound-specific assessment level; CAL, common assessment level. Pesticide type: H, herbicide; HD, herbicide degradate; I, insecticide; ID, insecticide degradate. Symbol: -

Pesticide 

class

Assessment 

level

Fall Spring

Number of 

pesticide 

detections in 

class/number 

of detections 

(percentage)

Number of 

different 

pesticides 

detected in 

class

Pesticides detected

Number of 

pesticide 

detections in 

class/number 

of detections 

(percentage)

Number of 

different 

pesticides 

detected in 

class

Pesti

Triazine

NAL 299/436

(68.6)

8 Atrazine (H), cyanazine (H), metribuzin 

(H), prometon (H), simazine (H), deisopropylatrazine (HD), deethylatrazine (HD), hydroxyatrazine (HD)

300/410

(73.2)

8 Atrazine (H), cyana

metribuzin (H), pdeisopropylatraz(HD).

CSAL 139/227

(61.2)

7 Atrazine (H), cyanazine (H), prometon (H), 

simazine (H), deisopropylatrazine (HD), deethylatrazine (HD), hydroxyatrazine (HD)

130/208

(62.5)

7 Atrazine (H), cyana

prometon (H), sirazine (HD), dee

CAL 53/111

(47.7)

4 Atrazine (H), prometon (H), simazine (H), 

deethylatrazine (HD)

49/109

(45)

6 Atrazine (H), cyana

simazine (H), dedeethylatrazine (

Urea

NAL 46/436

(10.6)

6 Chlorimuron (H), diuron (H), metsulfuron 

(H), sulfometuron (methyl) (H), tebuthiuron (H), 3,4-Dichloroaniline (HD)

44/410

(10.7)

2 Diuron (H), tebuthi

CSAL 34/227

(15.0)

3 Diuron (H), tebuthiuron (H), 3,4-Dichloroaniline (HD) 37/208 (17.8) 2 Diuron (H), tebuthi

CAL 16/111

(14.4)

2 Diuron (H), tebuthiuron (H) 20/109

(18.3)

2 Diuron (H), tebuthi

Pyridinecarboxylic 

acid

NAL 28/436

(6.4)

2 Clopyralid (H), picloram (H) 24/410

(5.9)

2 Clopyralid (H), pic

CSAL 22/227

(9.7)

2 Clopyralid (H), picloram (H) 17/208

(8.2)

2 Clopyralid (H), pic

CAL 19/111

(17.1)

2 Clopyralid (H), picloram (H) 20/109

(18.3)

2 Clopyralid (H), pic
Table 3. Pesticide detections relative to pesticide class in ground-water samples collected during fall and spring in Wyoming, 1995–2006.—Continu

[Pesticide classes listed in order of decreasing detection frequency by jointly ranking fall and spring detections by class with no assessment level. Assessment level: NAL, no ascompound-specific assessment level; CAL, common assessment level. Pesticide type: H, herbicide; HD, herbicide degradate; I, insecticide; ID, insecticide degradate. Symbol: -

Pesticide 

class

Assessment 

level

Fall Spring

Number of 

pesticide 

detections in 

class/number 

of detections 

(percentage)

Number of 

different 

pesticides 

detected in 

class

Pesticides detected

Number of 

pesticide 

detections in 

class/number 

of detections 

(percentage)

Number of 

different 

pesticides 

detected in 

class

Pesti

Miscellaneous 

(herbicide)

NAL 21/436

(4.8)

7 Bentazon (H), bromacil (H), bromoxynil 

(H), DCPA (H), flumetsulam (H), norflurazon (H), oxyfluorfen (H)

16/410

(3.9)

5 Bentazon (H), brom

camba (H), norfl

CSAL 13/227

(5.7)

5 Bentazon (H), bromacil (H), bromoxynil 

(H), DCPA (H), flumetsulam (H)

12/208

(5.8)

3 Bentazon (H), brom

CAL 9/111

(8.1)

4 Bentazon (H), bromacil (H), bromoxynil 

(H), flumetsulam (H)

11/109

(10.1)

3 Bentazon (H), brom

Carbamate

NAL 15/436

(3.4)

6 Triallate (H), carbaryl (I), carbofuran (I), 

aldicarb sulfone (ID), aldicarb sulfoxide (ID), 3-hydroxycarbofuran (ID)

11/410

(2.7)

3 Triallate (H), aldica

sulfoxide (ID).

CSAL 8/227

(3.5)

3 Carbofuran (I), aldicarb sulfone (ID), aldicarb sulfoxide (ID) 7/208 (3.4) 3 Triallate (H), aldica

sulfoxide (ID).

CAL 9/111

(8.1)

2 Aldicarb sulfone (ID), aldicarb sulfoxide (ID) 8/109 (7.3) 2 Aldicarb sulfone (I

Amide

NAL 8/436

(1.8)

1 Metolachlor (H) 9/410

(2.2)

2 Alachlor (H), meto

CSAL 3/227

(1.3)

1 Metolachlor (H) 2/208

(1.0)

1 Metolachlor (H).

CAL 0 0 -- 0 0 --

Organochlorine

NAL 4/436

(0.9)

3 Triclopyr (H), alpha-endosulfan (I), dieldrin (I) 1/410 (0.2) 1 Triclopyr (H).

CSAL 1/227

(0.4)

1 Triclopyr (H) 1/208

(0.5)

1 Triclopyr (H).

CAL 1/111

(0.9)

1 Triclopyr (H) 1/109

(0.9)

1 Triclopyr (H).
Table 3. Pesticide detections relative to pesticide class in ground-water samples collected during fall and spring in Wyoming, 1995–2006.—Continu

[Pesticide classes listed in order of decreasing detection frequency by jointly ranking fall and spring detections by class with no assessment level. Assessment level: NAL, no ascompound-specific assessment level; CAL, common assessment level. Pesticide type: H, herbicide; HD, herbicide degradate; I, insecticide; ID, insecticide degradate. Symbol: -

Pesticide 

class

Assessment 

level

Fall Spring

Number of 

pesticide 

detections in 

class/number 

of detections 

(percentage)

Number of 

different 

pesticides 

detected in 

class

Pesticides detected

Number of 

pesticide 

detections in 

class/number 

of detections 

(percentage)

Number of 

different 

pesticides 

detected in 

class

Pesti

Pyrazole

NAL 4/436

(0.9)

2 Fipronil sulfide (ID), fipronil sulfone (ID) 0 0 --

CSAL 2/227

(0.9)

1 Fipronil sulfide (ID) 0 0 --

CAL 0 0 -- 0 0 --

Dinitroaniline

NAL 4/436

(0.9)

3 Benfluralin (H), oryzalin (H), trifluralin (H) 0 0 --

CSAL 1/227

(0.4)

1 Oryzalin (H) 0 0 --

CAL 1/111

(0.9)

1 Oryzalin (H) 0 0 --

Chlorophenoxy 

(acid)

NAL 3/436

(0.7)

3 2,4–D (H); 2,4–D methyl ester (H); dichlorprop (H) 3/410 (0.7) 2 2,4–D (H); dichlorp

CSAL 3/227

(1.3)

3 2,4–D (H); 2,4–D methyl ester (H); dichlorprop (H) 0 0 --

CAL 3/111

(2.7)

3 2,4–D (H); 2,4–D methyl ester (H); dichlorprop (H) 0 0 --

Miscellaneous 

(insecticide)

NAL 2/436

(0.5)

2 Imidacloprid (I), cis-permethrin (I) 1/410

(0.2)

1 Imidacloprid (I).

CSAL 1/227

(0.4)

1 Imidacloprid (I) 1/208

(0.5)

1 Imidacloprid (I).

CAL 0 0 -- 0 0 --

Organophosphate

NAL 2/436

(0.5)

1 Malathion (I) 1/410

(0.2)

1 Diazinon (I).

CSAL 0 0 -- 1/208

(0.5)

1 Diazinon (I).

CAL 0 0 -- 0 0 --
Figure 5. Pesticide concentrations and detections relative to pesticide class in ground-water samples collected in Wyoming, 

1995–2006. A, Fall; B, Spring. Pesticide classes are listed in order of decreasing detection frequency by jointly ranking fall and spring 

detections with no assessment level.

0.0001 0.001 0.01 0.1 1 10 100

CONCENTRATION, IN MICROGRAMS PER LITER

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

PERCENTAGE OF DETECTIONS BY PESTICIDE CLASS 

AND ASSESSMENT LEVEL

 PESTICIDE CLASS

EXPLANATION

Detection frequency greater than common 

 assessment level

Median concentration of two or more 

 detections Number of detections in pesticide class/total number of pesticide 

 detections at applicable assessment level (table 3) 

Detection frequency greater than compound-

 specific assessment levels

No assessment level applied

Percentage of detections by pesticide class and assessment level

Triazine

Urea

Pyridinecarboxylic acid

Miscellaneous (herbicide)

Carbamate

Amide

Chlorophenoxy acid

Organochlorine

Pyrazole

Dinitroaniline

Miscellaneous (insecticide)

Organophosphate

Concentration greater than common 

 assessment level

Concentration greater than compound-

 specific assessment levels

No assessment level applied

Detected concentration associated with 

 pesticide class and assessment level

A Fall

B Spring

Triazine

Urea

Pyridinecarboxylic acid

Miscellaneous (herbicide)

Pyrazole

Dinitroaniline

Miscellaneous (insecticide)

Organophosphate

Organochlorine

Chlorophenoxy acid

Carbamate

Amide

299/436

139/227

53/111

46/436

34/227

16/111

28/436

22/227

19/111

21/436

13/227

9/111

15/436

8/227

9/111

8/436

3/227

0/111

4/436

2/227

0/111

4/436

1/227

1/111

3/436

3/227

3/111

4/436

1/227

1/111

2/436

1/227

0/111

2/436

0/227

0/111

300/410

130/208

49/109

44/410

37/208

20/109

24/410

17/208

20/109

16/410

12/208

11/109

11/410

7/208

8/109

9/410

2/208

0/109

0/410

0/208

0/109

0/410

0/208

0/109

3/410

0/208

0/109

1/410

1/208

1/109

1/410

1/208

0/109

1/410

1/208

0/109

0/109

Summary of Pesticide Occurrence 25
26 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006

Detected Concentrations and Calculated 

Detection Frequencies

Concentrations of all detected pesticides and calculated 

detection frequencies are summarized for fall (table 4) and 

spring (table 5). Detected pesticides also are identified by 

type. Detection frequencies, concentration ranges, and median 

values differed greatly in some cases by assessment level. 

In general, the number of pesticides detected and the 

frequency of detection were greater in the fall than in the 

spring. Considering only detections with original censoring (NAL), 44 different pesticides were detected in water 

from wells sampled in the fall (table 4), whereas 27 different 

pesticides were detected in water from wells sampled in spring 

(table 5). The number of pesticides detected was smaller and 

did not differ as much by season when censored to individual 

pesticide CSALs. Twenty-eight different pesticides were 

detected at concentrations greater than the CSALs in water 

from wells sampled in fall, whereas 21 different pesticides 

were detected in water from wells sampled in spring. 

Figure 6 shows the concentrations and frequencies of 

pesticides detected most often—pesticides detected with original censoring (NAL) in 1 percent or more of wells sampled in 

either the fall or the spring. Using figure 6, graphical comparisons can be made between individual pesticides detected in 

the same season or between the same pesticide in both seasons 

with and without censoring.

Many pesticides were detected infrequently (tables 4 

and 5). Thirteen of 28 pesticides detected (about 46 percent) 

at concentrations greater than the CSALs in water from wells 

sampled in the fall were detected only in one well. Ten of 

21 pesticides detected (about 48 percent) at concentrations 

greater than the CSALs in water from wells sampled in the 

spring were detected only in one well. Considering only detections using the CSALs, 14 of 20 focal pesticides identified in 

the SMP were detected at least once in water sampled from 

wells during either the fall or spring. 

Considering only concentrations greater than the CSALs, 

only five pesticides were each detected more than 5 percent of 

the time in water from wells sampled in either fall or spring 

(tables 4 and 5; fig. 6). Four of these five frequently detected 

pesticides were herbicides (atrazine, prometon, tebuthiuron, and picloram, listed in order of decreasing detection 

frequency), whereas the fifth frequently detected pesticide was 

an herbicide degradate (3,4-dichloroaniline). Three of these 

pesticides (atrazine, prometon, and tebuthiuron) have properties indicating high potential to be transported to ground water 

as they are moderately to highly soluble in water, adsorb only 

weakly to soil particles, and are persistent in soils and water 

(on the basis of soil and water half-lives; Gilliom and others, 

2006, appendix 2, table A). With the exception of prometon 

and 3,4-dichloroaniline, all of these frequently detected pesticides were identified as focal pesticides of concern in the SMP. 

The “large” detection frequency calculated for the herbicide 

degradate 3,4-dichloroaniline (about 9 percent, table 4) is the 

result of infrequent analysis as the pesticide was analyzed in 

water from only 11 wells sampled. 

Atrazine was the pesticide detected most frequently (as a 

parent compound) in water from wells sampled in either fall 

or spring at concentrations greater than the CSAL (tables 4 

and 5; fig. 6). Atrazine was detected in water from 82 of 

294 wells (about 28 percent) sampled and analyzed for the 

compound in the fall and in water from 78 of 284 wells (about 

27 percent) sampled and analyzed for the compound in the 

spring. Several atrazine degradates also were detected in water 

at concentrations greater than the respective CSALs, although 

much less often than the parent compound—deethylatrazine 

[13 of 296 wells sampled in the fall (about 4 percent) and 12 

of 295 wells (about 4 percent) sampled in the spring], deisopropylatrazine [1 of 96 wells (about 1 percent) sampled in the 

fall and 1 of 121 wells (less than 1 percent) sampled in the 

spring], and hydroxyatrazine, which was only detected in 1 of 

70 wells (1.4 percent) sampled during the fall. Atrazine is a 

selective triazine herbicide used primarily on corn. Nationally, 

atrazine is the most frequently detected pesticide in ground 

water underlying agricultural areas (Gilliom and others, 

2006). Degradates of atrazine and other pesticides commonly 

are found as frequently as or more frequently than the parent 

compounds in ground water in the United States (Gilliom, 

2001; Gilliom and others, 2006).

In the baseline study reported herein, four pesticide 

degradates were detected, although the corresponding parent 

pesticide was not detected. Degradates of the insecticides 

aldicarb (aldicarb sulfone and aldicarb sulfoxide) and fipronil 

(fipronil sulfone and fipronil sulfide) were detected infrequently (tables 4 and 5; fig. 6), and neither parent compound 

was detected in water from any wells sampled during this 

study. However, the two fipronil sulfone detections should be 

viewed with caution because both detections were recoded 

as nondetections when recensored to the CSAL (table 4). 

Pesticide degradates can be as persistent and mobile in water 

as their parent compounds, and in some cases, degradates are 

more persistent and more mobile in water than their parent 

compounds (Gilliom and others, 2006, and references therein).

Individual pesticides detected in both the fall and spring 

at a frequency equal to or greater than 1 percent using the 

CSALs were compared statistically using the CTA to assess 

seasonal differences (table 6). For all pesticides tested, the 

proportion of samples in which pesticides were detected at 

concentrations greater than respective CSALs (detection 

frequency) was not significantly different between fall and 

spring (all p-values > 0.05, CTA). 
Table 4. Summary of pesticide detections and concentrations in ground-water samples collected during the fall in Wyoming, 1995–2006.—Continue

[Pesticides listed in order of decreasing detection frequency with no assessment level applied. Pesticides in bold type are focal pesticides. Abbreviations: USEPA, U.S. Environμg/L, micrograms per liter; MCL, Maximum Contaminant Level; LHA, Lifetime Health Advisory; RSD4, Risk-Specific Dose at 10-4 Cancer Risk. Assessment level: NAL, no acompound-specific assessment level; CAL, common assessment level. Pesticide type: H, herbicide; HD, herbicide degradate; I, insecticide; ID, insecticide degradate. Symbol: -cable]

Pesticide

Pesticide 

type

Number of detections/

number of analyses

Detection frequency 

(percentage)

Detected concentration range 

(μg/L)

Median of d

concentr

(μg/L

NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL NAL CSA

Atrazine H 95/296 82/294 9/296 32.1 27.9 3.0 0.002–0.890 0.004–0.890 0.070–0.890 0.008 0.0Prometon H 93/295 39/295 33/295 31.5 13.2 11.2 0.001–2.88 0.05–2.88 0.07–2.88 .02 .1Deethylatrazine HD 81/296 13/296 10/296 27.4 4.4 3.4 0.001–0.81 0.05–0.81 0.07–0.81 .006 .12 Tebuthiuron H 32/294 31/294 14/294 10.9 10.5 4.8 0.004–1.97 0.01–1.97 0.07–1.97 .04 .04 Oxyfluorfen H 1/11 0/11 0/11 9.1 0 0 0.007 -- -- -- -- 3,4-Dichloroaniline HD 1/11 1/11 0/11 9.1 9.1 0 0.007 0.007 -- -- -- alpha-Endosulfan I 1/11 0/11 0/11 9.1 0 0 0.003 -- -- -- -- Picloram H 24/284 20/281 17/282 8.5 7.1 6.0 0.03–9.50 0.06–9.50 0.07–9.50 .20 .28 Simazine H 23/295 2/295 1/295 7.8 .7 .3 0.002–0.093 0.058–0.093 0.093 .006 .07Sulfometuron, methyl H 3/71 0/71 0/71 4.2 0 0 0.020–0.022 -- -- .021 -- Bromacil H 9/291 8/286 6/286 3.1 2.8 2.1 0.04–8.07 0.06–8.07 0.15–8.07 .60 .70 Hydroxyatrazine HD 2/70 1/70 0/70 2.9 1.4 0 0.007–0.02 0.02 -- .01 -- Flumetsulam H 2/71 2/71 1/71 2.8 2.8 1.4 0.06–0.08 0.06–0.08 0.08 .07 .07 Fipronil sulfide ID 2/71 2/71 0/71 2.8 2.8 0 0.007 0.007 -- .007 .00Fipronil sulfone ID 2/71 0/71 0/71 2.8 0 0 0.009–0.011 -- -- .010 -- Diuron H 8/292 2/291 2/291 2.7 .7 .7 0.01–0.19 0.08–0.19 0.08–0.19 .04 .14 Metolachlor H 8/295 3/275 0/295 2.7 1.1 0 0.002–0.024 0.009–0.024 -- .006 .0Deisopropylatrazine HD 2/96 1/96 0/96 2.1 1.0 0 0.01–0.05 0.05 -- .03 -- Aldicarb sulfoxide ID 5/285 3/280 5/259 1.8 1.1 1.9 0.07–2.47 0.14–2.47 0.07–2.47 .14 .25 2,4–D methyl ester H 1/71 1/71 1/71 1.4 1.4 1.4 0.737 0.737 0.737 -- -- Chlorimuron H 1/71 0/71 0/71 1.4 0 0 0.008 -- -- -- -- Clopyralid H 4/284 2/282 2/134 1.4 .7 1.5 0.02–22.6 0.45–22.6 0.45–22.6 .24 11.5 DCPA H 4/293 1/293 0/293 1.4 .3 0 0.002–0.004 0.004 -- .002 -- Metsulfuron H 1/71 0/71 0/71 1.4 0 0 0.03 -- -- -- -- Imidacloprid I 1/71 1/71 0/71 1.4 1.4 0 0.029 0.029 -- -- -- Aldicarb sulfone ID 4/285 3/281 4/136 1.4 1.1 2.9 0.09–0.30 0.14–0.30 0.09–0.30 .14 .14 Bentazon H 3/284 1/284 1/284 1.1 0.4 0.4 0.03–0.14 0.14 0.14 .04 -- 
Table 4. Summary of pesticide detections and concentrations in ground-water samples collected during the fall in Wyoming, 1995–2006.—Continue

[Pesticides listed in order of decreasing detection frequency with no assessment level applied. Pesticides in bold type are focal pesticides. Abbreviations: USEPA, U.S. Environμg/L, micrograms per liter; MCL, Maximum Contaminant Level; LHA, Lifetime Health Advisory; RSD4, Risk-Specific Dose at 10-4 Cancer Risk. Assessment level: NAL, no acompound-specific assessment level; CAL, common assessment level. Pesticide type: H, herbicide; HD, herbicide degradate; I, insecticide; ID, insecticide degradate. Symbol: -cable]

Pesticide

Pesticide 

type

Number of detections/

number of analyses

Detection frequency 

(percentage)

Detected concentration range 

(μg/L)

Median of d

concentr

(μg/L

NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL NAL CSA

Cyanazine H 2/295 1/276 0/276 .7 .4 0 0.007–0.018 0.018 -- .012 -- Trifluralin H 2/293 0/293 0/293 .7 0 0 0.004–0.008 -- -- .006 -- Carbaryl I 2/293 0/293 0/293 .7 0 0 0.003–0.004 -- -- .004 -- Carbofuran I 2/293 2/293 0/293 .7 .7 0 0.022–0.042 0.022–0.042 -- .032 0.03Dieldrin I 2/293 0/293 0/293 .7 0 0 0.007 -- -- .007 -- Malathion I 2/293 0/293 0/293 .7 0 0 0.007–0.011 -- -- .009 -- Triallate H 1/282 0/282 0/282 .4 0 0 0.001 -- -- -- -- 2,4–D H 1/290 1/288 1/196 .3 .3 .5 13.8 13.8 13.8 -- -- Benfluralin H 1/293 0/293 0/293 .3 0 0 0.008 -- -- -- -- Bromoxynil H 1/290 1/289 1/289 .3 .3 .3 0.22 0.22 0.22 -- -- Dichlorprop H 1/290 1/290 1/290 .3 .3 .3 0.07 0.07 0.07 -- -- Metribuzin H 1/295 0/295 0/295 .3 0 0 0.006 -- -- -- -- Norflurazon H 1/292 0/291 0/291 .3 0 0 0.02 -- -- -- -- Oryzalin H 1/291 1/227 1/135 .3 .4 .7 0.63 0.63 0.63 -- -- Triclopyr H 1/290 1/290 1/187 .3 .3 .5 18.6 18.6 18.6 -- -- cis-Permethrin I 1/293 0/293 0/293 .3 0 0 0.01 -- -- -- -- 3-Hydroxycarbofuran ID 1/291 0/290 0/290 .3 0 0 0.001 -- -- -- -- 

1Source: U.S. Environmental Protection Agency, 2006.
Table 5. Summary of pesticide detections and concentrations in ground-water samples collected during the spring in Wyoming, 1995–2006.

[Pesticides listed in order of decreasing detection frequency with no assessment level applied. Pesticides in bold type are focal pesticides. Abbreviations: USEPA, U.S. Environmmicrograms per liter; MCL, MCL, Maximum Contaminant Level; LHA, Lifetime Health Advisory; RSD4, Risk-Specific Dose at 10-4 Cancer Risk. Assessment level: NAL, no acompound-specific assessment level; CAL, common assessment level. Pesticide type: H, herbicide; HD, herbicide degradate; I, insecticide; ID, insecticide degradate. Symbol: --

Pesticide

Pesticide 

type

Number of detections/

number of analyses

Detection frequency 

(percentage)

Detected concentration range 

(μg/L)

Median of de

concentrations 

NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL NAL CSAL 

Atrazine H 95/295 78/284 10/295 32.2 27.5 3.4 0.001–0.900 0.004–0.900 0.090–0.900 0.008 0.010 Prometon H 84/294 35/294 27/294 26.8 11.9 9.2 0.002–1.24 0.05–1.24 0.07–1.24 .03 .11 Deethylatrazine HD 79/295 12/295 9/295 26.8 4.1 3.1 0.002–0.550 0.05–0.550 0.073–0.550 .007 .146 Tebuthiuron H 35/295 34/295 17/295 11.9 11.5 5.8 0.005–1.18 0.01–1.18 0.07–1.18 .06 .06 Deisopropylatrazine HD 12/121 1/121 1/121 9.9 .8 .8 0.01–0.19 0.19 0.19 .03 -- Simazine H 25/294 2/294 1/294 8.5 .7 .3 0.002–0.112 0.069–0.112 0.112 .008 .090 Picloram H 19/291 16/288 15/288 6.5 5.6 5.2 0.01–12.2 0.06–12.2 0.07–12.2 .30 .42 Bromacil H 12/294 10/288 9/288 4.1 3.5 3.1 0.04–5.38 0.06–5.38 0.10–5.38 .27 .44 Diuron H 9/294 3/294 3/294 3.1 1.0 1.0 0.01–2.69 0.07–2.69 0.07–2.69 .04 .11 Hexazinone H 1/35 1/35 0/35 2.9 2.9 0 0.06 0.06 -- -- -- Metolachlor H 8/294 2/259 0/294 2.7 .8 0 0.003–0.056 0.029–0.056 -- .006 .042 Clopyralid H 5/288 1/285 5/116 1.7 .4 4.3 0.07–22.0 22.0 0.07–22.0 .11 -- Aldicarb sulfone ID 5/292 3/288 5/120 1.7 1.0 4.2 0.08–0.36 0.17–0.36 0.08–0.36 .17 .35 Aldicarb sulfoxide ID 5/292 3/291 3/243 1.7 1.0 1.2 0.06–0.52 0.46–0.52 0.46–0.52 .46 .47 Imidacloprid I 1/71 1/70 0/70 1.4 1.4 0 0.032 0.032 -- -- -- 2,4–D H 2/294 0/292 0/182 .7 0 0 0.04 -- -- .04 -- Cyanazine H 2/294 1/259 1/259 .7 .4 .4 0.008–0.13 0.13 0.13 .07 -- Metribuzin H 2/294 0/294 0/294 .7 0 0 0.007–0.010 -- -- .008 -- Triallate H 1/284 1/259 0/284 .4 .4 0 0.005 0.005 -- -- -- Alachlor H 1/294 0/294 0/294 .3 0 0 0.004 -- -- -- -- Bentazon H 1/294 1/294 1/294 .3 .3 .3 0.59 0.59 0.59 -- -- DCPA H 1/294 0/294 0/294 .3 0 0 0.001 -- -- -- -- Dicamba H 1/292 1/292 1/292 .3 .3 .3 1.11 1.11 1.11 -- -- Dichlorprop H 1/294 0/293 0/293 .3 0 0 0.01 -- -- -- -- Norflurazon H 1/294 0/293 0/293 .3 0 0 0.01 -- -- -- -- Triclopyr H 1/294 1/294 1/197 .3 .3 .5 1.51 1.51 1.51 -- -- Diazinon I 1/294 1/294 0/294 .3 .3 0 0.016 0.016 -- -- -- 

1Source: U.S. Environmental Protection Agency, 2006.
0.001 0.01 0.1 1 10 100 1,000

CONCENTRATION, IN MICROGRAMS PER LITER

0 5 10 15 20 25 30 35

DETECTION FREQUENCY GREATER THAN 

APPLICABLE ASSESSMENT LEVEL, IN PERCENT

EXPLANATION

MCL

LHA

LHA

MCL

MCL

LHA

RSD4

LHA

LHA

LHA

MCL

MCL

Concentration greater than

 common assessment level

Median concentration of two or

 more detections

Number of detections/number of samples 

 at applicable assessment level (tables 4 and 5)

No assessment level applied

Concentration greater than

 compound-specific assessment

 level

Detection frequency greater than

 common assessment level

Detection frequency greater than

 compound-specific assessment

 level

No assessment level applied

Detected concentration associated

 with assessment level

LHA

Fipronil sulfone

Fipronil sulfide

Aldicarb sulfone

Aldicarb sulfoxide

Imidacloprid

alpha-Endosulfan

3,4-Dichloroaniline

Bentazon

Metsulfuron

Chlorimuron

2,4–D methyl ester

DCPA

Flumetsulam

Clopyralid

Sulfometuron, methyl

Hexazinone

Metolachlor

Diuron

Bromacil

Oxyfluorfen

Picloram

Simazine

Tebuthiuron

Prometon

Atrazine

PESTICIDE DETECTED

HERBICIDES

HERBICIDE

DEGRADATES

INSECTICIDE

DEGRADATES INSECTICIDES

Hydroxyatrazine

Deisopropylatrazine

Deethylatrazine

Compound-specific assessment level 0/71

U.S. Environmental Protection Agency (2006) 

 Maximum Contaminant Level (MCL)

Common assessment level

U.S. Environmental Protection Agency (2006) 

 Lifetime Health Advisory (LHA)

MCL

LHA

U.S. Environmental Protection Agency (2006) 

 Risk-Specific Dose at 10-4 cancer risk 

 (RSD4)

RSD4

Detection frequency associated

 with assessment level

Pesticide names in blue are focal pesticides. 

Pesticide names in black are nonfocal pesticides. 

A Fall

95/296

82/294

9/296

93/295

39/295

33/295

32/294

31/294

14/294

23/295

2/295

1/295

24/284

20/281

17/282

1/11

0/11

0/11

9/291

8/286

6/286

8/292

2/291

2/291

8/295

3/275

0/295

0/11

0/11

0/11

3/71

0/71

0/71

4/284

2/282

2/134

2/71

2/71

1/71

4/293

1/293

0/293

1/71

1/71

1/71

1/71

0/71

0/71

3/284

1/284

1/284

81/296

13/296

10/296

2/96

1/96

0/96

1/11

1/11

0/11

2/70

1/70

0/70

1/11

0/11

0/11

1/71

1/71

0/71

5/285

3/280

5/259

4/285

3/281

4/136

2/71

2/71

0/71

2/71

0/71

0/71

1/71

0/71

0/71

Figure 6. Concentration and detection frequency of selected pesticides in ground-water samples collected in Wyoming, 1995–2006. 

A, Fall; B, Spring. Pesticides are grouped by pesticide type and are listed in order of decreasing detection frequency by jointly ranking 

fall and spring detections with no assessment level.

30 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006
Figure 6. Concentration and detection frequency of selected pesticides in ground-water samples collected in Wyoming, 1995–2006. 

A, Fall; B, Spring. Pesticides are grouped by pesticide type and are listed in order of decreasing detection frequency by jointly ranking 

fall and spring detections with no assessment level.—Continued

MCL

LHA

LHA

MCL

MCL

LHA

LHA

MCL

MCL

LHA

RSD4

LHA

LHA

0.001 0.01 0.1 1 10 100 1,000 0 5 10 15 20 25 30 35

CONCENTRATION, IN MICROGRAMS PER LITER DETECTION FREQUENCY GREATER THAN 

APPLICABLE ASSESSMENT LEVEL, IN PERCENT

95/295

78/284

10/295

84/294

35/294

27/294

35/295

34/295

17/295

25/294

2/294

1/294

19/291

16/288

15/288

0/10

0/10

0/10

12/294

10/288

9/288

9/294

3/294

3/294

8/294

2/259

0/294

1/35

1/35

0/35

0/71

0/71

0/71

5/288

1/285

5/116

0/71

0/71

0/71

1/294

0/294

0/294

0/69

0/69

0/69

1/294

1/294

1/294

79/295

12/295

9/295

12/121

1/121

1/121

0/10

0/10

0/10

0/71

0/71

0/71

0/10

0/10

0/10

1/71

1/70

0/70

5/292

3/291

3/243

5/292

3/288

5/120

0/83

0/83

0/83

0/83

0/83

0/83

0/71

0/71

0/71

0/71

0/71

0/71

Fipronil sulfone

Fipronil sulfide

Aldicarb sulfone

Aldicarb sulfoxide

Imidacloprid

alpha-Endosulfan

3,4-Dichloroaniline

Bentazon

Metsulfuron

Chlorimuron

2,4–D methyl ester

DCPA

Flumetsulam

Clopyralid

Sulfometuron, methyl

Hexazinone

Metolachlor

Diuron

Bromacil

Oxyfluorfen

Picloram

Simazine

Tebuthiuron

Prometon

Atrazine

PESTICIDE DETECTED

HERBICIDES

HERBICIDE

DEGRADATES

INSECTICIDE

DEGRADATES INSECTICIDES

Hydroxyatrazine

Deisopropylatrazine

Deethylatrazine

B Spring

Summary of Pesticide Occurrence 31
Table 6. Results of contingency-table analysis (CTA) comparing 

selected pesticide detections in ground-water samples collected 

during fall and spring in Wyoming, 1995–2006.

[Only individual pesticides detected in both the fall and spring at a frequency 

equal to or greater than 1 percent using CSALs were tested. Statistical test 

(significance indicated by p-value <0.05) between fall and spring samples. H, 

herbicide; CSAL, compound-specific assessment level; HD, herbicide degradate; ID, insecticide degradate; I, insecticide]

Tested pesticide Pesticide 

type

Assessment 

level used

Probability 

(p-value)

Atrazine H CSAL 0.9828

Prometon H CSAL .7208

Deethylatrazine HD CSAL .9931

Tebuthiuron H CSAL .8038

Picloram H CSAL .5532

3,4-Dichloroaniline H CSAL .9610

Bromacil H CSAL .8224

Diuron H CSAL .9908

Aldicarb sulfoxide ID CSAL .7166

Aldicarb sulfone ID CSAL .7038

Imidacloprid I CSAL .4826

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

Detected Pesticide Concentrations and 

Comparison to Standards or Health Advisories

Concentrations of detected pesticides generally were 

small (less than 1 μg/L), although a few infrequent detections 

at larger concentrations were noted (tables 4 and 5; fig. 6). 

Nationally, pesticides detected in ground water typically are 

found at similar small concentrations (Gilliom and others, 

2006). Ten different pesticides (prometon, tebuthiuron, picloram, bromacil, aldicarb sulfoxide, clopyralid, 2,4–D, diuron, 

dicamba, and triclopyr, listed in order of decreasing detection frequency) were detected at concentrations greater than 

1 μg/L at least once in water from at least one well sampled 

in either fall or spring (tables 4 and 5). The largest pesticide 

concentration measured (22.6 μg/L, table 4) in water from 

wells sampled in either fall or spring was a detection of the 

herbicide clopyralid. 

Pesticide concentrations presented in this report are 

compared with several USEPA drinking-water standards or 

health advisories for finished (treated) water established in 

the Safe Drinking Water Act. The USEPA has established 

standards for physical properties and chemical constituents in 

drinking water that may have adverse effects on human health 

or that may cause cosmetic effects (for example, skin or tooth 

discoloration) or aesthetic effects (for example, color, taste, or 

odor), and these standards have been adopted by the WDEQ as 

Wyoming’s ground-water protection standards. The Maximum 

Contaminant Level (MCL) is legally enforceable and healthbased and is the maximum permissible level for a constituent 

in drinking water that is delivered to a user of a public-water 

system (U.S. Environmental Protection Agency, 2006). A 

health advisory is a nonenforceable level that establishes 

acceptable constituent concentrations for different exposure 

periods (1 day, 10 days, long-term, and lifetime). A Lifetime 

Health Advisory (LHA) is the concentration of a chemical that 

would not result in any known or anticipated adverse noncarcinogenic health effects over a lifetime of exposure (70 years; 

U.S. Environmental Protection Agency, 2006). The RiskSpecific Dose (RSD) is a concentration of a chemical with a 

specific risk level under certain exposure conditions over a 

lifetime (70 years; U.S. Environmental Protection Agency, 

2006). USEPA standards and health advisory levels serve as 

technical guidance to evaluate the suitability of water collected 

from private wells for human consumption. In addition, pesticide detections were evaluated by the GPSC and compared to 

Wyoming ground-water-quality standards in accordance with 

the SMP (Wyoming Ground-water and Pesticides Strategy 

Committee, 1999).

All detected pesticide concentrations were smaller than 

USEPA drinking-water standards or health advisories where 

applicable (many pesticides did not have standards or advisories; tables 4 and 5; fig. 6). Most pesticide detections were 

an order of magnitude smaller than USEPA standards or 

health advisories. One detection of the insecticide degradate 

aldicarb sulfoxide was measured in the fall at a concentration 

(2.47 μg/L, table 4) within the same order of magnitude as the 

USEPA MCL (4 μg/L); however, this detection was about five 

times larger than the next largest sample concentration, which 

was about an order of magnitude less than the MCL.

Detected Pesticides in Relation to Use

All parent pesticides detected during the baseline groundwater sampling were compared to a survey describing 1994 

agricultural pesticide use in Wyoming (Ferrell and others, 

1996) (fig. 7). Although pesticide use is reported for the year 

1994 and prior to the beginning of the baseline ground-water 

sampling examined in this report (1995), the survey described 

in Ferrell and others (1996) was the last conducted and is the 

most recent describing Wyoming agricultural pesticide use. 

Ferrell and others (1996) reported agricultural pesticide use 

in relation to both cropland (row crops, small grains, hay, and 

alfalfa) and noncropland (improved pasture and rangeland). 

Figure 7 shows all parent pesticides detected with original 

censoring (NAL) in relation to reported 1994 agricultural use 

in Wyoming. Pesticides are grouped by type and are listed in 

order of decreasing detection frequency by jointly ranking fall 

and spring detections with no assessment level applied (NAL). 

Some detected parent pesticides shown in figure 7 were 

reported as nondetections when censored to their respective 

CSALs.

Seventeen of the 40 parent pesticides detected during the 

baseline ground-water sampling had no reported 1994 agricultural use in Wyoming (fig. 7). With the exception of picloram, 

four of the five frequently detected pesticides (atrazine through 

picloram in figure 7) had use reported for only two cropland 
Figure 7. Parent pesticides detected in ground-water samples collected during fall and spring, 1995–2006, relative to reported 1994 

agricultural use in Wyoming. Pesticides are grouped by type and are listed in order of decreasing detection frequency by jointly ranking 

fall and spring detections with original censoring (no assessment level applied). Reported 1994 agricultural pesticide use from Ferrell 

and others (1996).

Improved 

pasture PESTICIDE Sugarbeets Dry beans Corn Potatoes Barley Oats Wheat Hay 

(not alfalfa) Alfalfa

Rangeland

Prometon

Tebuthiuron

Simazine

Picloram

Oxyfluorfen

Bromacil

Diuron

Metolachlor

Hexazinone

Sulfometuron, methyl

Clopyralid

Flumetsulam

DCPA

2,4–D methyl ester

Chlorimuron

Metsulfuron

Bentazon

Cyanazine

2,4–D

Metribuzin

Triallate

Dichlorprop

Triclopyr

Norflurazon

Trifluralin

Dicamba

Bromoxynil

Oryzalin

Benfluralin1

alpha-Endosulfan

Aldicarb2

Imidacloprid

Carbaryl

Carbofuran

Dieldrin

Malathion

Diazinon

Pesticide used on 1 percent or more of total crop acres

Pesticide used on less than 1 percent of total crop acres

Row crops Small grains Hay and alfalfa

EXPLANATION

No reported agricultural pesticide use

Agricultural pesticide use

HERBICIDES INSECTICIDES

Alachlor

Atrazine

cis-Permethrin

1

Benfluralin is defined as benefin in Ferrell and others (1996).

2

Only aldicarb degradates (aldicarb sulfone and aldicarb sulfoxide) were detected.

Pesticide names in blue are focal pesticides. 

Pesticide names in black are nonfocal pesticides. 

Summary of Pesticide Occurrence 33
34 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006

(atrazine) or two noncropland categories (tebuthiuron), or had 

no reported agricultural use (prometon and simazine). Of the 

23 parent pesticides detected with reported agricultural use in 

Wyoming, clopyralid had use reported for the most land-use 

categories. Picloram likely had the largest agricultural use on 

the basis of application of the pesticide to 1 percent or more of 

total crop acres for six different agricultural categories (indicated by red dots in figure 7).

 Pesticide detections associated with noncropland 

(pasture and rangeland) use were noted (fig. 7). Eight different 

pesticides with reported noncropland use were detected. These 

pesticides had little reported 1994 agricultural use on the basis 

of application to less than 1 percent of total improved pasture 

or rangeland acres. Pesticides detected during this study, but 

with no identified agricultural use in the 1994 survey, may 

be attributed to infrequent use in Wyoming at the time of the 

1994 survey, nonagricultural use, or use in Wyoming subsequent to the survey (after 1994).

Most detected parent pesticides were reportedly used 

primarily for agricultural cropland purposes in 1994, but two 

of the pesticides detected most frequently during this study 

(prometon and tebuthiuron) either had no reported agricultural 

use (prometon) or limited noncropland use (tebuthiuron) in 

Wyoming (fig. 7). In the United States, both pesticides are 

associated primarily with noncropland use, and detections in 

ground water are more commonly associated with urban areas 

(Gilliom and others, 2006). Prometon is not registered for 

agricultural use in the United States, but the broad-spectrum 

herbicide is used to control weeds in noncropland areas such 

as rangelands, around buildings and storage areas, industrial 

sites, as well as along roads, fences, railroads, pipelines, power 

lines, and other rights-of-way (Meister, 2002; Gilliom and 

others, 2006). Prometon also can be used in and under asphalt 

to prevent weeds from growing (Ciba-Geigy Corporation, 

1994). Prometon has been found in ground water underlying 

both urban and agricultural areas in the United States, but 

more commonly in urban areas; detections in agricultural areas 

probably are the result of nonagricultural applications in these 

areas (Gilliom and others, 2006). Tebuthiuron is registered 

by the USEPA as a general-use herbicide, and like prometon, 

is used in noncropland areas as a broad-spectrum herbicide 

to control weeds, grasses, and woody plants (Meister, 2002; 

Cornell University Pesticide Management Education Program, 

2008).

Geography

Pesticide detections were examined in relation to 

Wyoming geography. To facilitate comparisons among different geographic areas, the State was divided into eight regional 

geographic areas on the basis of similar environmental setting 

using both natural (geography, physiography, hydrology, 

and geology) and anthropogenic (crops grown and likely 

agricultural pesticide use) characteristics. The eight regional 

geographic areas—Bighorn Basin, Central Basins, Green 

River Basin, High Plains/Casper Arch, Northwest Basins, 

Overthrust Belt, Powder River Basin, and Wind River Basin— 

are shown in figure 8 along with wells sampled during 

1995–2006.

Pesticides detected in each geographic area are listed 

and summarized in table 7. Sampled wells with no pesticide 

detections and with at least one pesticide detected using the 

CAL are shown in figure 8 in relation to geographic area. 

Regardless of assessment level and considering both seasons, 

the largest percentage of pesticide detections and the largest 

number of different pesticides detected occurred in samples 

from wells located in the Bighorn Basin and High Plains/

Casper Arch geographic areas (table 7). In fact, using the 

CAL, samples from wells in the Bighorn Basin and High 

Plains/Casper Arch geographic areas combined comprised 

about 69 percent of all detections in the fall, and about 

63 percent of all detections in the spring. Pesticide use for 

agricultural purposes in both geographic areas is likely among 

the largest as both geographic areas comprise two of three 

“major crop producing areas in Wyoming” (Wyoming Department of Agriculture, 1995, p. 20); the Wind River Basin is 

the third major crop-producing area. Pesticides were detected 

distinctly less often in samples from wells in other geographic 

areas. Considering only concentrations greater than the CAL, 

the proportion of samples with at least one pesticide detected 

was significantly different among the eight geographic areas 

for both the fall (p-value < 0.0001, CTA, table 8) and the 

spring (p-value < 0.0001, CTA, table 8). 

All pesticides detected in either the fall or spring are 

summarized in relation to the eight geographic areas in table 9. 

Considering only concentrations greater than their respective 

CSALs, only six herbicides (atrazine, prometon, tebuthiuron, 

picloram, bromacil, and diuron, listed in order of decreasing detection frequency) and one degradate (deethylatrazine) 

were detected in either the fall or spring in three or more 

different geographic areas (although some pesticides were not 

analyzed in samples from all eight geographic areas, table 9). 

Of these seven compounds, prometon was the only pesticide 

detected in either season in all eight geographic areas (samples 

collected in the spring, table 9). Considering only concentrations greater than their respective CSALs, 17 pesticides 

were detected in only one geographic area in the fall, and 

12 pesticides were detected in only one geographic area in the 

spring. The proportion of samples detected at concentrations 

greater than their respective CSALs (detection frequency) 

was significantly different among the eight geographic areas 

Relation of Pesticide Occurrence to 

Selected Factors

Various natural and anthropogenic factors that might 

affect the occurrence of pesticides in Wyoming’s ground 

water were evaluated and are described in this section of the 

report. Pesticide occurrence is qualitatively and quantitatively 

summarized and evaluated in relation to geography, land use, 

hydrogeology, soils, and nitrate and selected water-quality 

characteristics.
PARK CROOK

BIG

HORN

SHERIDAN

WASHAKIE

JOHNSON

CAMPBELL

TETON

HOT

SPRINGS

FREMONT

LINCOLN

UINTA

SWEETWATER

NATRONA

CONVERSE

NIOBRARA

WESTON

GOS

PLATTE

ALBANY

CARBON

LARAMIE

SUBLETTE

Lu

Cody

Basin

Casper

Lander

Laramie

Rawlins

Douglas

Jackson

Worland

Buffalo

Cheyenne

Evanston

Kemmerer

Pinedale

Gillette

Sundance

Sheridan

Wheatland

Newcast

GreenRiver

Thermopolis

YELLOWSTONE

NATIONAL

PARK 90

25

25

80

90

111°

110°

109° 108° 107° 106° 105°

41°

42°

43°

44°

45°

0

0

50

50

100 MILES

100 KILOMETERS

Base from U.S. Geological Survey digital data, 2003, 1:2,000,000Albers Equal-Area Conic projectionStandard parallels 29°30′N and 45°30′N, central meridian 107°30′W

River

Sweetwater

PathfinderReservoir

Seminoe Reservoir

BoysenReservoir

Keyhole

Reservoir

GlendoReservoir

Guernsey Reservoir

Fontenelle Reservoir

WheatlandReservoir

River

River

Platte North

Laramie

Cheyenne

River

Ri

Belle

Little Missouri

River

Fourche

River

Tongue

Powder

R

Lake DeSmet

Bighorn  Lake

Bighorn Shoshone

R Buffalo Bill

Reservoir

River

South

Fork

Greybull

River

River

Wind

River

Snake

River

Greys

River

Fork New

Green

Fork

River

Hams

River

Henrys Fork

River

River

Big Sandy

North Platte

River

River

Popo Agie

Medicine Bow

Fork

B

Blacks

ear R Smiths Fork

River Salt Afton

Saratoga

Glenrock

Riverton

Rock Springs

Lovell

Powell

Yellowstone

Lake

Jackson

Lake

Flaming GorgeReservoir

Gros Ventre River

AlcovaReservoir

Ocean Lake

Figure 8. Geographic areas and ground-water wells sampled in Wyoming during 1995–2006.
Table 7. Pesticide detections in ground-water samples collected during fall and spring relative to eight regional geographic areas in 

Wyoming, 1995–2006.—Continued

[Geographic areas listed in order of decreasing pesticide detection frequency by jointly ranking fall and spring detections with no assessment level applied.

Assessment level: NAL, no assessment level applied; CSAL, compound-specific assessment level; CAL, common assessment level. 

Pesticide type: H, herbicide; HD, herbicide degradate; I, insecticide; ID, insecticide degradate. Symbol: --, not applicable]

Geographic 

area

Assessment 

level

Fall

Number of 

pesticides 

detected in 

geographic 

area/number 

of pesticide 

detections in 

all geographic 

areas

 (percentage)

Number of 

different 

pesticides 

detected in 

geographic 

area

Pesticides detected

Bighorn Basin

NAL 167/436 (38.3) 21 Atrazine (H), bentazon (H), bromacil (H), bromoxynil (H), cyanazine (H), DCPA (H), diuron (H), metolachlor (H), metribuzin (H), norflurazon (H), picloram (H), prometon (H), simazine (H), tebuthiuron (H), trifluralin (H), deethylatrazine (HD), 

deisopropylatrazine (HD), carbaryl (I), carbofuran (I), aldicarb 

sulfone (ID), aldicarb sulfoxide (ID)

CSAL 70/227 (30.8) 12 Atrazine (H), bentazon (H), bromacil (H), bromoxynil (H), metolachlor (H), picloram (H), prometon (H), tebuthiuron (H), deethylatrazine (HD), carbofuran (I), aldicarb sulfone (ID), aldicarb 

sulfoxide (ID)

CAL 28/111 (25.2) 9 Atrazine (H), bentazon (H), bromoxynil (H), picloram (H), prometon (H), tebuthiuron (H), deethylatrazine (HD), aldicarb sulfone (ID), aldicarb sulfoxide (ID)

High Plains/

Casper Arch

NAL 109/436 (25.0) 15 2,4–D (H), 2,4–D methyl ester (H), atrazine (H), bromacil (H), 

clopyralid (H), cyanazine (H), diuron (H), metolachlor (H), 

oryzalin (H), picloram (H), prometon (H), simazine (H), tebuthiuron (H), triclopyr (H), deethylatrazine (HD)

CSAL 78/227 (34.4) 14 2,4–D (H), 2,4–D methyl ester (H), atrazine (H), bromacil (H), 

clopyralid (H), cyanazine (H), metolachlor (H), oryzalin (H), 

picloram (H), prometon (H), simazine (H), tebuthiuron (H), triclopyr (H), deethylatrazine (HD)

CAL 49/111 (44.1) 12 2,4–D (H), 2,4–D methyl ester (H), atrazine (H), bromacil (H), 

clopyralid (H), oryzalin (H), picloram (H), prometon (H), simazine (H), tebuthiuron (H), triclopyr (H), deethylatrazine (HD)

Powder River 

Basin

NAL 44/436 (10.1) 19 Atrazine (H), bromacil (H), DCPA (H), diuron (H), metsulfuron (H), 

oxyfluorfen (H), picloram (H), prometon (H), simazine (H), sulfometuron (H), tebuthiuron (H), 3,4-dichloroaniline (HD), deethylatrazine (HD), alpha-endosulfan (I), carbaryl (I), dieldrin (I), 

imidacloprid (I), fipronil sulfide (ID), fipronil sulfone (ID) 

CSAL 16/227 (7.0) 9 Atrazine (H), bromacil (H), DCPA (H), picloram (H), prometon (H), 

tebuthiuron (H), 3,4-dichloroaniline (HD), imidacloprid (I), fipronil sulfide (ID)

CAL 7/111 (6.3) 3 Bromacil (H), picloram (H), prometon (H) 

36 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006
Geographic 

area

Assessment 

level

Spring

Number of 

pesticides 

detected in 

geographic 

area/number 

of pesticide 

detections in 

all geographic 

areas 

(percentage)

Number of 

different 

pesticides 

detected in 

geographic 

area

Pesticides detected

Bighorn Basin

NAL 169/410 (41.2) 18 Atrazine (H), bentazon (H), bromacil (H), cyanazine (H), DCPA (H), 

diuron (H), hexazinone (H), metolachlor (H), metribuzin (H), 

norflurazon (H), picloram (H), prometon (H), simazine (H), 

tebuthiuron (H), deethylatrazine (HD), deisopropylatrazine (HD), 

aldicarb sulfone (ID), aldicarb sulfoxide (ID).

CSAL 71/208 (34.1) 12 Atrazine (H), bentazon (H), bromacil (H), hexazinone (H), metolachlor (H), picloram (H), prometon (H), tebuthiuron (H), deethylatrazine (HD), deisopropylatrazine (HD), aldicarb sulfone (ID), 

aldicarb sulfoxide (ID).

CAL 30/109 (27.5) 10 Atrazine (H), bentazon (H), bromacil (H), picloram (H), prometon (H), tebuthiuron (H), deethylatrazine (HD), deisopropylatrazine (HD), aldicarb sulfone (ID), aldicarb sulfoxide (ID).

High Plains/

Casper Arch

NAL 108/410 (26.3) 16 2,4–D (H), alachlor (H), atrazine (H), bromacil (H), clopyralid (H), 

cyanazine (H), diuron (H), metolachlor (H), metribuzin (H), 

picloram (H), prometon (H), simazine (H), tebuthiuron (H), triclopyr (H), deethylatrazine (HD), diazinon (I).

CSAL 64/208 (30.8) 13 Atrazine (H), bromacil (H), clopyralid (H), cyanazine (H), diuron (H), metolachlor (H), picloram (H), prometon (H), simazine (H), tebuthiuron (H), triclopyr (H), deethylatrazine (HD), 

diazinon (I).

CAL 39/109 (35.8) 10 Atrazine (H), bromacil (H), clopyralid (H), diuron (H), picloram (H), 

prometon (H), simazine (H), tebuthiuron (H), triclopyr (H), deethylatrazine (HD).

Powder River 

Basin

NAL 40/410 (9.8) 12 Atrazine (H), bromacil (H), clopyralid (H), dicamba (H), diuron (H), 

picloram (H), prometon (H), simazine (H), tebuthiuron (H), triallate (H), deethylatrazine (HD), imidacloprid (I).

CSAL 20/208 (9.6) 9 Atrazine (H), bromacil (H), dicamba (H), picloram (H), prometon (H), simazine (H), tebuthiuron (H), triallate (H), imidacloprid (I).

CAL 12/109 (11.0) 6 Bromacil (H), clopyralid (H), dicamba (H), picloram (H), prometon (H), tebuthiuron (H).

Table 7. Pesticide detections in ground-water samples collected during fall and spring relative to eight regional geographic areas in 

Wyoming, 1995–2006.—Continued

[Geographic areas listed in order of decreasing pesticide detection frequency by jointly ranking fall and spring detections with no assessment level applied.

Assessment level: NAL, no assessment level applied; CSAL, compound-specific assessment level; CAL, common assessment level. 

Pesticide type: H, herbicide; HD, herbicide degradate; I, insecticide; ID, insecticide degradate. Symbol: --, not applicable]

Relation of Pesticide Occurrence to Selected Factors 37
Table 7. Pesticide detections in ground-water samples collected during fall and spring relative to eight regional geographic areas in 

Wyoming, 1995–2006.—Continued

[Geographic areas listed in order of decreasing pesticide detection frequency by jointly ranking fall and spring detections with no assessment level applied.

Assessment level: NAL, no assessment level applied; CSAL, compound-specific assessment level; CAL, common assessment level. 

Pesticide type: H, herbicide; HD, herbicide degradate; I, insecticide; ID, insecticide degradate. Symbol: --, not applicable]

Geographic 

area

Assessment 

level

Fall

Number of 

pesticides 

detected in 

geographic 

area/number 

of pesticide 

detections in 

all geographic 

areas

 (percentage)

Number of 

different 

pesticides 

detected in 

geographic 

area

Pesticides detected

Central Basins

NAL 36/436 (8.3) 16 Atrazine (H), bromacil (H), chlorimuron (H), clopyralid (H), 

dichlorprop (H), diuron (H), flumetsulam (H), picloram (H), 

prometon (H), simazine (H), sulfometuron (H), tebuthiuron (H), 

deethylatrazine (HD), deisopropylatrazine (HD), hydroxyatrazine (HD), malathion (I)

CSAL 26/227 (11.5) 12 Atrazine (H), bromacil (H), clopyralid (H), dichlorprop (H), 

diuron (H), flumetsulam (H), picloram (H), prometon (H), 

tebuthiuron (H), deethylatrazine (HD), deisopropylatrazine (HD), 

hydroxyatrazine (HD)

CAL 16/111 (14.4) 10 Atrazine (H), bromacil (H), clopyralid (H), dichlorprop (H), diuron (H), flumetsulam (H), picloram (H), prometon (H), tebuthiuron (H), deethylatrazine (HD)

Wind River 

Basin

NAL 37/436 (8.5) 8 Atrazine (H), DCPA (H), picloram (H), prometon (H), simazine (H), 

tebuthiuron (H), deethylatrazine (HD), 3-hydroxycarbofuran (ID) 

CSAL 17/227 (7.5) 4 Atrazine (H), picloram (H), prometon (H), tebuthiuron (H) 

CAL 2/111 (1.8) 2 Picloram (H), prometon (H) 

Green River 

Basin

NAL 28/436 (6.4) 17 Atrazine (H), benfluralin (H), bromacil (H), clopyralid (H), diuron (H), picloram (H), prometon (H), simazine (H), sulfometuron (H), tebuthiuron (H), triallate (H), trifluralin (H), deethylatrazine (HD), cis-permethrin (I), dieldrin (I), fipronil sulfide (ID), 

fipronil sulfone (ID) 

CSAL 13/227 (5.7) 6 Atrazine (H), bromacil (H), diuron (H), prometon (H), tebuthiuron (H), fipronil sulfide (ID)

CAL 6/111 (5.4) 4 Bromacil (H), diuron (H), prometon (H), tebuthiuron (H) 

Overthrust Belt

NAL 11/436 (2.5) 6 Atrazine (H), picloram (H), prometon (H), simazine (H), tebuthiuron (H), deethylatrazine (HD)

CSAL 7/227 (3.1) 5 Atrazine (H), picloram (H), prometon (H), simazine (H), tebuthiuron (H)

CAL 3/111 (2.7) 2 Picloram (H), prometon (H) 

Northwest 

Basins

NAL 4/436 (0.9) 2 Prometon (H), malathion (I)

CSAL 0/227 (0) 0 -- 

CAL 0/111 (0) 0 -- 

38 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006
Geographic 

area

Assessment 

level

Spring

Number of 

pesticides 

detected in 

geographic 

area/number 

of pesticide 

detections in 

all geographic 

areas 

(percentage)

Number of 

different 

pesticides 

detected in 

geographic 

area

Pesticides detected

Central Basins

NAL 39/410 (9.5) 12 2,4–D (H), atrazine (H), bromacil (H), clopyralid (H), dichlorprop (H), diuron (H), picloram (H), prometon (H), simazine (H), 

tebuthiuron (H), deethylatrazine (HD), deisopropylatrazine (HD).

CSAL 22/208 (10.6) 7 Atrazine (H), bromacil (H), diuron (H), picloram (H), prometon (H), 

tebuthiuron (H), deethylatrazine (HD).

CAL 13/109 (11.9) 6 Bromacil (H), clopyralid (H), diuron (H), picloram (H), prometon (H), tebuthiuron (H).

Wind River 

Basin

NAL 25/410 (6.1) 6 Atrazine (H), picloram (H), prometon (H), simazine (H), tebuthiuron (H), deethylatrazine (HD).

CSAL 14/208 (6.7) 4 Atrazine (H), picloram (H), prometon (H), tebuthiuron (H).

CAL 3/109 (2.8) 3 Picloram (H), prometon (H), tebuthiuron (H).

Green River 

Basin

NAL 19/410 (4.6) 6 Bromacil (H), clopyralid (H), diuron (H), picloram (H), prometon (H), tebuthiuron (H).

CSAL 12/208 (5.8) 5 Bromacil (H), diuron (H), picloram (H), prometon (H), tebuthiuron (H).

CAL 9/109 (8.3) 6 Bromacil (H), clopyralid (H), diuron (H), picloram (H), prometon (H), tebuthiuron (H).

Overthrust Belt

NAL 8/410 (2.0) 3 Atrazine (H), prometon (H), simazine (H).

CSAL 4/208 (1.9) 2 Atrazine (H), prometon (H).

CAL 2/109 (1.8) 1 Prometon (H).

Northwest 

Basins

NAL 2/410 (0.5) 1 Prometon (H).

CSAL 1/208 (0.5) 1 Prometon (H).

CAL 1/109 (0.9) 1 Prometon (H).

Table 7. Pesticide detections in ground-water samples collected during fall and spring relative to eight regional geographic areas in 

Wyoming, 1995–2006.—Continued

[Geographic areas listed in order of decreasing pesticide detection frequency by jointly ranking fall and spring detections with no assessment level applied.

Assessment level: NAL, no assessment level applied; CSAL, compound-specific assessment level; CAL, common assessment level. 

Pesticide type: H, herbicide; HD, herbicide degradate; I, insecticide; ID, insecticide degradate. Symbol: --, not applicable]

Relation of Pesticide Occurrence to Selected Factors 39
Table 8. Results of contingency-table analysis (CTA) comparing selected pesticide detections in ground-water samples 

collected during fall and spring from eight regional geographic areas in Wyoming, 1995–2006.

[Statistically significant differences (p-value<0.05) among eight regional geographic areas (table 7) indicated in bold type; CAL, common 

assessment level; CSAL, compound-specific assessment level; H, herbicide; HD, herbicide degradate; ID, insecticide degradate; I, insecticide; 

<, less than; --, not applicable]

 Tested variable Pesticide type Assessment level 

used

Probability (p-value)

Fall Spring

Percentage of wells with at least one 

pesticide detected

-- CAL <0.0001 <0.0001

Atrazine H CSAL <.0001 <.0001

Prometon H CSAL .0399 .3051

Deethylatrazine HD CSAL .0008 .0182

Tebuthiuron H CSAL .0406 .0070

Picloram H CSAL .4016 .4704

Deisopropylatrazine HD CSAL .4281 --

3,4-Dichloroaniline H CSAL .7662 --

Bromacil H CSAL .7849 .1964

Diuron H CSAL -- .4839

Metolachlor H CSAL .7618 --

Hexazinone H CSAL -- .8139

Aldicarb sulfoxide ID CSAL .1436 .1704

Aldicarb sulfone ID CSAL .1517 .1777

Hydroxyatrazine HD CSAL .6052 --

Flumetsulam H CSAL .2628 --

Imidacloprid I CSAL .4356 .3982

Fipronil sulfide ID CSAL .4899 --

2,4–D methyl ester H CSAL .3396 --

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

for four pesticides (atrazine, prometon, deethylatrazine, and 

tebuthiuron, listed in order of decreasing detection frequency) 

detected in the fall and three pesticides (atrazine, deethylatrazine, and tebuthiuron) detected in the spring (p-values for all 

compounds < 0.05, CTA, table 8).

Land Use

Pesticide detections were examined in relation to land 

use. Land use within a 500-m (1,640-ft) radius surrounding 

each sampled well was classified using an enhanced version 

of the USGS 1992 National Land Cover Data (NLCD) set 

(Nakagaki and Wolock, 2005). The NLCD classified land use 

for each 30-by-30-m area of the United States. For the study 

described herein, a GIS was used to manipulate the NLCD 

to classify all land uses within the 500-m radius surrounding all sampled wells. Individual land uses classified within 

the 500-m radius then were used to classify land use into one 

of four predominant land-use categories using the following 

criteria: (1) agricultural [greater than (>) 50 percent agricultural land and less than or equal to (≤) 5 percent urban land]; 

(2) urban (> 25 percent urban land use and ≤ 25 percent 

agricultural land); (3) rangeland/undeveloped (≤ 5 percent 

urban land and ≤ 25 percent agricultural land); and (4) mixed 

(all other combinations of urban, agricultural, and rangeland/

undeveloped land). Each sampled well then was assigned to 

one of these four land-use categories to relate pesticide detections to overlying land use.

On the basis of reviews of previous studies and on USGS 

NAWQA guidance (Koterba, 1998, and references therein), 

a 500-m radius was assumed to represent the recharge area 

to sampled wells. This approach was used because information describing the rate and direction of ground-water flow 

necessary to determine the shape and size of recharge areas to 

each sampled well generally was not available; use of a 500-m 

radius has been found to work reasonably well when trying to 

relate land-use activities to ground-water quality in underlying shallow surficial aquifers (Koterba, 1998, and references 

therein). Pesticide detections in shallow ground water throughout the United States often are related to overlying land use 

(Gilliom and others, 2006).

Detected pesticide concentrations and the overall 

frequency of detection in samples (represented by the percentage of wells with at least one pesticide detected) are shown 
Relation of Pesticide Occurrence to Selected Factors 41

many different nonagricultural applications throughout all land 

uses in Wyoming.

The occurrence of some pesticides did not necessarily 

correspond to the predominant land use assigned to sampled 

wells (fig. 10). The detection of many pesticides most 

commonly associated with agricultural use in water from 

wells located in predominantly urban, mixed, and rangeland/

undeveloped areas was likely caused by at least one of three 

factors: (1) in most cases, the predominant land use assigned 

to each sampled well was not the only land use present within 

a 500-m radius of the well, and the detected pesticide may be 

due to the other less common land uses; (2) pesticides can be 

transported in ground water from other areas with different 

land use; and (3) pesticides can be transported in the atmosphere and (or) precipitation from other areas. 

The frequencies of pesticide detection among the four 

land-use categories were compared using the CSALs for 

individual pesticides detected in either the fall or spring at a 

frequency equal to or greater than 1 percent (table 10). The 

proportion of samples detected at concentrations greater than 

their respective CSALs (detection frequency) was significantly 

different among the four land-use categories for four pesticides (atrazine, prometon, tebuthiuron, and bromacil, listed 

in order of decreasing detection frequency) detected in both 

the fall and spring (p-values for all four compounds < 0.05, 

KWOCR, table 10). 

Hydrogeology

Four different hydrogeologic characteristics—aquifer 

type, water-level depth, well depth, and well type—were 

evaluated to examine their relation to the occurrence of pesticides in ground water. In some previous studies, these characteristics have been shown to affect the occurrence of pesticides 

in ground water (Barbash and Resek, 1996, and references 

therein).

Aquifer Type

On the basis of determined geologic formation and lithology, all sampled wells were assigned to one of two different aquifer types—unconsolidated deposits or consolidated 

bedrock. The frequency of pesticide detection was examined 

in relation to these two different types of aquifers. Water from 

wells completed in unconsolidated aquifers accounted for 

79 to 87 percent of pesticide detections (fig. 11) regardless 

of season or assessment level. Other investigators also have 

found pesticide detections to be generally more frequent in 

unconsolidated aquifers compared to consolidated (bedrock) 

aquifers (Barbash and Resek, 1996, and references therein). 

In the Wyoming baseline study, more sampled wells were 

completed in surficial aquifers in unconsolidated deposits 

(223 of 296 wells sampled) than in bedrock (73 of 296 wells 

sampled). Sampled unconsolidated aquifers were composed 

of clay, silt, sand, and (or) gravel of Quaternary age, whereas 

by land use in figure 9. Regardless of assessment level and 

season, pesticides were detected much more frequently in 

samples from wells located in predominantly urban areas than 

in samples from wells located in predominantly agricultural, 

mixed, or rangeland/undeveloped areas. Pesticides were 

detected distinctly less often in samples from wells located 

in predominantly rangeland/undeveloped areas (fig. 9). The 

frequency of pesticide detection in samples from wells located 

in a predominantly agricultural or mixed land-use areas was 

intermediate to that of urban and rangeland/undeveloped 

areas; however, using the CAL, the difference was more 

distinct, and the percentage of wells located in areas classified 

as either predominantly agricultural or mixed with at least one 

pesticide detected was less than one-half that of wells located 

in predominantly urban areas regardless of season. Considering only detections greater than 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 (p-value < 0.0001, KWOCR, table 10) and the spring 

(p-value < 0.0001, KWOCR, table 10).

Concentrations and frequencies of pesticides detected 

most often—pesticides detected with original censoring 

(NAL) in 1 percent or more of wells sampled in either fall 

or the spring—are shown by land-use category in figure 10. 

Using figure 10, graphical comparisons can be made 

between individual pesticides detected in the same season 

or between spring and fall detections of the same pesticide 

with and without recensoring. Considering only concentrations greater than their respective CSALs for either fall or 

spring, eight pesticides [bromacil, clopyralid, flumetsulam, 

2,4–D methyl ester, deisopropylatrazine (fall), 3,4-dichloroaniline, hydroxyatrazine, and imidacloprid, listed in order of 

decreasing detection frequency; fig. 10A] were detected only 

in water from wells located in predominantly urban areas, 

three pesticides (DCPA, bentazon, and aldicarb sulfone; fall, 

fig. 10A) were detected only in water from wells located in 

predominantly agricultural areas, and two pesticides (hexazinone and deisopropylatrazine; spring, fig. 10B) were detected 

only in water from wells located in predominantly mixed landuse areas. Considering only concentrations greater than their 

respective CSALs, no pesticide was detected in samples from 

wells located only in predominantly rangeland/undeveloped 

areas. Remaining pesticides detected at concentrations greater 

than their respective CSALs and shown in figure 10 were 

associated with sampled wells located in two to four different 

predominant land uses. 

As noted previously herein, two of the pesticides detected 

most frequently during this study (prometon and tebuthiuron) 

either have no reported agricultural use (prometon) or limited 

noncropland use (tebuthiuron) in Wyoming. As expected, 

both of these pesticides were detected most commonly in 

sampled wells located in predominantly urban areas, although 

many detections were associated with sampled wells located 

in predominantly agricultural or mixed areas. This was not 

surprising and likely reflects the use of both pesticides for 
Table 9. Pesticide detection frequencies and concentrations in ground-water samples collected during fall and spring relative to eight 

regional geographic areas in Wyoming, 1995–2006.—Continued

[Geographic areas where pesticides were detected are listed in order of decreasing frequency with no assessment level applied. Assessment level: NAL, no assessment 

level applied; CSAL, compound-specific assessment level; CAL, common assessment level. Geographic area: BH, Bighorn Basin; CB, Central Basins; GR, Green 

River Basin; HC, High Plains/Casper Arch; NW, Northwest Basins; OB, Overthrust Belt; PR, Powder River Basin; WR, Wind River Basin. Symbol: --, no detections or 

not applicable]

Fall 

Pesticide

Geographic 

area 

where 

detected

Number of detections/

number of analyses

Detection frequency in 

geographic area 

(percentage)

Detected concentration range

(micrograms per liter)

Median concentration of 

detections

(micrograms per liter)

Geographic 

area where 

pesticide 

was not 

analyzed

NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL 

Herbicides 

Atrazine BH 44/66 37/66 1/66 66.7 56.1 1.5 0.002–0.158 0.004–0.158 0.158 0.006 0.008 -- -- 

WR 10/20 9/20 0/20 50.0 45.0 0 0.003–0.026 0.004–0.026 -- .006 .006 -- -- 

HC 31/78 26/76 7/78 39.7 34.2 9.0 0.002–0.890 0.004–0.890 0.07–0.890 .011 .014 0.270 -- 

CB 3/21 3/21 1/21 14.3 14.3 4.8 0.015–0.173 0.015–0.173 0.173 .015 .015 -- -- 

OB 2/21 2/21 0/21 9.5 9.5 0 0.005–0.013 0.005–0.013 -- .009 .009 -- -- 

GR 2/28 2/28 0/28 7.1 7.1 0 0.006–0.007 0.006–0.007 -- .006 .006 -- -- 

PR 3/50 3/50 0/50 6.0 6.0 0 0.005–0.022 0.005–0.022 -- .005 .005 -- --

Prometon BH 31/66 8/66 7/66 47.0 12.1 10.6 0.003–0.49 0.06–0.49 0.08–0.49 .01 .15 .17 -- 

CB 7/20 6/20 6/20 35.0 30.0 30.0 0.01–2.88 0.07–2.88 0.07–2.88 .09 .12 .12 -- 

PR 16/50 2/50 1/50 32.0 4.0 2.0 0.003–0.07 0.06–0.07 0.07 .01 .06 .07 -- 

HC 20/78 16/78 14/78 25.6 20.5 17.9 0.001–1.40 0.06–1.40 0.08–1.40 .10 .11 .12 -- 

GR 7/28 3/28 2/28 25.0 10.7 7.1 0.01–1.05 0.05–1.05 0.09–1.05 .04 .09 .57 -- 

WR 5/20 2/20 1/20 25.0 10.0 5.0 0.01–0.08 0.05–0.08 0.08 .01 .06 -- -- 

NW 3/12 0/12 0/12 25.0 0 0 0.002–0.02 -- -- .01 -- -- -- 

OB 4/21 2/21 2/21 19.0 9.5 9.5 0.004–0.36 0.1–0.36 0.1–0.36 .06 .23 .23 -- 

Tebuthiuron CB 5/21 5/21 2/21 23.8 23.8 9.5 0.01–0.58 0.01–0.58 0.44–0.58 .03 .03 .51 -- 

WR 4/20 4/20 0/20 20.0 20.0 0 0.01–0.05 0.01–0.05 -- .02 .02 -- -- 

GR 5/28 5/28 2/28 17.9 17.9 7.1 0.02–0.43 0.02–0.43 0.33–0.43 .04 .04 .38 -- 

BH 9/66 9/66 6/66 13.6 13.6 9.1 0.01–1.97 0.01–1.97 0.08–1.97 .08 .08 .22 -- 

HC 7/76 6/76 4/76 9.2 7.9 5.3 0.004–0.57 0.03–0.57 0.07–0.57 .07 .16 .30 -- 

OB 1/21 1/21 0/21 4.8 4.8 0 0.01 0.01 -- -- -- -- -- 

PR 1/50 1/50 0/50 2.0 2.0 0 0.03 0.03 -- -- -- -- --

Simazine WR 4/20 0/20 0/20 20.0 0 0 0.002–0.011 -- -- .004 -- -- -- 

BH 11/66 0/66 0/66 16.7 0 0 0.003–0.011 -- -- .006 -- -- -- 

HC 4/78 1/78 1/78 5.1 1.3 1.3 0.006–0.093 0.093 0.093 .024 -- -- -- 

CB 1/20 0/20 0/20 5.0 0 0 0.008 -- -- -- -- -- -- 

OB 1/21 1/21 0/21 4.8 4.8 0 0.058 0.058 -- -- -- -- -- 

GR 1/28 0/28 0/28 3.6 0 0 0.011 -- -- -- -- -- -- 

PR 1/50 0/50 0/50 2.0 0 0 0.006 -- -- -- -- -- --

Picloram CB 3/21 3/21 1/21 14.3 14.3 4.8 0.06–0.09 0.06–0.09 0.09 .06 .06 -- -- 

HC 9/75 7/74 7/75 12.0 9.5 9.3 0.04–4.31 0.08–4.31 0.08–4.31 .24 .26 .26 -- 

PR 5/50 5/50 5/50 10.0 10.0 10.0 0.17–9.50 0.17–9.50 0.17–9.50 2.19 2.19 2.19 -- 

WR 2/20 2/20 1/20 10.0 10.0 5.0 0.06–1.73 0.06–1.73 1.73 .90 .90 -- -- 

OB 2/21 1/21 1/21 9.5 4.8 4.8 0.03–0.31 0.31 0.31 .17 -- -- -- 

GR 1/22 0/22 0/22 4.5 0 0 0.04 -- -- -- -- -- -- 

BH 2/63 2/61 2/61 3.2 3.3 3.3 0.07–0.65 0.07–0.65 0.07–0.65 .36 .36 .36 --

42 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006
Spring

Pesticide

Geographic 

area 

where 

detected

Number of detections/

number of analyses

Detection frequency in 

geographic area 

(percentage)

Detected concentration range

(micrograms per liter)

Median concentration of 

detections

(micrograms per liter)

Geographic 

area where 

compound 

was not 

analyzed 

NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL

Herbicides

Atrazine BH 48/66 38/66 1/66 72.7 57.6 1.5 0.002–0.180 0.004–0.180 0.180 0.007 0.010 0.180 --

HC 29/78 24/68 9/78 37.2 35.3 11.5 0.001–0.900 0.004–0.900 0.090–0.900 .010 .030 .147 --

WR 7/20 7/20 0/20 35.0 35.0 0 0.004–0.033 0.004–0.033 -- .008 .008 -- --

CB 6/21 4/20 0/21 28.6 20.0 0 0.002–0.030 0.005–0.030 -- .009 .016 -- --

OB 2/21 2/21 0/21 9.5 9.5 0 0.006–0.009 0.006–0.009 -- .008 .008 -- --

PR 3/49 3/49 0/49 6.1 6.1 0 0.004–0.009 0.004–0.009 -- .005 .005 -- --

Prometon CB 8/21 6/21 4/21 38.1 28.6 19.0 0.003–1.24 0.05–1.24 0.07–1.24 .06 .11 .26 --

BH 25/66 8/66 7/66 37.9 12.1 10.6 0.004–0.45 0.06–0.45 0.07–0.45 .03 .15 .18 --

PR 14/48 3/48 1/48 29.2 6.2 2.1 0.002–0.14 0.05–0.14 0.14 .02 .06 -- --

WR 5/20 2/20 1/20 25.0 10.0 5.0 0.01–0.11 0.06–0.11 0.11 .04 .08 -- --

GR 7/28 2/28 1/28 25.0 7.1 3.6 0.01–0.37 0.05–0.37 0.37 .03 .21 -- --

HC 19/78 11/78 10/78 24.4 14.1 12.8 0.002–1.24 0.05–1.24 0.07–1.24 .07 .11 .12 --

OB 4/21 2/21 2/21 19.0 9.5 9.5 0.005–0.42 0.11–0.42 0.11–0.42 .07 .26 .26 --

NW 2/12 1/12 1/12 16.7 8.3 8.3 0.02–0.14 0.14 0.14 .08 -- -- --

Tebuthiuron CB 7/21 7/21 4/21 33.3 33.3 19.0 0.01–1.18 0.01–1.18 0.30–1.18 .30 .30 .52 --

WR 4/20 4/20 1/20 20.0 20.0 5.0 0.01–0.09 0.01–0.09 0.09 .04 .04 -- --

GR 5/28 5/28 2/28 17.9 17.9 7.1 0.01–0.24 0.01–0.24 0.11–0.24 .05 .05 .18 --

BH 10/66 9/66 6/66 15.2 13.6 9.1 0.005–0.28 0.01–0.28 0.07–0.28 .08 .10 .16 --

HC 6/78 6/78 3/78 7.7 7.7 3.8 0.03–0.49 0.03–0.49 0.09–0.49 .07 .07 .32 --

PR 3/49 3/49 1/49 6.1 6.1 2.0 0.01–0.31 0.01–0.31 0.31 .02 .02 -- --

Simazine CB 3/21 0/21 0/21 14.3 0 0 0.003–0.010 -- -- .008 -- -- --

BH 9/66 0/66 0/66 13.6 0 0 0.002–0.014 -- -- .006 -- -- --

OB 2/21 0/21 0/21 9.5 0 0 0.003–0.036 -- -- .020 -- -- --

HC 7/78 1/78 1/78 9.0 1.3 1.3 0.007–0.112 0.112 0.112 .017 -- -- --

PR 3/48 1/48 0/48 6.2 2.1 0 0.006–0.069 0.069 -- .007 -- -- --

WR 1/20 0/20 0/20 5.0 0 0 0.012 -- -- .012 -- -- --

Picloram PR 6/48 6/48 6/48 12.5 12.5 12.5 0.14–12.2 0.14–12.2 0.14–12.2 .57 .57 .57 --

WR 2/20 1/20 1/20 10.0 5.0 5.0 0.01–1.26 1.26 1.26 .64 -- -- --

HC 6/78 4/78 3/78 7.7 5.1 3.8 0.04–2.45 0.06–2.45 0.07–2.45 .06 1.18 2.30 --

CB 1/19 1/19 1/19 5.3 5.3 5.3 0.21 0.21 0.21 .21 -- -- --

BH 3/66 3/66 3/66 4.5 4.5 4.5 0.07–0.46 0.07–0.46 0.07–0.46 .25 .25 .25 --

GR 1/27 1/24 1/24 3.7 4.2 4.2 0.38 0.38 0.38 .38 -- -- --

Table 9. Pesticide detection frequencies and concentrations in ground-water samples collected during fall and spring relative to eight 

regional geographic areas in Wyoming, 1995–2006.—Continued

[Geographic areas where pesticides were detected are listed in order of decreasing frequency with no assessment level applied. Assessment level: NAL, no assessment 

level applied; CSAL, compound-specific assessment level; CAL, common assessment level. Geographic area: BH, Bighorn Basin; CB, Central Basins; GR, Green River 

Basin; HC, High Plains/Casper Arch; NW, Northwest Basins; OB, Overthrust Belt; PR, Powder River Basin; WR, Wind River Basin. Symbol: --, no detections or not 

applicable]

Relation of Pesticide Occurrence to Selected Factors 43
Table 9. Pesticide detection frequencies and concentrations in ground-water samples collected during fall and spring relative to eight 

regional geographic areas in Wyoming, 1995–2006.—Continued

[Geographic areas where pesticides were detected are listed in order of decreasing frequency with no assessment level applied. Assessment level: NAL, no assessment 

level applied; CSAL, compound-specific assessment level; CAL, common assessment level. Geographic area: BH, Bighorn Basin; CB, Central Basins; GR, Green 

River Basin; HC, High Plains/Casper Arch; NW, Northwest Basins; OB, Overthrust Belt; PR, Powder River Basin; WR, Wind River Basin. Symbol: --, no detections or 

not applicable]

Fall 

Pesticide

Geographic 

area 

where 

detected

Number of detections/

number of analyses

Detection frequency in 

geographic area 

(percentage)

Detected concentration range

(micrograms per liter)

Median concentration of 

detections

(micrograms per liter)

Geographic 

area where 

pesticide 

was not 

analyzed

NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL 

Herbicides—Continued 

Oxyfluorfen PR 1/7 0/7 0/7 14.3 0 0 0.007 -- -- -- -- -- BH, CB, 

GR, NW, 

OB, WR

Bromacil CB 2/21 1/21 1/21 9.5 4.8 4.8 0.04–0.80 0.80 0.80 0.42 -- -- -- 

HC 4/77 4/76 3/76 5.2 5.3 3.9 0.06–8.07 0.06–8.07 0.15–8.07 .66 0.66 1.18 -- 

GR 1/27 1/27 1/27 3.7 3.7 3.7 1.96 1.96 1.96 -- -- -- -- 

PR 1/50 1/50 1/50 2.0 2.0 2.0 0.60 0.60 0.60 -- -- -- -- 

BH 1/63 1/62 0/62 1.6 1.6 0 0.06 0.06 -- -- -- -- -- 

Diuron CB 2/21 1/21 1/21 9.5 4.8 4.8 0.02–0.19 0.19 0.19 .10 -- -- -- 

GR 1/28 1/28 1/28 3.6 3.6 3.6 0.08 0.08 0.08 -- -- -- -- 

HC 2/77 0/76 0/76 2.6 0 0 0.04 -- -- .04 -- -- -- 

PR 2/50 0/50 0/50 4.0 0 0 0.03–0.05 -- -- .04 -- -- -- 

BH 1/63 0/63 0/63 1.6 0 0 0.01 -- -- -- -- -- -- 

Metolachlor BH 7/66 2/66 0/66 10.6 3.0 0 0.002–0.024 0.009–0.024 -- .004 .016 -- -- 

HC 1/78 1/59 0/78 1.3 1.7 0 0.01 0.01 -- -- -- -- -- 

Hexazinone -- -- -- -- -- -- -- -- -- -- -- -- -- BH, CB, 

GR, NW, 

OB, WR

Sulfometuron GR 1/12 0/12 0/12 8.3 0 0 0.021 -- -- -- -- -- NW, OB, 

WR

PR 1/15 0/15 0/15 6.7 0 0 0.02 -- -- -- -- -- -- 

CB 1/20 0/20 0/20 5.0 0 0 0.022 -- -- -- -- -- -- 

Clopyralid CB 2/21 1/21 1/20 9.5 4.8 5.0 0.02–0.45 0.45 0.45 .24 -- -- -- 

GR 1/22 0/21 0/12 4.5 0 0 0.04 -- -- -- -- -- -- 

HC 1/75 1/75 1/36 1.3 1.3 2.8 22.6 22.6 22.6 -- -- -- -- 

Flumetsulam CB 2/20 2/20 1/20 10.0 10.0 5.0 0.06–0.08 0.06–0.08 0.08 .07 .07 .08 NW, OB, 

WR

DCPA WR 2/20 0/20 0/20 10.0 0 0 0.002 -- -- .002 -- -- -- 

PR 1/50 1/50 0/50 2.0 2.0 0 0.004 0.004 -- -- -- -- --

BH 1/66 0/66 0/66 1.5 0 0 0.002 -- -- -- -- -- --

2,4–D methyl 

ester

HC 1/13 1/13 1/13 7.7 7.7 7.7 0.737 0.737 0.737 -- -- -- NW, OB, 

WR

44 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006
Table 9. Pesticide detection frequencies and concentrations in ground-water samples collected during fall and spring relative to eight 

regional geographic areas in Wyoming, 1995–2006.—Continued

[Geographic areas where pesticides were detected are listed in order of decreasing frequency with no assessment level applied. Assessment level: NAL, no assessment 

level applied; CSAL, compound-specific assessment level; CAL, common assessment level. Geographic area: BH, Bighorn Basin; CB, Central Basins; GR, Green River 

Basin; HC, High Plains/Casper Arch; NW, Northwest Basins; OB, Overthrust Belt; PR, Powder River Basin; WR, Wind River Basin. Symbol: --, no detections or not 

applicable]

Spring

Pesticide

Geographic 

area 

where 

detected

Number of detections/

number of analyses

Detection frequency in 

geographic area 

(percentage)

Detected concentration range

(micrograms per liter)

Median concentration of 

detections

(micrograms per liter)

Geographic 

area where 

compound 

was not 

analyzed 

NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL

Herbicides—Continued

Oxyfluorfen -- -- -- -- -- -- -- -- -- -- -- -- -- BH, CB, 

GR, NW, 

OB, WR

Bromacil GR 3/28 3/27 3/27 10.7 11.1 11.1 0.10–0.56 0.10–0.56 0.10–0.56 0.12 0.12 0.12 --

CB 2/21 2/21 2/21 9.5 9.5 9.5 0.47–0.61 0.47–0.61 0.47–0.61 .54 .54 .54 --

HC 4/78 3/77 2/77 5.1 3.9 2.6 0.05–5.38 0.06–5.38 0.80–5.38 .43 .80 3.09 --

BH 2/66 1/65 1/65 3.0 1.5 1.5 0.04–0.14 0.14 0.14 .09 -- -- --

PR 1/48 1/46 1/46 2.1 2.2 2.2 0.40 0.40 0.40 -- -- -- --

Diuron CB 2/21 1/21 1/21 9.5 4.8 4.8 0.05–0.11 0.11 0.11 .08 -- -- --

GR 2/28 1/28 1/28 7.1 3.6 3.6 0.04–0.07 0.07 0.07 .06 -- -- --

HC 3/78 1/78 1/78 3.8 1.3 1.3 0.01–2.69 2.69 2.69 .03 -- -- --

PR 1/48 0/48 0/48 2.1 0 0 0.02 -- -- .02 -- -- --

BH 1/66 0/66 0/66 1.5 0 0 0.02 -- -- .02 -- -- --

Metolachlor BH 7/66 1/41 0/66 10.6 2.4 0 0.003–0.029 0.029 -- .005 .029 -- --

HC 1/78 1/68 0/78 1.3 1.5 0 0.056 0.056 -- -- -- -- --

Hexazinone BH 1/25 1/25 0/25 4.0 4.0 0 0.06 0.06 -- -- -- -- CB, GR, 

NW, OB, 

WR

Sulfometuron -- -- -- -- -- -- -- -- -- -- -- -- -- NW, OB, 

WR

--

--

Clopyralid CB 1/18 0/18 1/17 5.6 0 5.9 0.07 -- 0.07 -- -- -- --

PR 2/46 0/46 2/12 4.3 0 16.7 0.08–0.13 -- 0.08–0.13 .10 -- .10 --

GR 1/27 0/24 1/12 3.7 0 8.3 0.11 -- 0.11 -- -- -- --

HC 1/78 1/78 1/39 1.3 1.3 2.6 22.0 22.0 22.0 -- -- -- --

Flumetsulam NW, OB, 

WR

DCPA BH 1/66 0/66 0/66 1.5 0 0 0.001 -- -- -- -- -- --

2,4–D methyl 

ester

-- -- -- -- -- -- -- -- -- -- -- -- -- NW, OB, 

WR

Relation of Pesticide Occurrence to Selected Factors 45
Table 9. Pesticide detection frequencies and concentrations in ground-water samples collected during fall and spring relative to eight 

regional geographic areas in Wyoming, 1995–2006.—Continued

[Geographic areas where pesticides were detected are listed in order of decreasing frequency with no assessment level applied. Assessment level: NAL, no assessment 

level applied; CSAL, compound-specific assessment level; CAL, common assessment level. Geographic area: BH, Bighorn Basin; CB, Central Basins; GR, Green 

River Basin; HC, High Plains/Casper Arch; NW, Northwest Basins; OB, Overthrust Belt; PR, Powder River Basin; WR, Wind River Basin. Symbol: --, no detections or 

not applicable]

Fall 

Pesticide

Geographic 

area 

where 

detected

Number of detections/

number of analyses

Detection frequency in 

geographic area 

(percentage)

Detected concentration range

(micrograms per liter)

Median concentration of 

detections

(micrograms per liter)

Geographic 

area where 

pesticide 

was not 

analyzed

NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL 

Herbicides—Continued 

Chlorimuron CB 1/20 0/20 0/20 5.0 0 0 0.008 -- -- -- -- -- NW, OB, 

WR

Metsulfuron PR 1/15 0/15 0/15 6.7 0 0 0.03 -- -- -- -- -- NW, OB, 

WR

Bentazon BH 3/63 1/63 1/63 4.8 1.6 1.6 0.03–0.14 0.14 0.14 0.04 -- -- -- 

-- 

Cyanazine BH 1/66 0/66 0/66 1.5 0 0 0.007 -- -- -- -- -- -- 

HC 1/78 1/59 0/59 1.3 1.7 0 0.018 0.018 -- -- -- -- -- 

2,4–D HC 1/75 1/74 1/56 1.3 1.4 1.8 13.8 13.8 13.8 -- -- -- -- 

-- 

Metribuzin BH 1/66 0/66 0/66 1.5 0 0 0.006 -- -- -- -- -- -- 

-- 

Triallate GR 1/28 0/28 0/28 3.6 0 0 0.001 -- -- -- -- -- -- 

Dichlorprop CB 1/21 1/21 1/21 4.8 4.8 4.8 0.07 0.07 0.07 -- -- -- -- 

Triclopyr HC 1/75 1/75 1/55 1.3 1.3 1.8 18.6 18.6 18.6 -- -- -- -- 

Norflurazon BH 1/63 0/63 0/63 1.6 0 0 0.02 -- -- -- -- -- -- 

Trifluralin GR 1/28 0/28 0/28 3.6 0 0 0.008 -- -- -- -- -- -- 

BH 1/66 0/66 0/66 1.5 0 0 0.004 -- -- -- -- -- -- 

Bromoxynil BH 1/63 1/63 1/63 1.6 1.6 1.6 0.22 0.22 0.22 -- -- -- -- 

Oryzalin HC 1/77 1/60 1/38 1.3 1.7 2.6 0.63 0.63 0.63 -- -- -- -- 

Benfluralin GR 1/28 0/28 0/28 3.6 0 0 0.008 -- -- -- -- -- -- 

Alachlor -- -- -- -- -- -- -- -- -- -- -- -- -- -- 

Dicamba -- -- -- -- -- -- -- -- -- -- -- -- -- -- 

46 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006
Spring

Pesticide

Geographic 

area 

where 

detected

Number of detections/

number of analyses

Detection frequency in 

geographic area 

(percentage)

Detected concentration range

(micrograms per liter)

Median concentration of 

detections

(micrograms per liter)

Geographic 

area where 

compound 

was not 

analyzed 

NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL

Herbicides—Continued

Chlorimuron -- -- -- -- -- -- -- -- -- -- -- -- -- NW, OB, 

WR

Metsulfuron -- -- -- -- -- -- -- -- -- -- -- -- -- NW, OB, 

WR

Bentazon BH 1/66 1/66 1/66 1.5 1.5 1.5 0.59 0.59 0.59 -- -- -- --

--

Cyanazine BH 1/66 0/41 0/41 1.5 0 0 0.008 -- -- -- -- -- --

HC 1/78 1/68 1/68 1.3 1.5 1.5 0.130 0.130 0.130 -- -- -- --

2,4–D CB 1/21 0/21 0/20 4.8 0 0 0.04 -- -- -- -- -- --

HC 1/78 0/78 0/51 1.3 0 0 0.04 -- -- -- -- -- --

Metribuzin BH 1/66 0/66 0/66 1.5 0 0 0.007 -- -- -- -- -- --

HC 1/78 0/78 0/78 1.3 0 0 0.010 -- -- -- -- -- --

Triallate PR 1/42 1/42 0/42 2.4 2.4 0 0.005 0.005 -- -- -- -- --

Dichlorprop CB 1/21 0/21 0/21 4.8 0 0 0.01 -- -- -- -- -- --

Triclopyr HC 1/78 1/78 1/62 1.3 1.3 1.6 1.51 1.51 1.51 -- -- -- --

Norflurazon BH 1/66 0/66 0/66 1.5 0 0 0.01 -- -- -- -- -- --

Trifluralin -- -- -- -- -- -- -- -- -- -- -- -- -- --

--

Bromoxynil -- -- -- -- -- -- -- -- -- -- -- -- -- --

Oryzalin -- -- -- -- -- -- -- -- -- -- -- -- -- --

Benfluralin -- -- -- -- -- -- -- -- -- -- -- -- -- --

Alachlor HC 1/78 0/78 0/78 1.3 0 0 0.004 -- -- -- -- -- --

Dicamba PR 1/48 1/48 1/48 2.1 2.1 2.1 1.11 1.11 1.11 -- -- -- --

Table 9. Pesticide detection frequencies and concentrations in ground-water samples collected during fall and spring relative to eight 

regional geographic areas in Wyoming, 1995–2006.—Continued

[Geographic areas where pesticides were detected are listed in order of decreasing frequency with no assessment level applied. Assessment level: NAL, no assessment 

level applied; CSAL, compound-specific assessment level; CAL, common assessment level. Geographic area: BH, Bighorn Basin; CB, Central Basins; GR, Green River 

Basin; HC, High Plains/Casper Arch; NW, Northwest Basins; OB, Overthrust Belt; PR, Powder River Basin; WR, Wind River Basin. Symbol: --, no detections or not 

applicable]

Relation of Pesticide Occurrence to Selected Factors 47
Table 9. Pesticide detection frequencies and concentrations in ground-water samples collected during fall and spring relative to eight 

regional geographic areas in Wyoming, 1995–2006.—Continued

[Geographic areas where pesticides were detected are listed in order of decreasing frequency with no assessment level applied. Assessment level: NAL, no assessment 

level applied; CSAL, compound-specific assessment level; CAL, common assessment level. Geographic area: BH, Bighorn Basin; CB, Central Basins; GR, Green 

River Basin; HC, High Plains/Casper Arch; NW, Northwest Basins; OB, Overthrust Belt; PR, Powder River Basin; WR, Wind River Basin. Symbol: --, no detections or 

not applicable]

Fall 

Pesticide

Geographic 

area 

where 

detected

Number of detections/

number of analyses

Detection frequency in 

geographic area 

(percentage)

Detected concentration range

(micrograms per liter)

Median concentration of 

detections

(micrograms per liter)

Geographic 

area where 

pesticide 

was not 

analyzed

NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL 

Herbicide degradates 

Deethylatrazine BH 39/66 1/66 1/66 59.1 1.5 1.5 0.001–0.107 0.107 0.107 0.004 -- -- -- 

WR 9/20 0/20 0/20 45.0 0 0 0.004–0.020 -- -- .005 -- -- -- 

HC 25/78 11/78 8/78 32.1 14.1 10.3 0.002–0.810 0.05–0.810 0.07–0.810 .034 .180 .220 -- 

CB 2/21 1/21 1/21 9.5 4.8 4.8 0.012–0.120 0.120 0.120 .066 -- -- -- 

PR 4/50 0/50 0/50 8.0 0 0 0.004–0.022 -- -- .006 -- -- -- 

OB 1/21 0/21 0/21 4.8 0 0 0.003 -- -- .003 -- -- --

GR 1/28 0/28 0/28 3.6 0 0 0.006 -- -- .006 -- -- --

DeisopropylatrazineBH 1/11 0/11 0/11 9.1 0 0 0.01 -- -- .01 -- -- NW, OB, 

WR

CB 1/20 1/20 0/20 5.0 5.0 0 0.05 0.05 -- .05 -- -- -- 

3,4-DichloroanilinePR 1/7 1/7 0/7 14.3 14.3 0 0.007 0.007 -- -- -- -- BH, CB, 

GR, NW, 

OB, WR

HydroxyatrazineCB 2/19 1/19 0/19 10.5 5.3 0 0.007–0.020 0.020 -- .014 -- -- NW, OB, 

WR

Insecticides 

alpha-Endosulfan

PR 1/7 0/7 0/7 14.3 0 0 0.003 -- -- -- -- -- BH, CB, 

GR, NW, 

OB, WR

Imidacloprid PR 1/15 1/15 0/15 6.7 6.7 0 0.029 0.029 -- -- -- -- NW, OB, 

WR

Carbaryl PR 1/50 0/50 0/50 2.0 0 0 0.003 -- -- -- -- -- -- 

BH 1/66 0/66 0/66 1.5 0 0 0.004 -- -- -- -- -- --

Carbofuran BH 2/66 2/66 0/66 3.0 3.0 0 0.022–0.042 0.022–0.042 -- .032 .032 -- -- 

Dieldrin GR 1/28 0/28 0/28 3.6 0 0 0.007 -- -- -- -- -- -- 

PR 1/50 0/50 0/50 2.0 0 0 0.007 -- -- -- -- -- --

Malathion NW 1/12 0/12 0/12 8.3 0 0 0.011 -- -- -- -- -- -- 

CB 1/20 0/20 0/20 5.0 0 0 0.007 -- -- -- -- -- --

Diazinon -- -- -- -- -- -- -- -- -- -- -- -- -- -- 

cis-Permethrin GR 1/28 0/28 0/28 3.6 0 0 0.01 -- -- .01 -- -- -- 

48 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006
Spring

Pesticide

Geographic 

area 

where 

detected

Number of detections/

number of analyses

Detection frequency in 

geographic area 

(percentage)

Detected concentration range

(micrograms per liter)

Median concentration of 

detections

(micrograms per liter)

Geographic 

area where 

compound 

was not 

analyzed 

NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL

Herbicide degradates

Deethylatrazine BH 37/66 2/66 2/66 56.1 3.0 3.0 0.002–0.121 0.100–0.121 0.100–0.121 0.008 0.110 0.110 --

HC 26/78 9/78 7/78 33.3 11.5 9.0 0.002–0.550 0.060–0.550 0.073–0.550 .013 .190 .210 --

WR 6/20 0/20 0/20 30.0 0 0 0.003–0.024 -- -- .005 -- -- --

CB 6/21 1/21 0/21 28.6 4.8 0 0.002–0.050 0.050 -- .005 -- -- --

PR 4/49 0/49 0/49 8.2 0 0 0.004–0.017 -- -- .007 -- -- --

DeisopropylatrazineBH 11/36 1/36 1/36 30.6 2.8 2.8 0.01–0.19 0.19 0.19 .03 -- -- NW, OB, 

WR

CB 1/20 0/20 0/20 5.0 0 0 0.01 -- -- .01 -- -- --

3,4-Dichloroaniline-- -- -- -- -- -- -- -- -- -- -- -- -- BH, CB, 

GR, NW, 

OB, WR

Hydroxyatrazine-- -- -- -- -- -- -- -- -- -- -- -- -- NW, OB, 

WR

Insecticides

alpha-Endosulfan

-- -- -- -- -- -- -- -- -- -- -- -- -- BH, CB, 

GR, NW, 

OB, WR

Imidacloprid PR 1/14 1/14 0/14 7.1 7.1 0 0.032 0.032 -- -- -- -- NW, OB, 

WR

Carbaryl -- -- -- -- -- -- -- -- -- -- -- -- -- --

Carbofuran -- -- -- -- -- -- -- -- -- -- -- -- -- --

Dieldrin -- -- -- -- -- -- -- -- -- -- -- -- -- --

Malathion -- -- -- -- -- -- -- -- -- -- -- -- -- --

Diazinon HC 1/78 1/78 0/78 1.3 1.3 0 0.016 0.016 -- -- -- -- --

cis-Permethrin -- -- -- -- -- -- -- -- -- -- -- -- -- --

Table 9. Pesticide detection frequencies and concentrations in ground-water samples collected during fall and spring relative to eight 

regional geographic areas in Wyoming, 1995–2006.—Continued

[Geographic areas where pesticides were detected are listed in order of decreasing frequency with no assessment level applied. Assessment level: NAL, no assessment 

level applied; CSAL, compound-specific assessment level; CAL, common assessment level. Geographic area: BH, Bighorn Basin; CB, Central Basins; GR, Green River 

Basin; HC, High Plains/Casper Arch; NW, Northwest Basins; OB, Overthrust Belt; PR, Powder River Basin; WR, Wind River Basin. Symbol: --, no detections or not 

applicable]

Relation of Pesticide Occurrence to Selected Factors 49
Table 9. Pesticide detection frequencies and concentrations in ground-water samples collected during fall and spring relative to eight 

regional geographic areas in Wyoming, 1995–2006.—Continued

[Geographic areas where pesticides were detected are listed in order of decreasing frequency with no assessment level applied. Assessment level: NAL, no assessment 

level applied; CSAL, compound-specific assessment level; CAL, common assessment level. Geographic area: BH, Bighorn Basin; CB, Central Basins; GR, Green 

River Basin; HC, High Plains/Casper Arch; NW, Northwest Basins; OB, Overthrust Belt; PR, Powder River Basin; WR, Wind River Basin. Symbol: --, no detections or 

not applicable]

Fall 

Pesticide

Geographic 

area 

where 

detected

Number of detections/

number of analyses

Detection frequency in 

geographic area 

(percentage)

Detected concentration range

(micrograms per liter)

Median concentration of 

detections

(micrograms per liter)

Geographic 

area where 

pesticide 

was not 

analyzed

NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL 

Insecticide degradates 

Aldicarb sulfoxideBH 5/63 3/61 5/61 7.9 4.9 8.2 0.07–2.47 0.14–2.47 0.07–2.47 0.14 0.25 0.14 -- 

Aldicarb 

sulfone

BH 4/63 3/62 4/51 6.3 4.8 7.8 0.09–0.3 0.14–0.3 0.09–0.3 .14 .14 .14 -- 

Fipronil sulfide GR 1/12 1/12 0/12 8.3 8.3 0 0.007 0.007 -- -- -- -- NW, OB, 

WR

PR 1/15 1/15 0/15 6.7 6.7 0 0.007 0.007 -- -- -- -- -- 

Fipronil sulfoneGR 1/12 0/12 0/12 8.3 0 0 0.011 -- -- -- -- -- NW, OB, 

WR

PR 1/15 0/15 0/15 6.7 0 0 0.009 -- -- -- -- -- -- 

3-HydroxycarbofuranWR 1/20 0/19 0/19 5.0 0 0 0.0007 -- -- -- -- -- -- 

50 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006
Spring

Pesticide

Geographic 

area 

where 

detected

Number of detections/

number of analyses

Detection frequency in 

geographic area 

(percentage)

Detected concentration range

(micrograms per liter)

Median concentration of 

detections

(micrograms per liter)

Geographic 

area where 

compound 

was not 

analyzed 

NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL NAL CSAL CAL

Insecticide degradates

Aldicarb sulfoxideBH 5/66 3/66 3/66 7.6 4.5 4.5 0.06–0.52 0.46–0.52 0.46–0.52 0.46 0.47 0.47 --

Aldicarb 

sulfone

BH 5/66 3/66 5/37 7.6 4.5 13.5 0.08–0.36 0.17–0.36 0.08–0.36 .17 .35 .17 --

Fipronil sulfide -- -- -- -- -- -- -- -- -- -- -- -- -- NW, WR

--

Fipronil sulfone-- -- -- -- -- -- -- -- -- -- -- -- -- NW, WR

--

3-Hydroxycarbofuran-- -- -- -- -- -- -- -- -- -- -- -- -- --

Table 9. Pesticide detection frequencies and concentrations in ground-water samples collected during fall and spring relative to eight 

regional geographic areas in Wyoming, 1995–2006.—Continued

[Geographic areas where pesticides were detected are listed in order of decreasing frequency with no assessment level applied. Assessment level: NAL, no assessment 

level applied; CSAL, compound-specific assessment level; CAL, common assessment level. Geographic area: BH, Bighorn Basin; CB, Central Basins; GR, Green River 

Basin; HC, High Plains/Casper Arch; NW, Northwest Basins; OB, Overthrust Belt; PR, Powder River Basin; WR, Wind River Basin. Symbol: --, no detections or not 

applicable]

Relation of Pesticide Occurrence to Selected Factors 51
Figure 9. Pesticide concentrations and percentage of wells with at least one pesticide detected relative to land-use category in 

Wyoming, 1995–2006. A, Fall; B, Spring.

15/43

12/72

EXPLANATION

Mixed

Rangeland/undeveloped

Urban

Agriculture

Pesticide detection with no assessment level

Pesticide detection greater than common assessment level

Pesticide detection greater than compound-specific 

 assessment levels

NAL

CSAL

CAL

Number of wells with at least one pesticide detected/number 

 of wells sampled by land-use category at applicable 

 assessment level

2/42

Detected concentration by

 land-use category

Mixed

Rangeland/undeveloped

Urban

Agriculture

Percentage of wells with at least one

 pesticide detected by land-use 

 category

NAL

CSAL

CAL

53/108

52/73

40/72

45/108

43/73

9/43

25/72

19/108

33/73

4/43

8/42

14/72

56/108

57/73

33/72

39/108

44/73

5/42

27/72

15/108

35/73

2/42

NAL

CSAL

CAL

PERCENTAGE OF WELLS WITH AT LEAST ONE PESTICIDE DETECTED

BY LAND-USE CATEGORY

0.0001 0.001 0.01 0.1 1 10 100 0 10 20 30 40 50 60 70 80

CONCENTRATION, IN MICROGRAMS PER LITER

A Fall

B Spring

Median concentration of 

 detections

52 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006
Relation of Pesticide Occurrence to Selected Factors 53

sampled bedrock aquifers were composed of siliciclastic 

sedimentary rocks primarily of Tertiary age. Most sampled 

wells completed in bedrock aquifers were composed of rocks 

of Tertiary age because they are present at or near land surface 

throughout much of Wyoming (Love and Christiansen, 1985). 

Regardless of assessment level, the percentage of wells 

with at least one pesticide detected was larger for wells 

completed in unconsolidated aquifers than in bedrock aquifers (fig. 12). Considering only concentrations greater than 

the CAL, the proportion of wells with at least one pesticide 

detected was significantly different between unconsolidated 

and bedrock aquifers sampled in the fall (p-value = 0.0444, 

CTA, table 11) but not in the spring (p-value = 0.1217, CTA, 

table 11). 

The frequencies of pesticide detection between the 

two types of aquifers were compared using the CSALs for 

individual pesticides detected in either the fall or spring at a 

frequency equal to or greater than 1 percent (table 11). For 

all pesticides tested, the proportion of samples detected at 

concentrations greater than their respective CSALs (detection frequency) was not significantly different between 

aquifer types for the fall or spring (all p-values > 0.05, CTA); 

however, the least insignificant differences were for prometon 

and tebuthiuron. The detection of these two pesticides probably is the primary source of the overall statistically significant 

difference between aquifer types in the fall.

Water-Level Depth

The relation between pesticides and water depth was 

investigated by comparing measured water levels with the 

frequency of pesticide detections at concentrations greater 

than applicable assessment levels. Measured water levels 

ranged from about 2 to 113 feet (ft) below land surface and 

were grouped into four depth categories (2–15, 15.1–33, 

33.1–64, and 64.1–113 ft below land surface) using the Jenks 

method prior to statistical testing using the KWOCR. Most 

sampled aquifers were believed to be unconfined, and therefore, measured water levels likely represented the water table 

in most cases. 

The overall detection frequency of pesticides among the 

four water-level depth categories was examined. Considering 

only concentrations greater than the CAL, the proportion of 

wells with at least one pesticide detected was not significantly 

different among the four water-level depth categories for 

the fall (p-value = 0.4108, KWOCR, table 11) or the spring 

(p-value = 0.9330, KWOCR, table 11). 

The frequencies of pesticide detection among waterlevel-depth categories were compared using the CSALs for 

individual pesticides detected in either the fall or spring at a 

frequency equal to or greater than 1 percent (table 11). Some 

individual pesticides were not detected in all four water-leveldepth categories, and in this case, pesticide detections were 

examined in relation to three depth intervals. In most cases, 

no clear increase or decrease in pesticide detection frequencies was noted as water-level depth increased. For all tested 

pesticides except deethylatrazine, the proportion of samples 

detected at concentrations greater than their respective CSALs 

(detection frequency) was not significantly different among 

the water-level-depth categories for the fall or spring (all 

p-values > 0.05, KWOCR, table 11). For deethylatrazine, the 

proportion of samples detected at concentrations greater than 

the CSAL decreased as water level increased among the four 

water-level-depth categories, and the difference was significant for the spring (p-value = 0.0093, KWOCR, table 11) but 

not for the fall; however, the fall p-value was very close to 

statistical significance (p-value = 0.0636, KWOCR, table 11).

Frequencies of pesticide detection and pesticide concentrations have been reported to decrease with increasing depth 

to the water table in a few studies (Barbash and Resek, 1996, 

and references therein; Gilliom and others, 2006). Greater 

depths to the water table indicate greater unsaturated zone 

thickness, which generally increases the time required for 

pesticides applied at the land surface to reach the water table. 

This increased traveltime allows for increased opportunities 

to affect parent pesticide transport through processes such as 

transformation, sorption, dispersion, and volatilization. An 

inverse relation between pesticide detection and water-table 

depth was not observed in this study (with the exception of 

the pesticide deethylatrazine), and this result is similar to most 

other studies examining the relation. In fact, Barbash and 

Resek (1996, p. 283) noted that “most of the reviewed studies 

did not detect significant relations between pesticide occurrence and water-table depth.”

Well Depth

The relation between pesticides and well depth was 

investigated by comparing measured or reported well depths 

with the frequency of pesticide detection using applicable 

assessment levels. Measured or reported well depths ranged 

from about 7 to 200 ft below land surface and were grouped 

into four well-depth categories (7–29, 30–56, 57–91, and 

92–200 ft below land surface) using the Jenks method prior to 

statistical testing using the KWOCR. 

The overall detection frequency of pesticides among 

the four well-depth categories was examined. Considering 

only concentrations greater than the CAL, the percentage of 

wells with at least one pesticide detected generally decreased 

with increasing depth, and the difference was significant for 

well-depth categories for the fall (p-value = 0.0002, KWOCR, 

table 11) and the spring (p-value = 0.0001, KWOCR, 

table 11). 

The frequencies of pesticide detection among well-depth 

categories were compared using the CSALs for individual 

pesticides detected in either the fall or spring at a frequency 

equal to or greater than 1 percent. In most cases, pesticide 

detection frequencies decreased as well depth increased, 

although detection frequencies for a few pesticides increased 

between one or two well-depth intervals (even though the 

detection frequency generally decreased from the shallowest 

to deepest well-depth interval). The proportion of samples 
Figure 10. Selected pesticide concentrations and detections in ground-water samples collected relative to land use, Wyoming, 

1995–2006. A, Fall; B, Spring. Pesticides are grouped by pesticide type and are listed in order of decreasing detection frequency by 

jointly ranking fall and spring detections with no assessment level.

0.001

CONCENTRATION, IN MICROGRAMS PER LITER

0 10 20 30 40 50 60 70 80 90 100

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

0.001 0.01 0.1 1 10 100

EXPLANATION

Mixed

Rangeland/undeveloped

Urban

Agriculture

Pesticide detection with no assessment level

Pesticide detection greater than common assessment level

Pesticide detection greater than compound-specific 

 assessment level

NAL

CSAL

CAL

Detected concentration 

 associated with land-use

 category

Mixed

Rangeland/undeveloped

Urban

Agriculture

Percentage of pesticide detections 

 associated with land-use

 category

PERCENTAGE OF PESTICIDE DETECTIONS 

ASSOCIATED WITH LAND-USE CATEGORY 

AND ASSESSMENT LEVEL 

A Fall

95/296

82/294

9/296

93/295

39/295

33/295

32/294

31/294

14/294

23/295

2/295

1/295

24/284

20/281

17/282

1/11

0/11

0/11

9/291

8/286

6/286

8/292

2/291

2/291

8/295

3/275

0/295

0/11

0/11

0/11

3/71

0/71

0/71

4/284

2/282

2/134

2/71

2/71

1/71

4/293

1/293

0/293

1/71

1/71

1/71

1/71

0/71

0/71

3/284

1/284

1/284

81/296

13/296

10/296

2/96

1/96

0/96

1/11

1/11

0/11

2/70

1/70

0/70

1/11

0/11

0/11

1/71

1/71

0/71

5/285

3/280

5/259

4/285

3/281

4/136

2/71

2/71

0/71

2/71

0/71

0/71

1/71

0/71

0/71

Number of detections/number of samples at applicable 

 assessment level

0/71

Fipronil sulfone

Fipronil sulfide

Aldicarb sulfone

Aldicarb sulfoxide

Imidacloprid

alpha-Endosulfan

3,4-Dichloroaniline

Bentazon

Metsulfuron

Chlorimuron

2,4–D methyl ester

DCPA

Flumetsulam

Clopyralid

Sulfometuron, methyl

Hexazinone

Metolachlor

Diuron

Bromacil

Oxyfluorfen

Picloram

Simazine

Tebuthiuron

Prometon

Atrazine

PESTICIDE DETECTED

HERBICIDES

HERBICIDE

DEGRADATES

INSECTICIDE

DEGRADATES INSECTICIDES

Hydroxyatrazine

Deisopropylatrazine

Deethylatrazine

Pesticide names in blue are focal pesticides. 

Pesticide names in black are nonfocal pesticides. 

54 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006
0.001 0.01 0.1 1 10 100

CONCENTRATION, IN MICROGRAMS PER LITER

0 10 20 30 40 50 60 70 80 90 100

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

NAL

CSAL

CAL

B Spring

95/295

78/284

10/295

84/294

35/294

27/294

35/295

34/295

17/295

25/294

2/294

1/294

19/291

16/288

15/288

0/10

0/10

0/10

12/294

10/288

9/288

9/294

3/294

3/294

8/294

2/259

0/294

1/35

1/35

0/35

0/71

0/71

0/71

5/288

1/285

5/116

0/71

0/71

0/71

1/294

0/294

0/294

0/69

0/69

0/69

1/294

1/294

1/294

79/295

12/295

9/295

12/121

1/121

1/121

0/10

0/10

0/10

0/71

0/71

0/71

0/10

0/10

0/10

1/71

1/70

0/70

5/292

3/291

3/243

5/292

3/288

5/120

0/83

0/83

0/83

0/83

0/83

0/83

0/71

0/71

0/71

0/71

0/71

0/71

PERCENTAGE OF PESTICIDE DETECTIONS 

ASSOCIATED WITH LAND-USE CATEGORY 

AND ASSESSMENT LEVEL 

Fipronil sulfone

Fipronil sulfide

Aldicarb sulfone

Aldicarb sulfoxide

Imidacloprid

alpha-Endosulfan

3,4-Dichloroaniline

Bentazon

Metsulfuron

Chlorimuron

2,4–D methyl ester

DCPA

Flumetsulam

Clopyralid

Sulfometuron, methyl

Hexazinone

Metolachlor

Diuron

Bromacil

Oxyfluorfen

Picloram

Simazine

Tebuthiuron

Prometon

Atrazine

PESTICIDE DETECTED

HERBICIDES

HERBICIDE

DEGRADATES

INSECTICIDE

DEGRADATES INSECTICIDES

Hydroxyatrazine

Deisopropylatrazine

Deethylatrazine

Figure 10. Selected pesticide concentrations and detections in ground-water samples collected relative to land use, Wyoming, 

1995–2006. A, Fall; B, Spring. Pesticides are grouped by pesticide type and are listed in order of decreasing detection frequency by 

jointly ranking fall and spring detections with no assessment level.—Continued

Relation of Pesticide Occurrence to Selected Factors 55
Table 10. Results of contingency-table analysis (CTA) or Kruskal-Wallis test for ordered categorical responses 

(KWOCR) comparing selected pesticide detections in ground-water samples collected during fall and spring and landuse categories in Wyoming, 1995–2006.

[Statistically significant differences (p-value<0.05) between four land-use categories (agricultural, urban, rangeland/undeveloped, and mixed) 

indicated in bold type; CAL, common assessment level; CSAL, compound-specific assessment level; H, herbicide; HD, herbicide degradate; 

ID, insecticide degradate; I, insecticide; <, less than; --, not applicable]

Tested variable Pesticide type Assessment level 

used

Probability (p-value)

Fall Spring

Percentage of wells with at least 

one pesticide detected

-- CAL <0.0001 <0.0001

Atrazine H CSAL .0509 .0535

Prometon H CSAL <.0001 <.0001

Deethylatrazine HD CSAL .4419 .3556

Tebuthiuron H CSAL <.0001 <.0001

Picloram H CSAL .5424 .3428

Deisopropylatrazine HD CSAL .4836 --

3,4-Dichloroaniline H CSAL .8214 --

Bromacil H CSAL <.0001 <.0001

Diuron H CSAL -- .2787

Metolachlor H CSAL .5334 --

Hexazinone H CSAL -- .2490

Aldicarb sulfoxide ID CSAL .5768 .5737

Aldicarb sulfone ID CSAL .1713 .5764

Hydroxyatrazine HD CSAL .6558 --

Flumetsulam H CSAL .3675 --

Imidacloprid I CSAL .6690 .6558

Fipronil sulfide ID CSAL .4738 --

2,4–D methyl ester H CSAL .6690 --

56 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006
Figure 11. Pesticide detections in ground-water samples collected relative to aquifer type, Wyoming, 1995–2006. A, Fall; B, Spring.

PERCENTAGE OF DETECTIONS BY AQUIFER TYPE AND ASSESSMENT LEVEL

EXPLANATION

Pesticide detections with no assessment level applied

Pesticide detections greater than common assessment level

Pesticide detections greater than compound-specific assessment levels

NAL

CSAL

CAL

 Unconsolidated deposits

Aquifer type

 Bedrock

0 10 20 30 40 50 60 70 80 90 100

A Fall

B Spring

NAL

CSAL

CAL

NAL

CSAL

CAL

Relation of Pesticide Occurrence to Selected Factors 57
Figure 12. Pesticide concentrations and percentage of wells with at least one pesticide detected relative to aquifer type, Wyoming, 

1995–2006. A, Fall; B, Spring.

 Unconsolidated deposits

 Bedrock

 Unconsolidated deposits

 Bedrock

EXPLANATION

Pesticide detection with no assessment level

Pesticide detection greater than common assessment level

Pesticide detection greater than compound-specific 

 assessment levels

NAL

CSAL

CAL

Number of wells with at least one pesticide detected/

 number of wells sampled by aquifer type at applicable 

 assessment level

11/73

Detected concentration 

 by aquifer type

Percentage of wells with at least one

 pesticide detected by aquifer

 type

NAL

CSAL

CAL

32/73

25/73

10/73

130/223

96/223

58/223

32/73

22/73

11/73

55/222

123/222

93/222

NAL

CSAL

CAL

PERCENTAGE OF WELLS WITH AT LEAST ONE PESTICIDE 

DETECTED BY AQUIFER TYPE

0.0001 0.001 0.01 0.1 1 10 100 0 10 20 30 40 50 60

CONCENTRATION, IN MICROGRAMS PER LITER

A Fall

B Spring

Median concentration

58 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006
Table 11. Results of contingency-table analysis (CTA) or Kruskal-Wallis test for ordered categorical responses (KWOCR) comparing selected pestic

water samples collected during fall and spring and hydrogeologic characteristics in Wyoming, 1995–2006.[Statistically significant differences (p-value<0.05) indicated in bold type; CAL, common assessment level; CSAL, compound-specific assessment level; H, herbicide; HD, herbdegradate; I, insecticide; <, less than; --, not applicable]

 Tested variable Pesticide type Assessment level used

Hydrogeologic characteristic

Aquifer type1 Water-level depth2 Well depth3 

Probability (p-value) Probability (p-value) Probability (p-value) 

Fall Spring Fall Spring Fall Spring 

Percentage of wells 

with at least one pesticide detected

-- CAL 0.0444 0.1217 0.4108 0.9330 0.0002 0.0001 

Atrazine H CSAL .1659 .2194 .9863 .8562 .7420 .2053 Prometon H CSAL .2095 .0882 .3609 .6587 .0001 .0124 Deethylatrazine HD CSAL .8466 .7171 .0636 .0093 .5884 .3660 Tebuthiuron H CSAL .0708 .0982 .3491 .5331 .0023 .0002 Picloram H CSAL .7399 .7664 .6511 .2686 .3326 .8210 Deisopropylatrazine HD CSAL .7952 -- .9334 -- .7055 -- 3,4-Dichloroaniline H CSAL .3869 -- .7515 -- .7530 -- Bromacil H CSAL .6880 .9584 .6765 .6505 .0013 .0002 Diuron H CSAL -- .7515 -- .4633 -- .5943 Metolachlor H CSAL .7449 -- .2238 -- .5251 -- Hexazinone H CSAL -- .5119 -- .1850 -- .5355 Aldicarb sulfoxide ID CSAL .7185 .7446 .6872 .9494 .4683 .9308 Aldicarb sulfone ID CSAL .7399 .7470 .6890 .9494 .5348 .9317 Hydroxyatrazine HD CSAL .9309 -- .9459 -- .7391 -- Flumetsulam H CSAL .5628 -- .8634 -- .4802 -- Imidacloprid I CSAL .9205 .8844 .9473 .7952 .7485 .7212 Fipronil sulfide ID CSAL .3882 -- .8505 -- .6745 -- 2,4–D methyl ester H CSAL .9205 -- .9473 -- .7485 -- 

1Differences between unconsolidated and bedrock.2Differences among four categories (2–15, 15.1–33, 33.1–64, and 64.1–113 feet below land surface).3Differences among four categories (7–29, 30–56, 57–91, and 92–200 feet below land surface).4Differences among three categories (domestic-supply, stock, and other).
60 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006

detected at concentrations greater than their respective CSALs 

(detection frequency) decreased as well depth increased for 

three pesticides (prometon, tebuthiuron, and bromacil, listed 

in order of decreasing detection frequency), and the difference 

was significant for both fall and spring (p-values for all three 

compounds < 0.05, KWOCR, table 11).

The inverse relation between pesticide detection and 

well depth seen in the baseline study has been observed in 

other studies. Frequencies of pesticide detection and pesticide concentrations generally decreased with increasing well 

depths in most studies that examined this relation (Barbash 

and Resek, 1996, and references therein). In fact, Barbash and 

Resek (1996, p. 290) comprehensively reviewed many different studies examining pesticides in ground water, and they 

found that “well depth was the factor most commonly found to 

be correlated with pesticide detections in ground water,” and 

“inverse relations between well depths and pesticide detection frequencies and concentrations have been documented 

in many locations.” A greater well depth generally indicates 

greater depth of the water table and well-screen interval below 

the water table. Both characteristics generally increase the 

time required for pesticides applied at the land surface to reach 

the water table. This increased traveltime allows for increased 

opportunities to affect parent pesticide transport through 

processes such as transformation, sorption, dispersion, and 

volatilization. 

Well Type

During this study, ground-water-quality samples were 

collected from four different types of wells—domestic-supply 

wells (58 percent of sampled wells), monitoring wells 

(22 percent of sampled wells), stock wells (12 percent of 

sampled wells), and other miscellaneous wells (8 percent of 

sampled wells). The relation between pesticides and well type 

was investigated by comparing well type with the frequency 

of pesticide detection using applicable assessment levels. The 

different types of sampled wells were grouped into three welltype categories on the basis of water use—a domestic-supply 

well category, a stock well category, and “other” well category 

(composed of monitoring and other miscellaneous wells)—for 

statistical testing using the CTA.

The overall detection frequency of pesticides between 

the three well-type categories was examined. Pesticides were 

always detected much more frequently in the “other” and 

domestic-supply well categories than in the stock well category. In most cases, pesticides were detected more frequently 

in the “other” well category than in the domestic-supply well 

category, although detection frequencies were very similar 

for some pesticides. Considering only concentrations greater 

than the CAL, the proportion of wells with at least one 

pesticide detected was greater for the “other” well category 

than for the domestic-supply well and stock well categories 

for both fall and spring, and the difference was significant 

for both fall (p-value = 0.0002, CTA, table 11) and spring 

(p-value = 0.0001, CTA, table 11). 

The frequencies of pesticide detection between well-type 

categories were compared using the CSALs for individual 

pesticides detected in either the fall or spring at a frequency 

equal to or greater than 1 percent. The proportion of samples 

detected at concentrations greater than their respective CSALs 

(detection frequency) was larger for the “other” well category 

than for the domestic-supply well and stock well categories 

for most pesticides tested in table 11, and the difference 

was significant for three pesticides (prometon, tebuthiuron, and bromacil, listed in order of decreasing detection 

frequency) in both the fall and spring (p-values for all three 

compounds < 0.05, CTA, table 11) and one pesticide (diuron) 

in the spring (p-value = 0.0272, CTA, table 11). 

Differences in pesticide detection frequencies among 

different types of wells have been observed in other studies, 

most likely due to the different types of construction, hydrogeologic setting, and pumping patterns associated with a 

specific type of well (Barbash and Resek, 1996, and references 

therein). In this study, the difference in detection frequency 

among well types may be due to differences in well depth 

among the types of wells sampled. Well depth, as noted previously, is the factor most commonly found to correlate with 

pesticide detections in ground water. Detection frequencies for 

most pesticides in this study were largest for the “other” well 

category, and monitoring wells comprised 74 percent of the 

wells in this category. Median water-level depth was similar 

among all three well categories, but the median well depth of 

the “other” well category (20 ft), consisting of many shallow 

monitoring wells, was less than one-half that of the domesticsupply well category (50 ft) and stock well category (52.5 ft). 

Pesticide detection frequencies among different types of wells 

are often largest in monitoring wells, and in summarizing 

previous studies, Barbash and Resek (1996, p. 293) noted 

that “monitoring wells usually exhibit the highest detection 

frequencies, since they are typically shallow, and often screen 

close to the water table.”

Soils

Pesticide detections were examined in relation to selected 

soil properties from the State Soil Geographic (STATSGO) 

database (U.S. Department of Agriculture, 1991). STATSGO 

data were used instead of the larger scale Soil Survey 

Geographic (SSURGO) database (U.S. Department of Agriculture, 1995) because SSURGO coverage was not available for 

all areas in Wyoming. The STATSGO data used in this study 

were from a digital data set compiled by Schwarz and Alexander (1995) for a national model of water quality. Three different soil characteristics selected from this data set—organic 

matter content, soil permeability, and an index developed from 

soil hydrologic groupings—were evaluated in relation to the 

occurrence of pesticides in the underlying ground water. A GIS 

was used to manipulate the digital data set to quantify these 
Relation of Pesticide Occurrence to Selected Factors 61

soil characteristics within a 500-m radius surrounding each 

sampled well (selection and use of 500-m radius described 

previously in the “Land Use” section of the report). 

In some previous studies, soil characteristics have 

been related to the occurrence of pesticides in ground water 

(Barbash and Resek, 1996, and references therein). Consequently, pesticides detected during this study were compared 

to organic matter content and soil permeability, as well as a 

soil hydrologic index derived from several soil properties.

Organic Matter Content

Organic matter content is the amount of organic material 

in the soil, in percentage by weight (Schwarz and Alexander, 

1995). The relation between pesticides and organic matter 

content was investigated by comparing organic matter content 

with the frequency of pesticide detection using applicable 

assessment levels. Organic matter content was grouped into 

four different ordinal categories (0–0.3, 0.31–0.5, 0.51–0.8, 

and 0.81–1.4 percent by weight) using the Jenks method prior 

to statistical testing using the KWOCR.

The overall detection frequency of pesticides among 

the four organic-matter-content categories was examined. 

Considering only concentrations greater than the CAL, the 

proportion of wells with at least one pesticide detected was not 

significantly different among the four organic-matter-content 

categories for fall or spring (both p-values > 0.05, KWOCR, 

table 12).

The frequencies of pesticide detection among the four 

organic-matter-content categories were compared using the 

CSALs for individual pesticides detected in either the fall or 

spring at a frequency equal to or greater than 1 percent. For 

most pesticides, no clear trend of increasing or decreasing 

frequency of detection was noted among the four categories 

as organic matter content increased. In fact, detection frequencies for many pesticides both increased and decreased among 

the four organic-matter-content categories as organic matter 

content increased. The proportion of samples detected at 

concentrations greater than their respective CSALs (detection frequency) was significantly different among the four 

organic-matter-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 (p-values 

for all compounds < 0.05, KWOCR, table 12). The proportion of atrazine detections at concentrations greater than the 

CSAL was close to statistical significance (p-value = 0.0885, 

table 12) among the four organic-matter-content categories 

for samples collected in the spring. However, although the 

proportion of samples detected at concentrations greater than 

their respective CSALs (detection frequency) was significantly 

different among the four organic-matter-content categories for 

the four pesticides detected in the fall, detection frequencies 

for tebuthiuron in both fall and spring increased and decreased 

among the four organic-matter-content categories as organic 

matter content increased, whereas detection frequencies for 

atrazine, flumetsulam, and fipronil sulfide increased among 

the four soil-permeability categories as soil permeability 

increased.

Barbash and Resek (1996) indicate that organic matter 

(organic carbon content) is one of the most important soil 

characteristics that affects transport of pesticides through 

the root zone and into the subsurface and underlying ground 

water. These investigators noted that organic carbon content 

and soil permeability are “two factors” that “influence pesticide behavior in the subsurface in a concerted fashion” and 

“whereas lower permeability restricts pesticide movement 

by slowing the downward migration of the bulk soil solution, the presence of larger amounts of organic matter reduces 

the mass of pesticide reaching greater depths through the 

combined effects of enhanced microbial activity and hydrophobic sorption” (Barbash and Resek, 1996, p. 274). In this 

study, however, detection frequencies for many pesticides 

both increased and decreased among the four organic-mattercontent categories as organic matter content increased, including one pesticide (tebuthiuron) for which the proportion of 

samples detected at concentrations greater than their respective CSALs (detection frequency) was significantly different 

among the four organic-matter-content categories. This may 

be due to the fact that the effects of organic matter content (as 

well as soil permeability and other soil properties) often are 

not correlated with pesticide detection in many studies because 

the relation “may only be discernible among areas that exhibit 

substantial contrasts in these properties, or where differences 

in soil properties are not overwhelmed by variations in other 

factors” (Barbash and Resek, 1996, p. 277). 

Soil Permeability

Soil permeability is determined using laboratory 

measurements and is defined as the amount of water that will 

move downward through a unit area of saturated soil in unit 

time, under unit hydraulic gradient, measured in inches per 

hour (Schwarz and Alexander, 1995). The relation between 

pesticides and soil permeability was investigated by comparing soil permeability with the frequency of pesticide detection 

using applicable assessment levels. Soil permeability was 

grouped into four different ordinal categories (0.17–2.30, 

2.31–4.26, 4.27–6.62, and 6.63–12.4 in/h) using the Jenks 

method prior to statistical testing using the KWOCR. 

The overall detection frequency of pesticides was examined among the four soil-permeability categories. Considering 

only concentrations greater than the CAL, the proportion of 

wells with at least one pesticide detected was significantly 

different among the four soil-permeability categories for fall 

(p-value = 0.0395, KWOCR, table 12) but not spring (p-value 

> 0.05, KWOCR, table 12). 

The frequencies of pesticide detection among the four 

soil-permeability categories were compared using the CSALs 

for individual pesticides detected in either the fall or spring 

at a frequency equal to or greater than 1 percent (table 12). 

For most pesticides, no clear trend of increasing or decreasing 
Table 12. Results of Kruskal-Wallis test for ordered categorical responses (KWOCR) comparing selected pesticide detections in 

ground-water samples collected during fall and spring and soil characteristics in Wyoming, 1995–2006.

[Statistically significant differences (p-value<0.05) indicated in bold type; CAL, common assessment level; CSAL, compound-specific assessment level; H, 

herbicide; HD, herbicide degradate; ID, insecticide degradate; I, insecticide; <, less than; --, not applicable]

Tested variable Pesticide 

type

Assessment 

level used

 Soil characteristic

 Organic-matter content1 Soil permeability2 Soil hydrologic index3

Probability (p-value) Probability (p-value) Probability (p-value)

Fall Spring Fall Spring Fall Spring

Percentage of wells 

with at least one 

pesticide detected

-- CAL 0.2600 0.3177 0.0395 0.2727 0.0887 0.6260

Atrazine H CSAL .0032 .0885 .2746 .0103 .0003 <.0001

Prometon H CSAL .6663 .6219 .1132 .8366 .4637 .4473

Deethylatrazine HD CSAL .3989 .7439 .0126 .1558 <.0001 .0001

Tebuthiuron H CSAL .0175 .0234 .0253 .0095 .3401 .0828

Picloram H CSAL .1783 .8056 .6436 .2621 .4048 .5448

Deisopropylatrazine HD CSAL .1146 -- .1188 -- .1718 --

3,4-Dichloroaniline H CSAL .2636 -- .6592 -- .6259 --

Bromacil H CSAL .6968 .4962 .2257 .4465 .6640 .8568

Diuron H CSAL -- .3087 -- .7602 -- .3574

Metolachlor H CSAL .3226 -- .3747 -- .7643 --

Hexazinone H CSAL -- .7165 -- .6823 -- .9607

Aldicarb sulfoxide ID CSAL .7141 .7346 .2989 .2706 .7349 .7286

Aldicarb sulfone ID CSAL .3130 .7344 .0301 .2786 .2130 .7281

Hydroxyatrazine HD CSAL .1850 -- .2228 -- .3373 --

Flumetsulam H CSAL .0306 -- .6448 -- .7260 --

Imidacloprid I CSAL .3522 .3600 .8446 .8507 .4972 .4753

Fipronil sulfide ID CSAL .0227 -- .4331 -- .0727 --

2,4–D methyl ester H CSAL .3522 -- .8446 -- .7647 --

1

Differences among four categories (0–0.3, 0.31–0.5, 0.51–0.8, and 0.81–1.4 percent by weight).

2

Differences among four categories (0.17–2.30, 2.31–4.26, 4.27–6.62, and 6.63–12.4 inches per hour).

3

Differences among three or four soil-hydrologic-index categories.

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

frequency of detection was noted among the four categories 

as soil permeability increased. In fact, detection frequencies 

for many pesticides both increased and decreased among 

the four soil-permeability categories as soil permeability 

increased. The proportion of samples detected at concentrations greater than their respective CSALs (detection 

frequency) was significantly different among the four soilpermeability categories for three pesticides (deethylatrazine, 

tebuthiuron, and aldicarb sulfone, listed in order of decreasing 

detection frequency) detected in the fall and two pesticides 

(atrazine and tebuthiuron) detected in the spring (p-values 

for all compounds < 0.05, KWOCR, table 12). However, 

although the proportion of samples detected at concentrations 

greater than their respective CSALs (detection frequency) 

was significantly different among the four soil-permeability 

categories for these four pesticides, detection frequencies for 

deethylatrazine and tebuthiuron both increased and decreased 

among the four soil-permeability categories as soil permeability increased, whereas detection frequencies for both atrazine 

and aldicarb sulfone both increased among the four soilpermeability categories as soil permeability increased.

As previously mentioned, Barbash and Resek (1996) 

indicated that soil permeability is one of the most important 

soil characteristics that affects transport of pesticides through 

the root zone and into the subsurface and underlying ground 

water. In this study, detection frequencies for many pesticides 

both increased and decreased among the four soil-permeability 

categories as soil permeability increased, even for pesticides 

where the proportion of samples detected at concentrations 

greater than their respective CSALs (detection frequency) 

was significantly different among the four soil-permeability 

categories. This may be due to the fact that the effects of 
Relation of Pesticide Occurrence to Selected Factors 63

soil permeability (as well as soil organic matter content and 

other soil properties) often are not correlated with pesticide 

detection in many studies because the relation “may only be 

discernible among areas that exhibit substantial contrasts in 

these properties, or where differences in soil properties are 

not overwhelmed by variations in other factors” (Barbash and 

Resek, 1996, p. 277). 

Soil Hydrologic Index

The soil hydrologic index was developed by Schwarz and 

Alexander (1995) from soil hydrologic groups presented in the 

STATSGO data set. The soil hydrologic group is a letter rating 

that combines several soil hydrologic characteristics and is 

the minimum steady-ponded infiltration rate for bare ground. 

Soil hydrologic groups consist of four different categories 

(A through D), with category A having the largest saturated 

hydraulic conductivity. The soil hydrologic index converts the 

four hydrologic groups into unitless numeric codes through 

coding transformations, where A = 1 (rapid infiltration rates, 

deep soils, well-drained to excessively drained sands and 

gravels), B = 2 (moderate infiltration rates, deep and moderately deep, moderately well and well-drained soils with 

moderately coarse textures), C = 3 (slow infiltration rates, 

soils with layers impeding downward movement of water, or 

soils with moderately fine or fine textures), and D = 4 (very 

slow infiltration rates, soils are clayey, have a high water table, 

or are shallow with an impervious layer). Mixtures of soil 

hydrologic groups are assigned the value 4. A GIS was used to 

manipulate the digital data set to quantify the indices within a 

500-m radius surrounding all sampled wells; wells not located 

exclusively within one group were assigned an index number 

determined through proportional weighting on the basis of 

percentages within the 500-m radius. 

The relation between pesticides and soil hydrologic index 

was investigated by comparing the soil hydrologic index with 

the frequency of pesticide detection using applicable assessment levels. The soil hydrologic index was grouped into three 

or four different ordinal categories using the Jenks method 

prior to statistical testing using the KWOCR. 

The overall detection frequency of pesticides was examined among four soil-hydrologic-index categories. Considering 

only concentrations greater than the CAL, the proportion of 

wells with at least one pesticide detected was not significantly 

different among the four soil-hydrologic-index categories for 

fall and spring (all p-values > 0.05, KWOCR, table 12). 

The frequencies of pesticide detection among three or 

four soil-hydrologic-index categories were compared using 

the CSALs for individual pesticides detected in either the fall 

or spring at a frequency equal to or greater than 1 percent 

(table 12). The number of soil-hydrologic-index categories 

varied by pesticide (either three or four categories). For most 

pesticides, no clear trend of increasing or decreasing frequency 

of detection was noted among the four categories. In fact, 

detection frequencies for many pesticides both increased and 

decreased among the four soil-hydrologic-index categories. 

The proportion of samples detected at concentrations greater 

than their respective CSALs (detection frequency) generally 

decreased for two pesticides (atrazine and deethylatrazine) 

as the soil hydrologic index increased among the three or 

four soil-hydrologic-index categories, and the difference 

was significant for both fall and spring (p-values for both 

compounds < 0.05, KWOCR, table 12). Pesticides often can 

be detected more commonly beneath coarser soils with greater 

permeability (represented by a smaller soil hydrologic index 

number) (Barbash and Resek, 1996). 

Nitrate and Selected Water-Quality 

Characteristics

One water-quality constituent (nitrite plus nitrate as nitrogen) and three different water-quality characteristics (specific 

conductance, pH, and dissolved oxygen) were examined in 

relation to the occurrence of pesticides in ground water. In 

some previous studies, these constituents and characteristics 

have been shown to be related to the occurrence of pesticides 

in ground water (Barbash and Resek, 1996, and references 

therein).

Nitrate

Water from most wells sampled and analyzed for pesticides during this study also was analyzed for selected nutrients, including nitrogen species such as nitrite plus nitrate (as 

nitrogen). Because nitrite was detected infrequently at very 

small concentrations and generally composed very little of 

the nitrite plus nitrate as nitrogen concentrations, nitrite plus 

nitrate concentrations as nitrogen are referred to as “nitrate” 

herein. The most common anthropogenic sources of nitrogen 

in shallow ground water are from leaching of fertilizers from 

agricultural lands, animal manure, or septic-system effluent 

(Alley, 1993, and references therein). 

The relation between pesticides and nitrate was investigated by comparing measured nitrate concentrations indicative 

of ground-water contamination from anthropogenic activities 

with the frequency of pesticide detection using applicable 

assessment levels. Concentrations of nitrate in ground water in 

the conterminous United States greater than 3 milligrams per 

liter (mg/L) (Madison and Brunett, 1985), 2 mg/L (Mueller 

and Helsel, 1996), or 1.1 mg/L (Nolan and Hitt, 2003) have 

been reported to be the result of anthropogenic activities. 

Measured nitrate concentrations were grouped into two ordinal 

categories—concentrations less than and greater than or equal 

to the 1.1-mg/L threshold value reported by Nolan and Hitt 

(2003) to represent nitrate contamination of shallow ground 

water by anthropogenic activities.

The overall detection frequency of pesticides between the 

two nitrate concentration categories was examined. Considering only concentrations greater than the CAL, the proportion 

of wells with at least one pesticide detected was significantly 
64 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006

different between the nitrate concentration categories for both 

the fall and spring (p-values < 0.0001, KWOCR, table 13). 

The frequencies of pesticide detection between the 

two nitrate concentration categories were compared using 

the CSALs for individual pesticides detected in either 

the fall or spring at a frequency equal to or greater than 1 

percent (table 13). The proportion of samples detected at 

concentrations greater than their respective CSALs (detection frequency) was much larger for the “elevated” nitrate 

concentration category than for the other nitrate concentration 

category for almost every pesticide tested in table 13 (data not 

shown). The proportion of samples detected at concentrations 

greater than their respective CSALs (detection frequency) 

was significantly different 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 

(p-values for all compounds < 0.05, KWOCR, table 13). The 

proportion of tebuthiuron and picloram detections at concentrations greater than the CSAL were very close to statistical 

significance between the two nitrate concentration categories 

for samples collected in the spring (p-value = 0.0507 and 

p-value = 0.0558, respectively, KWOCR, table 13).

Many studies have found that pesticides are detected 

more frequently in ground water as nitrate concentrations 

increase (Barbash and Resek, 1996, and references therein). 

This relation has been attributed to application of fertilizers 

in conjunction with pesticides to produce crops in agricultural 

areas. Elevated nitrate concentrations indicative of anthropogenic activities have been found frequently in ground-water 

samples containing other contaminants such as pesticides and 

volatile organic compounds (Squillace and others, 2002).

Specific Conductance

Specific conductance was measured onsite in water 

from most sampled wells and analyzed for pesticides during 

this study. Measurements of specific conductance in water 

from wells sampled during this study were examined in 

relation to pesticide occurrence by comparing measured 

specific conductance with the frequency of pesticide detection above applicable assessment levels (table 13). Measured 

specific conductance values were grouped into two ordinal 

categories representing freshwater [specific conductance 

values ≤ 1,665 microsiemens per centimeter at 25 degrees 

Celsius (μS/cm)] and saline water (specific conductance 

values > 1,665 μS/cm) prior to statistical testing using the 

KWOCR. The specific conductance value of 1,665 μS/cm 

was assumed to represent the boundary between fresh and 

saline water by assuming multiplication of the value by 0.6 to 

convert values into representative dissolved-solids concentrations (salinity) (Hem, 1985).

The overall detection frequency of pesticides was 

examined between the two specific conductance categories. 

Considering only concentrations greater than the CAL, the 

proportion of wells with at least one pesticide detected was 

not significantly different between the two specific conductance categories for the fall and the spring (p-values > 0.05, 

KWOCR, table 13). 

The frequencies of pesticide detection between the two 

specific conductance categories were compared using the 

CSALs for individual pesticides detected in either the fall 

or spring at a frequency equal to or greater than 1 percent 

(table 13). The proportion of samples detected at concentrations greater than their respective CSALs (detection 

frequency) was much larger for the saline-water category than 

for the freshwater category for almost every pesticide tested in 

table 13. The proportion of samples detected at concentrations 

greater than their respective CSALs (detection frequency) was 

significantly different between the two specific conductance 

categories for one pesticide (tebuthiuron) in both the fall and 

spring and one pesticide (prometon) only in the fall (p-values 

for both compounds < 0.05, KWOCR, table 13). The percentage of picloram detections at concentrations greater than the 

CSAL was very close to statistical significance between the 

two specific conductance categories for samples collected 

in the fall (p-value = 0.0638, KWOCR, table 13). However, 

although the proportion of samples detected at concentrations 

greater than their respective CSALs (detection frequency) was 

significantly different between fresh and saline water for these 

three pesticides, detection frequencies for tebuthiuron (fall and 

spring detections) and prometon (fall detections) were larger 

for the saline-water category than for the freshwater category, 

whereas the detection frequency for picloram (fall detections) 

was larger for the freshwater category than for the saline-water 

category.

It is unclear what processes may be responsible for the 

observed relation between pesticide detection and specific 

conductance for several pesticides. On the basis of a limited 

literature search, few studies (or parts of studies) have examined the relation between dissolved solids (and by inference, 

specific conductance) and pesticide detection in ground water. 

Barbash and Resek (1996) did note that ionic strength is 

unlikely to have much effect on pesticide reactivity in most 

natural water. It is possible that this observed relation is a 

result of the statistical process or is due to co-occurrence 

of dissolved solids (specific conductance) with some other 

(unrecognized) factor. 

pH

The water-quality characteristic pH was measured onsite 

in water from most sampled wells and analyzed for pesticides during this study. Measurements of pH in water from 

wells sampled during this study were examined in relation 

to pesticide occurrence by comparing measured pH with the 

frequency of pesticide detection using applicable assessment 

levels. Measured pH values were grouped into three ordinal 

categories—acidic (pH < 6.8), circumneutral (6.8 ≤ pH ≤ 7.3), 

and alkaline (pH > 7.3)—prior to statistical testing using the 

KWOCR.
Table 13. Results of Kruskal-Wallis test for ordered categorical responses (KWOCR) comparing selected pesticide detections in ground-water samples collected 

during fall and spring and nitrate and selected water-quality characteristics in Wyoming, 1995–2006.[Statistically significant differences (p-value<0.05) indicated in bold type; CAL, common assessment level; CSAL, compound-specific assessment level; H, herbicide; HD, herbicide degradate; ID, insecticide degradate; I, insecticide; <, less than; ≤, less than or equal to; >, greater than; ≥, greater than or equal to; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; --, not applicable]

 Tested variable Pesticide type Assessment level used

Constituent or characteristic

Nitrate1 Specific conductance2 pH3 Dissolved oxygen4

Probability (p-value) Probability (p-value) Probability (p-value) Probability (p-value)

Fall Spring Fall Spring Fall Spring Fall Spring

Percent of wells with at least 

one pesticide detected

-- CAL <0.0001 <0.0001 0.6038 0.1022 0.0008 0.0022 0.5485 0.4246

Atrazine H CSAL <.0001 <.0001 .4460 .9558 .2448 .1281 .3370 .2020Prometon H CSAL <.0001 .0014 .0072 .0905 .0107 .0008 .3892 .4279Deethylatrazine HD CSAL .0005 .0007 .2329 .2822 .8077 .4808 .9694 .6193Tebuthiuron H CSAL .0351 .0507 .0070 .0061 .0020 .0072 .0416 .0567Picloram H CSAL .4565 .0558 .0638 .4029 .4764 .4311 .2645 .5132Deisopropylatrazine HD CSAL .3122 -- .2005 -- .5738 -- .2871 --3,4-Dichloroaniline H CSAL .3613 -- .3613 -- .2636 -- .4142 --Bromacil H CSAL .2031 .0237 .4365 .0854 .0002 <.0001 .6957 .1975Diuron H CSAL -- .6030 -- .9142 -- .0006 -- .9532Metolachlor H CSAL .6056 -- .2357 -- .8452 -- -- --Hexazinone H CSAL -- .5186 -- .5862 -- .7704 -- --Aldicarb sulfoxide ID CSAL .1008 .0943 .8383 .2513 .1431 .6001 -- --Aldicarb sulfone ID CSAL .0947 .0933 .2758 .2516 .7077 .6097 -- --Hydroxyatrazine HD CSAL .2070 -- .3103 -- .6489 -- .2943 --Flumetsulam H CSAL .7582 -- .3173 -- .6564 -- .2871 --Imidacloprid I CSAL .2153 .2123 .3173 .3031 .6564 .6578 .3476 .2899Fipronil sulfide ID CSAL .2366 -- .9840 -- .0704 -- .9289 --2,4–D methyl ester H CSAL .2153 -- .3173 -- .6564 -- .3476 --

1Difference between two categories, nitrate concentrations <1.1 mg/L and ≥1.1 mg/L.2Difference between two categories, freshwater (≤1,665 μS/cm) and saline water (>1,665 μS/cm).3Difference among three categories, acidic, pH <6.8; circumneutral, 6.8≤pH ≤7.3; alkaline, pH >7.3.4Difference between two categories, dissolved oxygen concentrations <1 mg/L and ≥ 1 mg/L.

Relation of Pesticide Occurrence to Selected Factors 65
66 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006

The overall detection frequency of pesticides was 

examined among the three pH categories. Considering only 

concentrations greater than the CAL, the proportion of wells 

with at least one pesticide detected was significantly different 

among the three pH categories for the fall (p-value = 0.0008, 

KWOCR, table 13) and the spring (p-value = 0.0022, 

KWOCR, table 13). 

The frequencies of pesticide detection among the three 

pH categories were compared using the CSALs for individual 

pesticides detected in either the fall or spring at a frequency 

equal to or greater than 1 percent (table 13). For many pesticides, no clear trend of increasing or decreasing frequency 

of detection was noted among the three categories as pH 

increased. In fact, detection frequencies for many pesticides 

both increased and decreased among the three pH categories as pH increased. The proportion of samples detected at 

concentrations greater than their respective CSALs (detection 

frequency) 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 one pesticide (diuron) 

detected in the spring (p-values for all compounds < 0.05, 

KWOCR, table 13). The proportion of fipronil sulfide detections at concentrations greater than the CSAL was very close 

to statistical significance among the three pH categories for 

samples collected in the fall (p-value = 0.0704, KWOCR, 

table 13). However, although the proportion of samples 

detected at concentrations greater than their respective CSALs 

(detection frequency) was significantly different among 

the three pH categories for these five pesticides, detection 

frequencies for prometon and tebuthiuron (spring detections) 

both increased and decreased among the three pH categories 

as pH increased, whereas detection frequencies for tebuthiuron, bromacil, diuron, and fipronil sulfide (fall detections 

listed in order of decreasing detection frequency) increased 

among the three pH categories as pH increased.

Differences in pH have been observed to directly or 

indirectly affect pesticide transformations and concentrations 

(Barbash and Resek, 1996, and references therein). Unfortunately, most studies (Barbash and Resek, 1996) have been 

conducted in the laboratory rather than onsite, so results of this 

study were not compared with similar studies.

Dissolved Oxygen

Dissolved oxygen (DO) was measured onsite in water 

from less than one-half of the sampled wells and analyzed for 

pesticides during this study. Measurements of DO in water 

from wells sampled during this study were examined in relation to pesticide occurrence by comparing measured DO with 

the frequency of pesticide detection greater than applicable 

assessment levels. The amount of DO in ground water can be 

used to infer the oxidation/reduction (redox) state of ground 

water. The redox state can affect the solubility and speciation 

of many dissolved constituents in ground water (Hem, 1985). 

Redox conditions can directly or indirectly affect pesticide 

transformations and concentrations (Barbash and Resek, 

1996, and references therein). Ground water with little to no 

DO indicates reducing (anoxic or anaerobic) or near-reducing 

conditions, and ground water with DO indicates oxygenated 

(oxic or aerobic) conditions. In this report, measured DO 

values less than 1 mg/L were assumed to represent anoxic 

conditions, whereas measured DO values equal to or greater 

than 1 mg/L were assumed to represent oxic conditions. 

Measured DO values then were grouped according to these 

values into two ordinal categories representing anoxic and 

oxic conditions. 

The overall detection frequency of pesticides was 

examined between the two DO categories. Considering only 

concentrations greater than the CAL, the proportion of wells 

with at least one pesticide detected was not significantly different between the two DO categories for the fall and the spring 

(p-values > 0.05, KWOCR, table 13). 

The frequencies of pesticide detection between the two 

DO categories were compared using the CSALs for individual 

pesticides detected in either the fall or spring at a frequency 

equal to or greater than 1 percent (table 13). The proportion of 

samples detected at concentrations greater than their respective CSALs (detection frequency) was significantly different 

between the two DO categories for one pesticide (tebuthiuron) 

in the fall (p-value = 0.0416, KWOCR, table 13). The proportion of tebuthiuron detections at concentrations greater than 

the CSAL was very close to statistical significance between 

the two DO categories for samples collected in the spring 

(p-value = 0.0567, KWOCR, table 13). Few studies have been 

conducted examining the relation between redox conditions 

and pesticides in ground water (Barbash and Resek, 1996).

Summary

To address concerns regarding ground-water contamination by pesticides and other agricultural chemicals, the U.S. 

Environmental Protection Agency (USEPA) developed and 

recommended a new Federal-State partnership in October 

1991 to address the potential risks posed to ground water 

by the use of pesticides. In response, the State of Wyoming 

formed the Ground-water and Pesticides Strategy Committee 

(GPSC) consisting of members of local, State, and Federal 

government, as well as industry and interest groups, to prepare 

the State of Wyoming Generic Management Plan (SMP) 

for Pesticides in Ground Water. The SMP includes information describing individuals and organizations involved with 

implementation of the SMP, ground-water contamination 

prevention, ground-water monitoring, and required responses 

if pesticides are detected in ground water. 

In Wyoming, little existing information was available 

describing pesticide occurrence in ground water. Consequently, the GPSC decided that a large component of the 

SMP would be to conduct “baseline ground-water sampling” 

to characterize current pesticide occurrence in Wyoming’s 
Summary 67

ground water. In addition, the GPSC identified 20 pesticides 

(18 parent pesticides and 2 pesticide degradates) to be of 

greatest interest during baseline ground-water sampling. 

The U.S. Geological Survey (USGS), in cooperation with the Wyoming Department of Agriculture and the 

Wyoming Department of Environmental Quality and acting 

on behalf of the GPSC, began statewide implementation of 

baseline ground-water sampling in 1995 and completed it 

in 2006. During 1995-2006, water samples were collected 

twice (during fall and spring) from 296 wells located in areas 

of “high” or “medium-high” ground-water vulnerability to 

pesticide contamination that were mapped for the entire State 

of Wyoming as part of the SMP. Ground-water samples were 

collected using standard USGS procedures.

Ground-water samples were analyzed using USGS laboratory methods with analytical reporting limits much smaller 

than typically used in routine pesticide monitoring of public 

drinking-water supplies. Consequently, it is likely that much 

more frequent rates of detection were obtained than would 

have been possible with less sensitive analytical methods. 

Reporting limits varied by pesticide, and many changed during 

the course of the study; therefore, pesticide detections were 

recensored to account for variations in reporting limits before 

data were summarized and analyzed. 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 (μg/L) for all pesticides and a compound-specific 

assessment level (CSAL) that differed by pesticide. 

Because of severe data censoring (fewer than 50 percent 

of the data were 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. Pesticide data were classified into two groups according to the characteristics selected 

for analysis and whether or not the pesticide was detected at 

concentrations greater than or less than the CSAL or CAL. 

The null hypothesis tested was that detection or nondetection 

of any pesticide was independent of the grouped characteristic 

selected for examination. Rejection of the null hypothesis at 

a probability (p-value) of 0.05 (alpha level) was considered 

evidence supporting the alternative hypothesis that there 

was a relation between the variable (pesticide) and the factor 

(grouped characteristic) tested.

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 in water from about 22 percent of 

wells sampled in the spring. The proportion of wells with at 

least one pesticide detected at concentrations greater than the 

CAL was not significantly different between fall and spring 

(p-value > 0.05). 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. Most mixtures were composed of 

two to four different pesticides. At least 74 percent or more 

of pesticides detected were classified as herbicides. Pesticides classified as triazines, ureas, pyridinecarboxylic acids, 

miscellaneous herbicides, or carbamates were most commonly 

detected. Considering only detections using the CAL, triazine 

pesticides were detected much more frequently than all other 

pesticide classes and comprised about 48 percent (fall) and 

45 percent (spring) of all detections. In addition, the number 

of different pesticides classified as triazines was the largest of 

all classes.

Twenty-eight different pesticides were detected at 

concentrations greater than the CSALs in water from wells 

sampled in fall, whereas 21 different pesticides were detected 

in water from wells sampled in spring. Many pesticides were 

detected infrequently because 13 of 28 pesticides detected 

(about 46 percent) at concentrations greater than the CSALs 

in water from wells sampled in the fall were detected only 

in one well. Ten of 21 pesticides detected (about 48 percent) 

at concentrations greater than the CSALs in water from 

wells sampled in the spring were detected only in one well. 

Considering only detections using the CSALs, 14 of 20 focal 

pesticides identified in the SMP were detected at least once 

in water sampled from wells during either the fall or spring. 

Considering only detections using the CSALs and pesticides 

analyzed for in more than 11 wells, only five pesticides were 

each detected more than 5 percent of the time in water from 

wells sampled in either fall or spring (atrazine, prometon, 

tebuthiuron, picloram, and 3,4-dichloroaniline, listed in order 

of decreasing detection frequency). Atrazine was the pesticide detected most frequently in water from wells sampled in 

either fall or spring at concentrations greater than the CSAL 

(about 28 percent of wells in the fall and about 27 percent 

in the spring). Individual pesticides detected in both the fall 

and spring at a frequency equal to or greater than 1 percent 

using the CSALs were compared statistically to assess 

seasonal differences. For all pesticides tested, the proportion 

of samples with concentrations greater than their respective 

CSALs (detection frequency) was not significantly different 

(p-value > 0.05) between fall and spring.

Concentrations of detected pesticides generally were 

small (< 1 μg/L), although a few infrequent detections at 

larger concentrations were noted. Ten different pesticides were 

detected at concentrations greater than 1 μg/L in water from 

at least one well sampled in either fall or spring. All detected 

pesticide concentrations were smaller than U.S. Environmental Protection Agency (USEPA) drinking-water standards or 

health advisories where applicable (many pesticides did not 

have standards or advisories). Most concentrations were at 

least an order of magnitude smaller than USEPA drinkingwater standards or health advisories. 

Regardless of assessment level and considering both 

seasons, the largest percentage of pesticide detections and 

the largest number of different pesticides detected were in 

water from wells located in the Bighorn Basin and High 

Plains/Casper Arch geographic areas of Wyoming (using the 

CAL, both areas combined comprised about 69 percent of all 

detections in the fall and about 63 percent of all detections in 
68 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006

the spring). Both geographic areas are major crop-producing 

areas in Wyoming. Pesticides were detected distinctly less 

often in water from wells in the other six geographic areas. 

Using the CAL, the proportion of wells with at least one 

pesticide detected was significantly different among the eight 

geographic areas for both the fall and the spring. Using the 

CSALs, only six herbicides (atrazine, prometon, tebuthiuron, 

picloram, bromacil, and diuron, listed in order of decreasing detection frequency) and one degradate (deethylatrazine) 

were detected in either the fall or spring in three or more 

different geographic areas. Of these seven compounds, 

prometon was the only pesticide detected in either season in 

all eight geographic areas. Using the CSALs, 17 pesticides 

were detected in only one geographic area in the fall, and 

12 pesticides were detected in only one geographic area in the 

spring. The proportion of samples detected at concentrations 

greater than their respective the CSALs (detection frequency) 

was significantly different among the eight geographic areas 

for four pesticides (atrazine, prometon, deethylatrazine, and 

tebuthiuron, listed in order of decreasing detection frequency) 

detected in the fall and three pesticides (atrazine, deethylatrazine, and tebuthiuron) detected in the spring.

Pesticide detection frequencies were evaluated in relation 

to four land-use categories—urban, agricultural, rangeland/

undeveloped, and mixed. Regardless of assessment level and 

season, 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 

water from wells located in predominantly rangeland/undeveloped areas. The frequency of pesticide detection in water from 

wells located in either predominantly agricultural or mixed 

land-use areas was intermediate to water from wells located 

in urban and rangeland/undeveloped areas; however, using 

the CAL, the difference was more distinct, and the percentage of wells with at least one pesticide detected located in 

either predominantly agricultural or mixed areas was less than 

one-half that of wells located in predominantly urban areas 

regardless of season. 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 (p-values < 0.05). 

Using the CSALs for either fall or spring, eight pesticides 

[listed in order of decreasing detection frequency—bromacil, 

clopyralid, flumetsulam, 2,4–D methyl ester, deisopropylatrazine (fall), 3,4-dichloroaniline, hydroxyatrazine, and 

imidacloprid] were detected in water from wells located in 

predominantly urban areas, three pesticides (listed in order 

of decreasing detection frequency—DCPA, bentazon, and 

aldicarb sulfone) were detected only in samples from wells 

located in predominantly agricultural areas, and two pesticides [hexazinone and deisopropylatrazine (spring)] were 

detected only in samples from wells located in predominantly 

mixed land-use areas. Using the CSALs, no pesticide was 

detected in water from wells located only in predominantly 

rangeland/undeveloped areas. Remaining pesticides detected 

at concentrations greater than their respective CSALs were 

associated with sampled wells located in two to four different predominant land uses. Two of the pesticides detected 

most frequently during this study (prometon and tebuthiuron) 

either have no reported agricultural use (prometon) or limited 

noncropland use (tebuthiuron) in Wyoming, and both were 

detected most commonly in sampled wells located in predominantly urban areas, although many detections were associated 

with sampled wells located in predominantly agricultural or 

mixed areas. This was not surprising and likely reflects the use 

of both pesticides for many different nonagricultural applications throughout all land uses in Wyoming. The occurrence of 

some pesticides did not necessarily correspond to the predominant land use assigned to sampled wells. The proportion of 

samples detected at concentrations greater than their respective CSALs (detection frequency) was significantly different 

among the four land-use categories for four pesticides (atrazine, prometon, tebuthiuron, and bromacil, listed in order of 

decreasing detection frequency) detected in both the fall and 

spring. 

The frequency of pesticide detection was 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, and considering only concentrations 

greater than 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. For all pesticides tested, the proportion of samples 

detected at concentrations greater than their respective CSALs 

(detection frequency) was not significantly different between 

unconsolidated and bedrock aquifers for the fall or spring. 

Using the CAL, the proportion of wells with at least 

one pesticide detected was not significantly different among 

four water-level depth categories for the fall or the spring 

(all p-values > 0.05). For all tested individual pesticides 

except deethylatrazine, the proportion of samples detected 

at concentrations greater than their respective CSALs 

(detection frequency) was not significantly different among 

the water-level depth categories for the fall or spring (all 

p-values > 0.05). For deethylatrazine, the proportion of 

samples detected at concentrations greater than the CSAL 

decreased as water level increased among four water-level 

depth categories, and the difference was significant for the 

spring but not for the fall. 

Using the CAL, the proportion of wells with at least one 

pesticide detected generally decreased with increasing well 

depth, and the difference was significant for the fall and the 

spring. The proportion of samples detected at concentrations 

greater than their respective CSALs (detection frequency) 

decreased as well depth increased for three pesticides (prometon, tebuthiuron, and bromacil, listed in order of decreasing 

detection frequency), and the difference was significant for 

both fall and spring. 
Summary 69

Pesticides were always detected much more frequently 

in the “other” and domestic-supply well categories than in the 

stock well category. In most cases, pesticides were detected 

more frequently in the “other” well category than in the 

domestic-supply well category, although detection frequencies were very similar for some pesticides. Using the CAL, 

the proportion of wells with at least one pesticide detected 

was larger for the “other” well category than for the domesticsupply well and stock well categories for both fall and spring, 

and the difference was significant for both fall and spring. 

The proportion of samples detected at concentrations greater 

than their respective CSALs (detection frequency) was larger 

for the “other” well category than for the domestic-supply 

well and stock well categories for most pesticides tested, and 

the difference was significant for three pesticides (prometon, tebuthiuron, and bromacil, listed in order of decreasing 

detection frequency) detected in both fall and spring and one 

pesticide (diuron) detected 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-m (1,640-ft) radius surrounding each sampled well. Using 

the CAL, the proportion of wells with at least one pesticide 

detected was not significantly different among four organicmatter-content categories for fall or spring. For most pesticides, no clear trend of increasing or decreasing frequency 

of detection was noted among the four categories as organic 

matter content increased. The proportion of samples detected 

at concentrations greater than their respective CSALs (detection frequency) was significantly different among the four 

organic-matter-content categories for four pesticides (atrazine, 

tebuthiuron, flumetsulam, and fipronil sulfide, in order of 

decreasing detection frequency) detected in the fall and in one 

pesticide (tebuthiuron) detected in the spring. 

Using the CAL, the proportion of wells with at least 

one pesticide detected was significantly different among the 

four soil-permeability categories for fall but not spring. For 

most pesticides, no clear trend of increasing or decreasing 

frequency of detection was noted among the four categories 

as soil permeability increased. The proportion of samples 

detected at concentrations greater than their respective CSALs 

(detection frequency) was significantly different among the 

four soil-permeability categories for three pesticides (deethylatrazine, tebuthiuron, and aldicarb sulfone, listed in order of 

decreasing detection frequency) detected in the fall and two 

pesticides (atrazine and tebuthiuron) detected in the spring. 

Using the CAL, the proportion of wells with at least one 

pesticide detected was not significantly different among the 

four soil-hydrologic-index categories (derived from STATSGO 

soil properties) for fall and spring. For most pesticides, no 

clear trend of increasing or decreasing frequency of detection 

was noted among the four soil-hydrologic-index categories. 

The proportion of samples detected at concentrations greater 

than their respective CSALs (detection frequency) generally 

decreased for two pesticides (atrazine and deethylatrazine) as 

the soil hydrologic index increased, and the difference was 

significant for both fall and spring. 

One water-quality constituent (nitrate) and three different 

water-quality 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 

both fall and spring. The proportion of samples detected at 

concentrations greater than their respective CSALs (detection frequency) was much larger for the “elevated” nitrate 

concentration category than for the other nitrate concentration 

category for almost every pesticide tested. The proportion of 

samples detected at concentrations greater than their respective CSALs (detection frequency) was significantly different 

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. 

 Using the CAL, the proportion of wells with at least 

one pesticide detected was not significantly different between 

two specific conductance categories representing freshwater 

[specific conductance values ≤1,665 microsiemens per centimeter at 25 degrees Celsius (μS/cm)] and saline water (specific 

conductance values >1,665 μS/cm) for the fall and the spring. 

The proportion of samples detected at concentrations greater 

than their respective CSALs (detection frequency) was much 

larger for the saline-water category than for the freshwater 

category for almost every pesticide tested. The proportion of 

samples detected at concentrations greater than their respective CSALs (detection frequency) was significantly different 

between the two specific conductance categories for one pesticide (tebuthiuron) in both the fall and spring and 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 (pH <6.8), circumneutral 

(6.8 ≤ pH ≤7.3), and alkaline (pH >7.3)—for both fall and 

spring. For many pesticides, no clear trend of increasing 

or decreasing frequency of detection was noted among the 

three categories as pH increased. The proportion of samples 

detected at concentrations greater than their respective CSALs 

(detection frequency) 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 one pesticide (diuron) detected in the spring. 

Using the CAL, the proportion of wells with at least one 

pesticide detected was not significantly different between 

the two dissolved oxygen (DO) categories—anoxic conditions (DO concentrations < 1 milligram per liter) and oxic 

conditions (DO concentrations ≥ 1 milligram per liter)—for 

the fall and the spring. The proportion of samples detected at 

concentrations greater than their respective CSALs (detection 
70 Occurrence of Pesticides in Ground Water of Wyoming, 1995–2006

frequency) was significantly different between the two DO 

categories for one pesticide (tebuthiuron) in the fall. 

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Helena Publishing Service Center

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Director, USGS Wyoming Water Science Center

2617 East Lincolnway Avenue, Suite B

Cheyenne, Wyoming 82001

(307) 778-2931

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