Wyoming Department of 

Agriculture

Wyoming Department of 

Environmental 

Quality

Alcova

Reservoir

High

Medium high

Medium

Medium low

Low

Mapped ground-water 

 vulnerability to 

 pesticide contamination

Sampled well

Rank of county’s 

 ground-water vulnera-

 bility to pesticide con-

 tamination (Wyoming 

 Ground-water and 

 Pesticides Strategy 

 Committee, 1999)

0

0

60 MILES

60 KILOMETERS

EXPLANATION MONTANA

WYOMING

Lusk

Cody

Basin

Casper

Lander

Laramie

Rawlins

Douglas

Jackson

Worland

Buffalo

Cheyenne

Evanston

Kemmerer

Pinedale

Gillette

Sundance

Sheridan

Wheatland

Newcastle

Torrington

Green

River

Thermopolis

YELLOWSTONE

NATIONAL

PARK

110° 109° 108° 107° 106° 105°

41°

42°

43°

44°

45°

River

Sweetwater

Pathfinder

Reservoir

Seminoe

 Reservoir

Boysen

Reservoir

Keyhole

Res

Glendo

Reservoir

Guernsey

 Reservoir

Fontenelle

 Reservoir

Wheatland

Reservoir

River

Platte

River

North

Laramie

Cheyenne

River

River

Belle

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 Fk

River

River

Big Sandy

River

River

Medicine

Fork

B

Blacks

ear R

Smiths Fk

R

Salt

Base from U.S. Geological Survey 

digital data, 2003, 1:2,000,000

Albers Equal-Area Conic projection

Standard parallels 29°30′N and 

45°30′N, central meridian 107°30′W

Afton

Saratoga

Glenrock

Riverton

Rock Springs

Lovell Powell

Yellowstone

Lake

Jackson

Lake

Flaming Gorge

Reservoir

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)

LARAMIE

(6)

GOSHEN

(3)

PLATTE

(9)

NIOBRARA

(23)

FREMONT

(4)

SUBLETTE

(22)

River

Powder Little

Bow

North

Platte

PopoAgie

(23)

111°

IDAHO UTAH

UTAH COLORADO

SOUTH DAKOTA NEBRASKA

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2009–3006

February 2009

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

Pesticides in Ground Water of Wyoming, 1995–2006

Printed on recycled paper

Figure 1. Aquifer vulnerability mapped across the State of Wyoming was based on hydrogeologic features and land use (Hamerlinck and 

Arneson, 1998). Two hundred and ninety-six wells were sampled to evaluate the presence of pesticides in Wyoming’s most vulnerable aquifers.

In 1991, members of local, State, and Federal governments, as well as industry and interest groups, formed the Ground-water 

and Pesticides Strategy Committee (GPSC) to prepare the State of Wyoming Generic Management Plan for Pesticides in Ground 

Water. Little existing information was available describing pesticide occurrence in ground water; therefore, statewide baseline 

ground-water sampling was considered a high priority by the GPSC.

The GPSC identified 20 pesticides and degradates for baseline ground-water sampling (referred to herein as focal pesticides). 

Sampling focused on the State’s most vulnerable ground water (Wyoming Ground-water and Pesticides Strategy Committee, 1999) 

as determined by Hamerlinck and Arneson (1998; fig. 1). Ground-water vulnerability is based on inherent sensitivity of the hydrogeology (such as a shallow water table or highly permeable aquifer materials) and overlying land use.

Ground-Water Sampling

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

Wyoming Departments of Agriculture and Environmental Quality, 

on behalf of the GPSC, conducted baseline sampling in Wyoming 

to characterize the large-scale occurrence of pesticides in ground 

water. During 1995–2006, sampling was conducted in each county, 

during which generally two water samples were collected (once in 

the fall and spring) from each of 296 wells. Five to 28 wells were 

selected for sampling in each county with site selection focused on 

areas where the ground water is most vulnerable, shown as areas of 

high or medium high vulnerability in figure 1.

The order of counties sampled was based on a ranking from 

the GPSC (fig. 1; Ground-water and Pesticides Strategy Committee, 

1999). Most wells selected for this study were either a domestic, 

monitoring, or stock well. The depths of all wells were between 7 

and 200 feet below land surface, with most well depths less than 

100 feet. Most (78 percent) wells were completed in unconsolidated 

aquifers, and 22 percent of wells were completed in consolidated 

(bedrock) aquifers (Bartos and others, 2009).

Ground-water samples were analyzed for the 20 focal pesticides (table 1), plus as many as 136 additional pesticides (Bartos 

and others, 2009) using USGS laboratory methods. These highly 
Table 1. Focal pesticides and nonfocal pesticides, as determined by the Wyoming Ground-water Pesticides Strategy Committee (1999), 

detected in Wyoming ground water, 1995–2006 (compiled from Bartos and others, 2009).

[Compounds in bold type were detected at concentrations greater than the CSAL. CSAL, compound-specific assessment level; μg/L, micrograms per liter; 

--, not applicable; NA, not assigned; USEPA, U.S. Environmental Protection Agency]

Common name Trade name

Type of 

pesticide

(Meister, 2002)

Number of detections 

(using CSAL)/number of 

samples 

(CSAL noted in 

parentheses, in μg/L)

Median concentration of 

two or more detections, or 

value if only one detection, 

greater than CSAL (μg/L)

USEPA standard or 

health advisory

 (μg/L)

Focal pesticides1 from Wyoming Ground-water and Pesticides Strategy Committee (1999)

Alachlor Lasso, Alanex, Shroad Herbicide 0/589 (0.05) -- 22

Aldicarb Temik Insecticide 0/577 (NA) -- 2,33

Aldicarb sulfone Aldicarb degradate, Standek, 

Aldoxycarb

Insecticide 6/569 (0.11) 0.24 2,32

Aldicarb sulfoxide Aldicarb degradate -- 6/571 (0.14) .47 2,34

Atrazine Aatrex, Atranex Herbicide 160/578 (0.004) .010 23

Bromacil Hyvar X Herbicide 18/574 (0.06) .52 470

Clopyralid Stinger, Lontrel Herbicide 3/567 (0.26) 22 (

7

)

Cyanazine Bladex Herbicide 2/535 (0.013) .074 41

2,4–D Dacamine, Weed-B-Gon Herbicide 51/580 (0.15) 13.8 270

DCPA Dacthal Herbicide 51/587 (0.004) .004 470

Dicamba Banvel, Banex Herbicide 51/575 (0.13) 1.1 44,000

cis- and trans-1,3-

Dichloropropene

Telone Nematicide 0/570 (NA) -- 640

Difenzoquat Avenge Herbicide No analytical method available -- (

7

)

Hexazinone Buckshot, Pronone, Velpar Herbicide 51/46 (0.05) .06 4400

Metolachlor Bicep, Dual Herbicide 5/534 (0.009) .024 4700

Metribuzin Lexone, Sencor Herbicide 0/589 (0.05) -- 470

Metsulfuron Ally, Escort Herbicide 0/142 (0.07) -- (7)

Picloram Tordon Herbicide 36/569 (0.06) .35 2500

Simazine Princep, Primatol, Aquazine Herbicide 4/589 (0.05) .08 24

Tebuthiuron Graslan, Spike Herbicide 65/589 (0.01) .05 4500

Nonfocal pesticides detected above applicable CSAL1

Bentazon Basagram, Bentazone Herbicide 2/578 (0.06) 0.36 4200

Bromoxynil Agristar, Brominal, Buctril Herbicide 51/580 (0.07) .22 (7)

Carbofuran Furadan, Futura Insecticide 2/587 (0.02) .032 240

2,4–D methyl ester -- Herbicide 51/140 (0.009) .74 (7)

Deethylatrazine Atrazine degradate -- 25/591 (0.05) .21 (7)

Deisopropylatrazine Atrazine/cyanazine/simazine 

degradate

-- 2/217 (0.05) .12 (7)

Diazinon Basudin, Spectracide, 

Knoxout

Insecticide, 

nematicide

5

1/587 (0.008) .016 41

3,4-Dichloroaniline Propanil degradate -- 51/21 (0.004) .007 (7)

Dichlorprop Weedone, Polymone Herbicide 51/583 (0.06) .07 (7)

Diuron Durashield, Karmex Herbicide 5/585 (0.06) .11 6200

Fipronil sulfide Fipronil degradate -- 2/154 (0.006) .007 (7)

Flumetsulam Broadstrike, Python Herbicide 2/142 (0.06) .07 (

7

)

Hydroxyatrazine Atrazine degradate -- 51/141 (0.016) .02 (7)

Imidacloprid Admire, Provado Insecticide 2/141 (0.02) .03 (7)

Oryzalin Surflan Herbicide 51/520 (0.31) .63 (7)

Prometon Pramitol, Gesafram Herbicide 74/549 (0.05) .11 4100

Triallate Far-Go, Avadex BW Herbicide 51/541 (0.003) .005 (7)

Triclopyr Garlon Herbicide 2/584 (0.36) 10.1 (7)

1

For a list of all pesticides analyzed for during the study, see Bartos and others, 2009, table 1.

2

MCL, USEPA Maximum Contaminant Level (U.S. Environmental Protection Agency, 2006).

3

The MCL for any combination of two or more of these three chemicals (aldicarb, aldicarb sulfoxide, and aldicarb sulfone) should not exceed 7 μg/L because of 

similar mode of action.

4

LHA, USEPA Lifetime Health Advisory Level (U.S. Environmental Protection Agency, 2006).

5

If a compound was only detected once, that value rather than a median is presented.

6

RSD4, USEPA Risk-Specific Dose at 10-4 Cancer Risk (U.S. Environmental Protection Agency, 2006).

7

At the time of publication, a standard or health advisory had not been established by the USEPA.
Alcova

Reservoir

Lusk

Cody

Basin

Casper

Lander

Laramie

Rawlins

Douglas

Jackson

Worland

Buffalo

Cheyenne

Evanston

Kemmerer

Pinedale

Gillette

Sundance

Sheridan

Wheatland

Newcastle

Torrington

Green

River

Thermopolis

110° 109° 108° 107° 106° 105°

41°

42°

43°

44°

45°

River

Sweetwater

Pathfinder

Reservoir

Seminoe

 Reservoir

Boysen

Reservoir

Keyhole

Reservoir

Glendo

Reservoir

Guernsey

 Reservoir

Fontenelle

 Reservoir

Wheatland

Reservoir

River

Platte

River

North

Laramie

Cheyenne

River

River

Belle

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 Fk

River

River

Big Sandy

River

River

Medicine

Fork

B

Blacks

ear R

Smiths Fk

R

Salt

0

0

60 MILES

60 KILOMETERS

Afton

Saratoga

Glenrock

Riverton

Rock Springs

Lovell Powell

Yellowstone

Lake

Jackson

Lake

Flaming Gorge

Reservoir

Gros Ventre River

River

Powder Little

Bow

North

Platte

PopoAgie

111°

CROOK

PARK BIG

HORN

SHERIDAN

WASHAKIE

JOHNSON

CAMPBELL

TETON

HOT

SPRINGS

FREMONT

LINCOLN

UINTA

SWEETWATER

NATRONA

CONVERSE NIOBRARA

WESTON

GOSHEN

PLATTE

ALBANY

CARBON

LARAMIE

SUBLETTE

Base from U.S. Geological Survey digital 

data, 2003, 1:2,000,000

Albers Equal-Area Conic projection

Standard parallels 29°30′N and 45°30′N, 

central meridian 107°30′W

YELLOWSTONE

NATIONAL

PARK

EXPLANATION

Geographic area and

 number of different

 pesticides detected

 using the common

 assessment level (CAL)

 in parentheses ()

Bighorn Basin (11)

Central Basins (10)

Green River Basin (6)

High Plains/Casper

 Arch (13)

Northwest Basins (1)

Overthrust Belt (2)

Powder River Basin (6)

Wind River Basin (4)

Well sampled for 

 pesticide

No pesticide detected

Pesticide detected at 

 concentration greater 

 than the CAL 

 (0.07 μg/L)

Figure 2. One or more pesticides were detected at concentrations greater than the CAL (0.07 μg/L) in water from 23 percent of the wells sampled during 1995–2006. The largest percentage of pesticide detections was in ground water from the High Plains/Casper Arch geographic area.

sensitive methods often have analytical reporting limits 100 to 

1,000 times lower than State and Federal drinking-water standards 

and guidelines for protecting water quality or routine pesticide 

monitoring of public drinking-water supplies. All concentrations 

of pesticides detected during the study were less than State and 

Federal standards and guidelines. Detections, therefore, do not necessarily indicate a concern to human health but rather help identify 

the environmental presence of pesticides and track their occurrence 

over time.

Maximum Contaminant Levels (MCLs). Concentrations of some 

unregulated compounds were compared to USEPA Lifetime Health 

Advisory Levels (LHAs) or toxicity information, if available. 

Sixteen of the detected compounds listed in table 1 do not have 

human-health benchmarks or adequate toxicity information for 

evaluating results in a human-health context.

Pesticide Detections—Statewide

Descriptions of pesticide detections are useful for understanding the nature of pesticides in ground water throughout the State 

and are as follows:

•	 No pesticide concentrations exceeded USEPA drinkingwater standards or health-advisory levels (available for 18 of 

32 detected pesticides; table 1).

•	 One or more pesticides were detected at concentrations greater 

than the CAL (0.07 μg/L) in water from about 23 percent of 

wells sampled in the State.

•	 Most detected pesticides (81 percent) were classified as herbicides or herbicide degradates.

•	 Thirty-two different pesticides were detected at concentrations 

greater than applicable CSALs.

•	 A greater number of pesticides were detected at concentrations 

greater than the CSAL during the fall (28 different compounds) 

compared to the spring (21 different compounds).

Pesticide Detections—Geographic Area

The State was divided into eight different regional geographic 

areas by Bartos and others (2009), on the basis of regional areas 

having similar natural characteristics (geography, physical features, 

Data Analysis

Results of baseline ground-water sampling were analyzed to 

describe pesticide occurrence (Bartos and others, 2009). Although 

the data were collected over a 12-year period, the data were not 

conducive to trend analyses, because most sites were only sampled 

during a 1-year period.

Most pesticide detections were at low levels—concentrations 

that approach laboratory analytical capabilities. When comparing 

detections of the same pesticide over time, all sample detections 

were recensored to a single reporting level (noted as the CSAL, 

compound-specific assessment level). In order to compare different 

compounds to each other, pesticide detections were again recensored to a common assessment level (CAL) of 0.07 microgram per 

liter (μg/L).

A screening-level assessment of the possible significance 

of detected compounds to human health was done and was based 

on a comparison of measured concentrations to available humanhealth benchmarks. Concentrations of regulated compounds were 

compared to U.S. Environmental Protection Agency (USEPA) 
Figure 4. Urban land use has the highest percentage of wells with 

at least one pesticide detected. Agricultural, mixed, and rangeland/

undeveloped land uses had fewer wells with at least one pesticide 

detection.

0 10 20 30 40 50 60

RANGELAND/

UNDEVELOPED

MIXED

URBAN

AGRICULTURAL

PERCENTAGE OF WELLS WITHIN A LAND-USE CATEGORY, WITH 

AT LEAST ONE PESTICIDE DETECTED AT CONCENTRATIONS

 GREATER THAN THE COMPOUND-SPECIFIC ASSESSMENT LEVEL

LAND-USE CATEGORY

16.5 percent

36.1 percent

59.6 percent

38.9 percent

0 5 10 15 20 25 30 35 40 45

WIND RIVER

BASIN

POWDER RIVER

BASIN

OVERTHRUST

BELT

NORTHWEST

BASINS

HIGH PLAINS/

CASPER ARCH

GREEN RIVER

BASIN

CENTRAL

BASINS

BIGHORN

BASIN

GEOGRAPHIC AREA

PERCENTAGE OF TOTAL PESTICIDE DETECTIONS (GREATER THAN 

THE COMMON ASSESSMENT LEVEL) IN GEOGRAPHIC AREA

2.3 percent

8.6 percent

2.3 percent

0.5 percent

40.0 percent

6.8 percent

13.2 percent

26.4 percent

Figure 3. Forty percent of all pesticide detections in Wyoming were 

in the High Plains/Casper Arch geographic area.

hydrology, and geology) and land use (fig. 2). Evaluation of pesticide occurrence in relation to geographic areas showed that:

•	 The largest percentages of pesticide detections were in water 

from the High Plains/Casper Arch and Bighorn Basin geographic 

areas, where 40 and 26 percent, respectively, of all detections 

occurred at concentrations greater than the CAL (fig. 3).

•	 The number of different pesticides detected at concentrations 

greater than the CAL was largest in water from the High Plains/

Casper Arch, Bighorn Basin, and Central Basins geographic 

areas (fig. 2).

•	 Prometon (a nonagricultural pesticide) was the only pesticide 

detected in all eight geographic areas.

Pesticide Detections—Hydrogeology

The frequency of pesticide detection was examined in relation 

to hydrogeology. This involved assessing the aquifer type, water 

level and well depth, well type, and soil characteristics. A notable 

outcome of this analysis was:

•	 The percentage of wells with at least one pesticide detected was 

larger for wells completed in unconsolidated aquifers than in 

bedrock aquifers regardless of assessment level.

Pesticide Detections—Land Use

Land use was mapped within a 500-meter radius of each 

sampled well and classified into one of four predominant land-use 

categories (agricultural, urban, rangeland/undeveloped, and mixed) 

so that pesticide detections in ground water could be examined in 

relation to overlying land use. The following information was noted 

in regard to land use:

•	 Pesticides were detected most frequently in water from wells 

located in areas classified as predominantly urban (59.6 percent) 

and agricultural (38.9 percent; fig. 4).

•	 Pesticides were detected least frequently in water from wells 

located in areas classified as predominantly rangeland/undeveloped, as only 16.5 percent of the water samples from these wells 

had a pesticide detection.

Future of Ground-Water Monitoring Program

The completion of baseline ground-water quality sampling 

and the analysis of that data have provided the State with an opportunity to move forward with monitoring Wyoming’s ground water. 

Information gained from this effort is helping to refine and guide a 

new, more focused ground-water monitoring program, with the ultimate goal of protecting Wyoming’s ground water while allowing 

responsible use of pesticides to continue (Wyoming Ground-water 

and Pesticides Strategy Committee, oral commun., 2008).

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

Layout and design by Suzanne C. Roberts

Publishing support provided by the USGS Helena Publishing Service Center

This study has been supported in part with a Section 319 grant from the U.S. 

Environmental Protection Agency to the Wyoming Department of Environmental 

Quality’s Non-Point Source Program.

The use of trade, product, industry, or firm names is for descriptive purposes 

only and does not imply endorsement by the U.S. Government.

U.S. Environmental Protection Agency, 2006, 2006 Edition of the 

drinking-water standards and health advisories: Washington, D.C., 

U.S. Environmental Protection Agency, Office of Water, EPA 822–

R–06–013, summer 2006, 11 p., accessed January 22, 2008, at 

http://www.epa.gov/waterscience/criteria/drinking/dwstandards.pdf

Wyoming Ground-water and Pesticides Strategy Committee, 1999, 

Wyoming generic State management plan for pesticides and ground 

water: prepared for Wyoming Department of Agriculture, Cheyenne, 

Wyo., 103 p.

Selected References

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.

Hamerlinck, J.D., and Arneson, C.S., eds., 1998, Wyoming ground 

water vulnerability assessment handbook, volume 2. Assessing 

ground-water vulnerability to pesticides: Laramie, University of 

Wyoming, Spatial Data and Visualization Center Publication SDVC 

98–01–2 [variously paged].

Meister, R.T., 2002, Farm chemicals handbook: Willoughby, Ohio, 

Meister Publishing Co. [variously paged].