Wyoming Department 

of Agriculture

Wyoming Department 

of Environmental 

Quality

Figure 1. Pathways of pesticide movement in the hydrologic cycle (modified from Barbash 

and Resek, 1996).

In cooperation with the Wyoming Department of Agriculture (WDA) and the Wyoming Department of 

Environmental Quality (WDEQ)

In 1991, members of local, State, and Federal governments, as well as industry and interest groups, formed the Groundwater and Pesticide Strategy Committee to prepare the State of Wyoming’s generic Management Plan for Pesticides in 

Ground Water. Part of this management plan is to sample and analyze Wyoming’s ground water for pesticides. In 1995, the 

U.S. Geological Survey, in cooperation with the Ground-water and Pesticide Strategy Committee, began statewide implementation of the sampling component of the State of Wyoming’s generic Management Plan for Pesticides in Ground Water. 

During 2005–2006, baseline monitoring was conducted in Niobrara and Weston Counties. This Fact Sheet describes and summarizes results of the baseline monitoring in Niobrara and Weston Counties.

Pesticides in Ground Water - Niobrara and Weston Counties, 

Wyoming, 2005–2006

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 2007–3024

March 2007

Pesticides in Ground Water

Synthetic organic pesticides are used to 

control weeds, insects, and other organisms in a wide variety of agricultural 

and nonagricultural settings. The use of 

pesticides has helped to make the United 

States the world’s largest producer of food 

(Barbash and Resek, 1996). Pesticide use, 

however, also has been accompanied by 

concerns about potential adverse effects 

on the environment and human health. 

A potential pathway for the transport of 

pesticides is through hydrologic systems, 

which supply water for both humans and 

natural ecosystems. Water is one of the 

primary ways pesticides are transported 

from an application area to other locations 

in the environment (fig. 1) (Barbash and 

Resek, 1996).

Pesticide contamination of ground 

water is a national issue because of the 

widespread use of pesticides, the expense 

and difficulty of remediating ground water, 

and the fact that ground water is used for 

drinking water by about 40 percent of the 

Nation’s population (Hutson and others, 

2004). Although application rates and the 

variety of pesticides used may be greater 

in urban areas, concern over their presence in ground water is especially acute in 

rural agricultural areas where more than 

95 percent of the population rely upon this 

resource for drinking water (Hutson and 

others, 2004).

Wyoming’s Pesticide Management 

Plan

The Ground-water and Pesticide Strategy Committee (GPSC) has developed 

the generic State Management Plan for 

Pesticides in Ground Water (SMP) for the 

State of Wyoming (Wyoming Groundwater and Pesticides Strategy Committee, 

1999). Wyoming was required by the 

U.S. Environmental Protection Agency to 

have an SMP in order for individuals and 

organizations to continue using certain 

pesticides in the State. The SMP includes 

information relating to individuals and 

organizations involved with implementation of the SMP, methods for preventing 

ground-water contamination, ground-water 

Printed on recycled paper
Herbicide

Table 1. Summary of pesticides and pesticide degradation products in ground-water samples from Niobrara and Weston Counties, 

September 2005 and April 2006.

[μg/L, micrograms per liter; E, value is estimated; --, no data; C, estimated value used in calculation; NA, not applicable]

Pesticide

Pesticide

trade name Pesticide action1

Number of

detections/

number of

samples2

Minimum

reporting level3

(μg/L)

Maximum

concentration

(μg/L)

Average

concentration

of detections

(μg/L)

Drinkingwater

standard4

(μg/L)

Focal pesticides detected in ground water from Niobrara and Weston Counties

Atrazine Aatrex Selective herbicide 2/22 0.007 E0.006 C0.006 3

Bromacil Hyvar XL Herbicide 3/22 .02 E.4 C.2 590

Clopyralid Stinger, Curtail Herbicide 2/22 .02 E.1 C.1 61,000

Metsulfuron Ally, Escort Herbicide 1/22 .03, .07 E.03 -- NA

Picloram Tordon Systemic herbicide 2/22 .03 3 2 500

Simazine Princep Selective herbicide 2/21 .005 E.007 C.006 4

Non-focal pesticides detected in ground water from Niobrara and Weston Counties

Diuron Farmco, Durashield Herbicide 2/22 0.02 0.05 C0.04 510

Oryzalin Surflan Selective herbicide 1/22 .01 .6 -- NA

Prometon Pramitol Non-selective herbicide 4/21 .01 .05 .03 5100

Focal pesticides not detected in ground water from Niobrara and Weston Counties

Alachlor, Aldicarb, Aldicarb Sulfone7, Aldicarb Sulfoxide7, Cyanazine, 2,4-D,

DCPA, Dicamba, Dichloropropene, Hexazinone, Metolachlor, Metribuzin, Tebuthiuron

Focal pesticides not analyzed in ground water from Niobrara and Weston Counties (no method of analysis available)

Difenzoquat

1

Meister (2002).

2

Each of the 11 wells was sampled twice. One well had only one analysis of 

simazine and prometon.

3

The minimum reporting level is the smallest concentration of a constituent that 

may be reliably reported by using a given analytical method. The minimum 

reporting level for metsulfuron was changed from 0.03 ug/L for late summer 

2005 samples to 0.07 ug/L for the spring 2006 samples by the USGS National 

Water Quality Laboratory.

4

U.S. Environmental Protection Agency Maximum Contaminant Level unless 

otherwise noted (U.S. Environmental Protection Agency, 2004).

5

U.S. Environmental Protection Agency Lifetime Health Advisory Level 

(U.S. Environmental Protection Agency, 2004).

6

Bob Benson, U.S. Environmental Protection Agency, written commun., 

December 30, 2003.

7

Degradation product of aldicarb.

monitoring, and the responses required if 

pesticides are detected in ground water.

One critical part of the SMP is groundwater monitoring. This ground-water 

monitoring program has two phases. 

The first phase, baseline monitoring, is 

designed to determine what pesticides, if 

any, have entered into a county’s ground 

water. The second phase, problem identification monitoring, is used to gather 

additional information about ground water 

near wells with samples having significant 

pesticide detections.

Baseline monitoring is prioritized by 

a county rank and the vulnerability of the 

county’s ground water to pesticide contamination. During the development of the 

SMP, the GPSC evaluated each county in 

Wyoming to determine the potential vulnerability of the county’s ground water to 

pesticide contamination. Each county was 

ranked according to the extent of cropland 

and urban areas in the county, as well as 

to the amount of pesticides sold within the 

county in 1991 (Wyoming Ground-water 

and Pesticides Strategy Committee, 1999).

A ground-water vulnerability map was 

prepared for the uppermost or shallowest 

aquifers (Hamerlinck and Arneson, 1998). 

A Geographic Information System was 

used to overlay digital information about 

the hydrogeology and land use. Ground 

water can be vulnerable to contaminants 

because of either inherent sensitivity of 

the hydrogeology (such as a shallow water 

table or highly permeable aquifer materials) or the combination of the sensitivity 

and associated land use. The map was 

used to select monitoring sites in each 

county. The baseline monitoring focuses 

on areas where the ground water is most 

vulnerable (fig. 2).

The GPSC selected 18 pesticides and 

2 degra­dation products as the focal pesticides to be analyzed for as part of the SMP 

(table 1). The analytical method used to 

detect the focal pesticides also can detect 

about 100 other pesticides and degradation products. Any additional pesticides 

that were detected are listed in table 1 

as non-focal pesticides. Water from all 

wells in the baseline monitoring program 

was analyzed for the pesticides listed in 

table 1, with the exception of difenzoquat, 

for which analytical methods are not available.

The goal of the ground-water sampling 

part of the SMP is to collect groundwater samples for pesticide analyses in all 

23 Wyoming counties. To date, sampling 

has been completed in these counties:

1995–1996: Goshen;

1997: Park;

1997–1998: Washakie;

1998–1999: Fremont, Lincoln, and 

Laramie;

1999–2000: Big Horn and Sheridan;

2000–2001: Platte, Johnson, and Crook;

2001–2002: Natrona, Sweetwater, and 

Teton;

2002–2003: Uinta;

2003–2004: Albany, Converse, and Hot 

Springs;

2004–2005: Carbon, Campbell, and 

Sublette;

2005–2006: Niobrara and Weston.

•

•

•

•

•

•

•

•

•

•

•
Figure 2. Vulnerability of ground water in Niobrara and Weston Counties, Wyoming, to 

pesticide contamination (from Hamerlinck and Arneson, 1998).

NW9

NW8

NW7

NW6

NW5

NW4

NW3

NW2

NW1

NW11

NW10

NW9

NW8

NW7

NW6

NW5

NW4

NW3

NW2

NW1

NW11

NW10

River

River Niobrara

Ch e e ey nn

Oil

Creek Creek

Beaver

Black

Lance

Thunder

Creek

Creek

Ol o d W

m an

Creek

Creek

Lodgepole

Newcastle

Upton

Lusk

SOUTH

DAKOTA

NEBRASKA

WYOMING

105°00' 104°45' 104°30' 104°15'

44°00'

43°45'

43°30'

43°15'

43°00'

42°45'

R. 67 W. R. 66 W. R. 65 W. R. 64 W. R. 63 W. R. 62 W. R. 61 W. R. 60 W.

R. 68 W.

T. 48 N.

T. 47 N.

T. 46 N.

T. 45 N.

T. 44 N.

T. 43 N.

T. 42 N.

T. 41 N.

T. 40 N.

T. 39 N.

T. 38 N.

T. 37 N.

T. 36 N.

T. 34 N.

T. 33 N.

T. 32 N.

T. 31 N.

15 MILES

0 5 10 15 KILOMETERS

0 5 10

270

270

270

272

450

116

585

85

85

18

18 20

20

85

16

16

85 Buffalo

Creek

Weston County

Niobrara County

Weston

County

Niobrara

County

EXPLANATION

Vulnerability to pesticide contamination

High

Medium high

Medium

Medium low

Low

Ground-water sampling site

 and number

WYOMING NW9

Base modified from Redistricting Census 2000 

TIGER/Line Files, U.S. Census Bureau, 2001

Public land survey system from Wyoming Water 

Resources Center digital data, 1994

Albers Equal-Area Conic projection

Standard parallels 29°30' and 45°30', 

central meridian 105°30' 

Ground-Water Monitoring in Niobrara and 

Weston Counties

Ground water in Niobrara County was ranked 

twenty-third and ground water in Weston County 

was ranked thirteenth most vulnerable to pesticide 

contamination in Wyoming (Wyoming Groundwater and Pesticide Strategy Committee, 1999). 

Because the land area in Niobrara and Weston 

Counties deemed vulnerable to ground-water 

pesticide contamination constituted a small fraction of the total land area, the GPSC combined the 

two counties into a single study. This facilitated 

location of suitable sampling sites, and was considered a reasonable compromise because of the 

similar geologic settings and land uses in the two 

counties. The vulnerability map by Hamerlinck 

and Arneson (1998) identifies unconsolidated 

Quaternary alluvial and terrace deposits in the 

counties and older consolidated units that underlie 

urban areas as the most vulnerable to pesticide 

contamination (shown as red or yellow on fig. 2).

Eleven wells were selected in Niobrara and 

Weston Counties (fig. 3) for baseline monitoring. 

All wells were located in areas that had high or 

medium high vulnerability to pesticide contamination (fig. 2). All wells were sampled twice, once 

in September 2005 and once in April 2006. Six of 

the 19 focal pesticides with available analyses and 

3 non-focal pesticides were detected in the wells 

sampled (table 1). Pesticides were detected in 6 

of the 11 wells sampled in Niobrara and Weston 

Counties (fig. 3); concentrations of each detected 

pesticide were equal to or less than 1/150 of the 

applicable drinking-water standard (U.S. Environmental Protection Agency, 2004) (table 1).

The most commonly detected pesticide (4 of 

21 samples) in Niobrara and Weston Counties 

was prometon. Prometon, the active ingredient 

in the general-use pesticide Pramitol, typically 

is detected in urban areas (Barbash and others, 

1999). Prometon was the most commonly detected 

pesticide in Albany, Campbell, Carbon, Converse, 

Crook, Johnson, Natrona, Sheridan, Sublette, 

Teton, and Uinta Counties.

Data Distribution and Availability

Sampling results have been provided to local 

groups interested in pesticides in ground water in 

Niobrara and Weston Counties. The information 

can be used by citizens and local governments to 

help understand current (2005–2006) conditions. 

Analytical results of sampling in Niobrara and 

Weston Counties can be found in Blajszczak and 

others (2006) and U.S. Geological Survey (2007). 

All water-quality data for Wyoming can be found 

at http://waterdata.usgs.gov/wy/nwis/qwdata. 

Analytical results and Fact Sheets for all counties 

are available from the U.S. Geological Survey, 
Figure 3. Location of wells sampled in Niobrara and Weston Counties, Wyoming, and 

notation of pesticide detection in samples from each well.

NW9

NW8

NW7

NW6

NW5

NW4

NW3

NW2

NW1

NW11

NW10

NW9

River

River Niobrara

Ch e e ey nn

Oil

Creek Creek

Beaver

Black

Lance

Thunder

Creek

Creek

Ol o d W

man

Creek

Creek

Lodgepole

Newcastle

Upton

Lusk

SOUTH

DAKOTA

NEBRASKA

WYOMING

105°00' 104°45' 104°30' 104°15'

44°00'

43°45'

43°30'

43°15'

43°00'

42°45'

R. 67 W. R. 66 W. R. 65 W. R. 64 W. R. 63 W. R. 62 W. R. 61 W. R. 60 W.

R. 68 W.

T. 48 N.

T. 47 N.

T. 46 N.

T. 45 N.

T. 44 N.

T. 43 N.

T. 42 N.

T. 41 N.

T. 40 N.

T. 39 N.

T. 38 N.

T. 37 N.

T. 36 N.

T. 34 N.

T. 33 N.

T. 32 N.

T. 31 N.

270

270

270

272

450

116

585

85

85

18

18 20

20

85

16

16

85 Buffalo

Creek

Quaternary alluvial and terrace deposits

EXPLANATION

No pesticide detected in samples

Pesticide detected in at least one sample

Sampling site number

Weston County

Niobrara County

15 MILES

0 5 10 15 KILOMETERS

Base modified from Redistricting Census 2000 0 5 10

TIGER/Line Files, U.S. Census Bureau, 2001

Public land survey system from Wyoming Water 

Resources Center digital data, 1994

Albers Equal-Area Conic projection

Standard parallels 29°30' and 45°30', 

central meridian 105°30' 

Wyoming Water Science Center by phone, 

email, or the Internet at http://wy.water.usgs.

gov/projects/pesticides/.

References

Barbash, J.E., and Resek, E.A., 1996, Pesticides in ground 

water—Distribution, trends, and governing factors: 

Chelsea, Mich., Ann Arbor Press, Pesticides in the 

Hydrologic System series, v. 2, 588 p.

Barbash, J.E., Thelin, G.P., Kolpin, D.W., and Gilliom, R.J., 

1999, Distribution of major herbicides in ground water 

of the United States: U.S. Geological Survey WaterResources Investigations Report 98–4245, 64 p.

Blajszczak, E.J., Mason, J.P., Watson, K.R., Roberts, S.C., 

and Miller, K.A., 2006, Water-resources data, Wyoming, 

water year 2005, Volume 2. Ground water: U.S. Geological Survey Water-Data Report WY–05–2, 173 p.

Hamerlinck, J.D., and Arneson, C.S., eds., 1998, Wyoming ground water vulnerability assessment handbook, 

Volume 2. Assessing ground water vulnerability to pesticides: University of Wyoming, Laramie, Spatial Data 

and Visualization Center Publication SDVC 98–01–2, 

variable pagination.

Hutson, S.S., Barber, N.L., Kenny, J.F., Linsey, K.S., 

Lumia, D.S., and Maupin, M.A., 2004, Estimated 

use of water in the United States in 2000: U.S. Geological Survey Circular 1268, 46 p.

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

Willoughby, Ohio, Meister Publishing Co., 

variable pagination.

U.S. Environmental Protection Agency, 2004, 2004 Edition 

of drinking water regulations and health advisories: EPA 

822–R–04–005, 12 p., accessed September 16, 2006, 

at http://www.epa.gov/WaterScience/criteria/drinking/

dwstandards.pdf

U.S. Geological Survey, 2007, Water-resources data for the 

United States, Water Year 2006: U.S. Geological Survey 

Water-Data Report WDR–US–2006, accessed April 1, 

2007, at http://pubs.water.usgs.gov/wdr2006

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, Wyoming, 103 p.

• Prepared by Cheryl A. Eddy-Miller

• Layout prepared by Suzanne C. Roberts

For more information, contact:

Roy Reichenbach

Wyoming Department of Agriculture

2219 Carey Avenue

Cheyenne, Wyoming 82002

(307) 777-6590

Kevin Frederick

Wyoming Department of Environmental Quality, 

Water Quality Division

Herschler Building West, 4th Floor

Cheyenne, Wyoming 82002

(307) 777-5985

Cheryl A. Eddy-Miller

U.S. Geological Survey

2617 E. Lincolnway, Suite B

Cheyenne, Wyoming 82001

(307) 775-9167

Email: cemiller@usgs.gov

This Fact Sheet was prepared by the U.S. Geological Survey (USGS), 

the Wyoming Department of Agriculture (WDA), and the Wyoming Department of Environmental Quality, Water Quality Division 

(WDEQ). All agencies are members of the Ground-water and Pesticide 

Strategy Committee.

This project has been funded in part with a Section 319 grant from the 

U.S. Environmental Protection Agency to 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.