In 1991, members of local, State, and Federal governments, as well as industry and interest groups, formed the Ground-water and Pesticide Strategy Committee to prepare the State of Wyoming's generic Management Plan for Pesticides in Ground Water (SMP). 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 implementation of the SMP by sampling wells in Goshen County, Wyoming. In 1997, baseline monitoring began in Park County.
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 largest
producer of food in the world and has provided other
benefits, but the use has also been accompanied by
concerns about their 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).
Pesticide contamination of ground water is a national issue because ground water is used for drinking water by about 50 percent of the Nation's population. Concern about pesticides in ground water is especially acute in rural agricultural areas where over 95 percent of the population relies upon ground water for drinking water.
Figure 1. Pathways of pesticide movement in the hydrologic cycle (modified from U.S. Geological Survey Fact Sheet FS 244-95).
In 1991, the Ground-water and Pesticide Strategy Committee (GPSC) began developing the generic State Management Plan for Pesticides in Ground Water for the State of Wyoming. The SMP will be required by the U.S. Environmental Protection Agency (EPA) in order for individuals to continue using certain pesticides in Wyoming. The SMP includes information relating to agencies and individuals involved with the implementation of the SMP, ground-water monitoring, methods of preventing ground-water contamination, and what the responses will be to detections of pesticides in ground water.
One critical part of the SMP is ground-water monitoring.
The ground-water sampling program has two phases. The first
phase involves baseline monitoring, which is an initial
survey of the pesticides detected in a county's ground
water. The second phase is problem identification
monitoring, which is used to gather more information about
the ground water near wells with significant pesticide
Baseline monitoring is directed by a county rank and the vulnerability of the ground water to pesticides. 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 pesticides. Each county was ranked based on the extent of cropland and urban areas in the county, as well as the amount of pesticides sold within the county in 1991.
A ground-water vulnerability map is prepared for the uppermost or shallowest aquifer. The map, a summation of seven maps describing the hydrogeology and land use, is used to assist in the selection of monitoring sites in each county. The monitoring focuses on areas where the ground water is most vulnerable.
The GPSC selected 18 pesticides of focus and 2 degradation products to be sampled as part of the SMP (table 1). An additional 66 pesticides and degradation products are included in USGS pesticide analyses, leading to possible detections of non-focal pesticides. Ground water from all wells in the baseline monitoring program was analyzed for the pesticides listed in table 1, with the exception of Difenzoquat and Metsulfuron.
Table 1. Baseline
monitoring for pesticides in Park County, 1997-98.
The ground-water sampling part of the SMP began in Goshen County in 1995. The goal of the sampling program is to collect ground-water samples for pesticide analyses in all 23 Wyoming counties.
vulnerability of ground water in Park County was
ranked second in Wyoming and sampling began in April
1997. The vulnerability map for Park County (fig. 2)
was created by the University of Wyoming Spatial Data
and Visualization Center. The alluvial and terrace
aquifers, usually located in stream valleys and on
benches, were identified as the most vulnerable to
pesticides in Park County.
Twenty-seven sites were selected for baseline monitoring in Park County. All wells were selected in the two most vulnerable rankings (areas in red and yellow). The wells were inspected and selected with the assistance of the three Park County Conservation Districts. All sites were sampled twice, either during the spring and late summer of 1997, or the late summer of 1997 and spring of 1998. These time periods were selected to correspond with deepest and shallowest water-table conditions.
(Click on image for a larger version, 167k jpg)
Figure 2. Vulnerability of Park County ground water to pesticide contamination (Wyoming Water Research Center, written commun., 1998).
Six of the 18 pesticides of focus and the two degradation products were detected in Park County (table 1). All levels of pesticides detected were less than two-thirds of the drinking water standard or its equivalent established by the EPA for domestic supplies (U.S. Environmental Protection Agency, 1996). Although these standards do not apply to private domestic wells, they do provide a reference to the acceptable amount of chemicals in drinking water.
|At least one
pesticide was detected in 21 of the 27 wells sampled
in the county (fig. 3). The pesticide with the highest
concentration was Aldicarb sulfoxide, a degradation
product of Aldicarb. More than 75 percent of the
detections were trace quantities. A trace quantity
indicates the pesticide was detected, but at a level
too small to quantify. Atrazine was the most commonly
detected pesticide, detected in 42 samples from 21 of
the 27 wells.
(Click on image for a larger version, 142k jpg)
Figure 3. Location
of wells sampled in Park County, and notation of
Farm Chemicals Handbook and Dictionary, 1996:
Willoughby, Ohio, Meister Publishing Co., variable
Wyoming Department of Agriculture
2219 Carey Avenue
Cheyenne, Wyoming 82002
Wyoming Department of Environmental Quality, Water Quality Division
4th Floor, Herschler Building
Cheyenne, Wyoming 82001
U.S. Geological Survey, WRD
2617 E. Lincolnway, Suite B
Cheyenne, Wyoming 82001
This document 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.
Prepared by: Cheryl A. Eddy-Miller
Layout by: Suzanne C. Roberts
The use of trade, product, industry, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
This project has been funded in part with a Section 319 grant from the U.S. Environmental Protection Agency to Wyo. Dept. of Environmental Quality's Non-Point Source Program.
This fact sheet is also available in pdf format: fs09798.pdf (1.26mb)