8277

• UCREFRP Catalog Number: 8277
• Author: Bauch, N. J. and N. E. Spahr.
• Title: Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996-98.
• USFW Year: 2000.
• USFW - Doc Type: Water-Resources Investigations Report 00-4005,
• Copyright Material: NO

Pesticides in Surface Waters of the Upper
Colorado River Basin, Colorado, 1996-98
By Nancy J. Bauch and Norman E. Spahr
Abstract
Forty-four river, stream, and drain sites in
the Upper Colorado River Basin in Colorado were
sampled during 1996-98 to determine the occur-
rence and distribution of pesticides in the basin. In
afixed-station study, 57 surface-water samples
were collected from October 1996 through
January 1998 at four sites. Each site was sampled
approximately monthly for up to a year, with
more frequent sampling during the spring and
summer growing season. In a synoptic study,
surface-water samples were collected at 43 sites
in the agricultural areas of the Grand Valley and
the Uncompahgre River Valley in May 1998.
Each site was sampled once.
Pesticide concentrations generally were low
and varied seasonally and across land uses.
Thirty-five pesticides were detected at least once
in the two studies, and 11 pesticides accounted for
almost 82 percent of the detections. Herbicides
were more commonly detected than insecticides,
and only the herbicides alachlor and atrazine were
detected in more than 50 percent of the samples.
Carbofuran was the most commonly detected
insecticide and was detected in 19 percent of the
samples. Pesticide detections increased and were
measured at higher concentrations in the summer
months and at the agriculture sites. All pesticide
concentrations were less than drinking-water
standards, and most complied with human-health
advisories and criteria for the protection of fresh-
water aquatic life. Pesticides in the Upper Colo-
rado River Basin in Colorado were detected
slightly less frequently and generally at lower
concentrations than in 20 National Water-Quality
Assessment Program study-unit basins that
collected water-quality data from 1992 through
1996. Results from surface-water sampling
conducted during 1996-98 in the Upper Colorado
River Basin in Colorado indicate that beneficial
uses of water were not being impaired by the
presence of pesticides in surface waters in the
basin.
INTRODUCTION
In 1991, the U.S. Geological Survey (USGS)
began full implementation of the National Water-
Quality Assessment (NAWQA) Program. The goals
of the NAWQA Program are to (1) describe current
water-quality conditions for a large part of the
Nation's freshwater streams, rivers, and aquifers;
(2) describe how water quality is changing over time;
and (3) improve understanding of the primary natural
and human factors that affect water-quality conditions
(Leahy and others, 1990). Many of the Nation's most
important river basins and aquifers, represented as
study units, are being investigated through the
NAWQA Program. The Upper Colorado River Basin
(UCOL) study unit (fig. 1), located primarily in
western Colorado, is one of the NAWQA study units.
As part of the NAWQA Program, investigations of the
hydrology and water quality of the UCOL began in
1994. The occurrence and distribution of pesticides in
surface water are two of the main topics of interest in
the NAWQA Program. Issues of concern include the
types and concentrations of pesticides in the Nation's
rivers and streams, seasonal distribution of pesticides,
relation of pesticide occurrences and concentrations to
land use, and potential of pesticide concentrations to
Abstract