ABSTRACT

Water samples from 58 rivers and streams across the United States were analyzed for pesticides as part of the National Water-Quality Assessment Program of the U.S. Geological Survey. The sampling sites represent 37 diverse agricultural basins, 11 urban basins, and 10 basins with mixed land use. Forty-six pesticides and pesticide degradation products were analyzed in approximately 2,200 samples collected from 1992 to 1995. The target compounds account for approximately 70 percent of national agricultural use in terms of the mass of pesticides applied annually.

All the target compounds were detected in one or more samples. Herbicides generally were detected more frequently and at higher concentrations than insecticides. Nationally, 11 herbicides, 1 herbicide degradation product, and 3 insecticides were detected in more than 10 percent of samples. The number of target compounds detected at each site ranged from 7 to 37. The herbicides atrazine, metolachlor, prometon, and simazine were detected most frequently; among the insecticides, carbaryl, chlorpyrifos, and diazinon were detected the most frequently. Distinct differences in pesticide occurrence were observed in streams draining the various agricultural settings. Relatively high levels of several herbicides occurred as seasonal pulses in corn-growing areas. Several insecticides were frequently detected in areas where the dominant crops consist of orchards and vegetables. The number of pesticides detected and their concentrations were lower in wheat-growing areas than in most other agricultural areas. In most urban areas, the herbicides prometon and simazine and the insecticides carbaryl, chlorpyrifos, diazinon, and malathion were commonly detected. Concentrations of pesticides rarely exceeded standards and criteria established for drinking water, but some pesticides commonly exceeded criteria established for the protection of aquatic life.

INTRODUCTION

The National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey (USGS) is designed to assess the status of and trends in the quality of the nation's surface- and ground-water resources and to link the status and trends with an understanding of the natural and human factors that affect the quality of water (Hirsch and others, 1988; Leahy and others, 1990; Gilliom and others, 1995). The study design balances the unique assessment requirements of individual hydrologic systems with a nationally consistent design structure that incorporates a multiscale, interdisciplinary approach. The building blocks of the national assessment are investigations in major hydrologic basins of the nation, referred to as study units (fig. 1). The goal for the first phase of investigation in each study unit is to characterize, in a nationally consistent manner, the broad-scale geographic and seasonal distributions of water-quality conditions in relation to major contaminant sources and background conditions.

The NAWQA study units include about 40 percent of the land area of the conterminous United States, encompass 60 to 70 percent of national water use, and include diverse hydrologic systems that differ widely in the natural and human factors that affect water quality. The study units are divided into three groups that are studied on a rotational schedule of 3-year periods of intensive data collection. About one-third of the study units are in the intensive data-collection phase at any given time, and the 9-year cycle is repeated perennially. The first complete cycle of intensive data collection in the study units began during 1992-93 and is scheduled to be completed in 2002.

The national assessment goals of NAWQA will be accomplished primarily in two ways. First, NAWQA will accumulate data from consistent and comparable perennial water-quality assessments for the most significant hydrologic systems of the nation; this will be stand alone as a major contribution to our knowledge of regional and national water-quality conditions. Second, NAWQA's National Synthesis Project will build on and expand the findings of individual study units by combining and interpreting the results from multiple study units and from historical information from the USGS and other agencies and researchers. National Synthesis analyses will produce regional and national assessments for priority water-quality issues.

The goal of Pesticide National Synthesis is to assess the extent and the nature of pesticide contamination of the nation's surface and ground water. The aggregated data from all study units will be analyzed to provide a national overview of the occurrence, distribution, and significance of pesticides in surface and ground water across a broad range of geographic, climatic, and land-use conditions.

This report is a national overview of the occurrence and distribution of pesticides in rivers and streams (hereafter referred to only as streams) in 19 of the 20 NAWQA study units that were intensively studied during 1991-95 (data from the Rio Grande study unit were not yet available). The assessment is based on analysis of 46 pesticide compounds in approximately 2,200 water samples from 58 streams sampled during 1992-95. All the data used for this report are available on the World Wide Web (U.S. Geological Survey, 1999). Specific objectives of this analysis are to

• Determine the identity and concentrations of pesticides detected in streams.
• Evaluate seasonal patterns of pesticide occurrence in streams.
• Compare pesticide occurrence in streams draining basins with differing land uses, including comparisons among basins with various agricultural crops and between agricultural and urban basins.
• Compare the transport of pesticides to streams among basins of differing land uses.
• Compare observed concentrations with standards and criteria established for the protection of the health of humans and aquatic biota.

Results from NAWQA investigations of pesticides in bed sediments of rivers and streams and in tissues of aquatic organisms currently are being analyzed; the initial results from NAWQA investigations of ground water are summarized in Kolpin and others (1998). Results from NAWQA pesticide studies during the past several decades have been summarized by Majewski and Capel (1995) for pesticides in the atmosphere, Barbash and Resek (1996) for ground water, and Larson and others (1997) for surface waters.

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