Agricultural Pesticide Applications and Observed Concentrations in Surface Waters from Four Drainage Basins in the Central Columbia Plateau, Washington and Idaho, 1993-94

As part of the U.S. Geological Survey's National Water-Quality Assessment Program, the use and occurrence of agricultural pesticides were investigated in four drainage basins two dominated by irrigated agriculture and two by dryland agriculture-in the Central Columbia Plateau of eastern Washington. For this study, 85 pesticides or pesticide metabolites were selected for analysis from a list of nearly 400 compounds commonly used in the United States. Pesticide-use data included estimates of the total quantity of herbicides, insecticides, and fungicides applied to croplands in each of the four drainage basins and reported times of application for selected pesticides. Pesticide-occurrence data included concentrations of pesticides in samples collected at one surface-water site at or near the outflow of each of the four drainage basins, where surface waters were sampled one to five times a month from March 1993 through May 1994. Of the 85 pesticides or pesticide metabolites targeted for analysis, a total of 45 different compounds were detected in samples from the four sites, ranging in concentration from at or near the limit of detection (as low as 0.001 microgram per liter) to a maximum of 8.1 micrograms per liter. None of the concentrations of pesticides exceeded the U.S. Environmental Protection Agency (USEPA) drinking water standards, but concentrations of five pesticides exceeded the USEPA freshwater-chronic criteria for the protection of aquatic life. Fourty-one different pesticides or pesticide metabolites were detected in surface waters sampled at the two sites representing irrigated agriculture drainage basins. The herbicides atrazine, DCPA, and EPTC were detected most frequently at the two sampling sites. Not all pesticides that were applied were detected, however. For example, disulfoton, phorate, and methyl parathion accounted for 15 percent of the insecticides applied in the two irrigated drainage basins, yet none of these pesticides were detected in samples from the two irrigated-agricultural sites. Concentrations of pesticides found in surface waters at the two sites representing irrigated agriculture did not exceed drinking water standards, but some concentrations of the insecticides carbaryl, chlorpyrifos, diazinon, and azinphos-methyl exceeded the freshwaterchronic criteria for the protection of aquatic life. Twenty-three different pesticides or pesticide metabolites were detected in samples from the two sites representing dryland agricultural drainage basins. Herbicides were the type of pesticides most heavily applied in these drainage basins, and the herbicides atrazine, triallate, and simazine were detected most frequently in samples. Some herbicides, for example triallate in the Palouse River drainage basin, were both heavily applied and frequently detected. Others, like atrazine and simazine, were not typically applied to cropland, but were frequently detected in surface-water samples. Several insecticides (Lindane, ethoprop, carbaryl, and azinphos-methyl) were detected in samples from the two sites, although they were not reported as commonly applied to croplands in the dryland agricultural drainage basins. Concentrations of pesticides found in surface waters at the two sites did not exceed drinking water guidelines, but concentrations of the insecticides diazinon and azinphos-methyl and the herbicide triallate exceeded freshwater-chronic criteria for the protection of aquatic life. INTRODUCTION Purpose and Scope The Central Columbia Plateau study unit is 1 of 60 study units being investigated by the National WaterQuality Assessment (NAWQA) Program of the U.S. Geological Survey (Hirsch and others, 1988; Leahy and others, 1990). The goals of NAWQA are to describe the status and trends in the quality of the Nation's groundand surface-water resources and to gain a better understanding of the natural and human factors that affect the quality of water resources. The 60 study units, which are distributed throughout the Nation, contribute to the overall goals of NAWQA (Gilliom and others, 1995) by providing waterquality information that is relevant to the study unit and that can be used in combination with information from other study units to assess water quality at regional and National scales. Because agriculture is the dominant land use in the Central Columbia Plateau study unit, the investigation focused on examining the relation between agricultural land uses and water quality. One aspect of that relation is the question of how pesticide use affects surfaceand ground-water resources. The presence of pesticides in water is a concern if concentrations exceed drinking-water standards or are at levels that may adversely affect aquatic life. Previous studies (Washington State Department of Social and Health Services, 1975,1976, 1977, 1978, 1979; Davis, 1993) have established that pesticides are sometimes present in surface water of the Central Columbia Plateau, but not enough sampling has been done recently to determine if their presence is typical and at what concentrations they occur. To gain more information about the presence of agricultural pesticides in surface waters, four sites, representing different agricultural land uses in the study unit, were sampled for pesticides one to five times a month for a period of 15 months. In studies of this type, the absence of readilyavailable information on pesticide use is an obstacle to understanding why some pesticides are found in hydrologic systems and others are not. For a pesticide to be present in water there must be a source of the pesticide; however, the presence of a source does not necessarily mean that the pesticide will be transported from the location of its application to a water body: a pesticide may volatilize, metabolize, or otherwise degrade before it reaches the water body. Pesticide-use data are essential to understanding how other factors, like degradation, affect the fate of a pesticide after it is applied. Therefore, obtaining data to determine pesticide use in the drainage basins of the sampled sites was another key element of this study. The purposes of this report are to (1) summarize concentrations of agricultural pesticides in surface water at four sites that were sampled one to five times a month from March 1993 through May 1994; (2) show graphically the relation between the concentrations of selected pesticides, streamflows, and the application periods of the pesticides; (3) present data on the quantities of pesticides used in the drainage basins of the sampled sites; and (4) document the methods used to collect and analyze the samples and the methods used to compile the pesticide-use data. Two of the four sampled sites were located in irrigated agricultural areas and two were located in dryland farming areas. Samples collected at the four sites were analyzed for about 85 pesticides, or target analytes, selected from a list of nearly 400 of the pesticides most commonly used in the United States. Pesticide concentration data and estimates of quantities of pesticides applied in the drainage basins of the four sites also are summarized in tables. Quality-control data including field-matrix and laboratory-reagent-spike results, field and equipment blanks, and replicate samples are summarized. Although some observations pertaining to the data are provided, this report contains no detailed analyses of the data. Description of the Study Unit The Central Columbia Plateau study unit is located in east-central Washington and northwestern Idaho (fig. 1). The approximately 13,000 square-mile area is bordered on the north by the Columbia River and the topographic divide in the headwaters of Crab Creek, on the east by the topographic divide in the headwaters of the Palouse River, on the south by the Snake River, and on the west by the Columbia River. The study unit has numerous land forms, including low-altitude mountains and rolling hills on the eastern side and a wide range of high-desert land forms throughout the western part of the study unit. The altitude of the land surface ranges from less than 300 feet above sea level near Pasco to nearly 5,000 feet above sea level in the mountains in the headwaters of the Palouse River. For a more complete description of the study unit, refer to Greene and others (1994). Four surface-water sites in the Central Columbia Plateau two sites whose contributing drainage basins are representative of irrigated agricultural land use and two representative of dryland agriculturewere selected for investigation of use and occurrence of pesticides.