The occurrence, distribution, and transport of pesticides in surface water of the Yakima River Basin were assessed using data collected during 1999­2000 as part of the U.S. Geological Survey National Water-Quality Assessment (NAWQA) Program. Samples were collected at 34 sites located throughout the basin in August 1999 using a Lagrangian sampling design. Samples also were collected weekly and monthly from May 1999 through January 2000 at three of the sites. This report includes data for 47 pesticide compounds from the analysis of filtered water using ocadecyl (C-18) solid-phase extraction and gas chromatography/mass spectrometry.

Twenty-five pesticide compounds were detected in samples collected during the study. Detection frequencies ranged from about 1 percent for ethalfluralin, ethoprophos, and lindane to 82 percent for atrazine. Maximum concentrations of azinphos-methyl, carbaryl, diazinon, para,para'-dichlorodiphenyldichloroethylene (p,p'-DDE), and lindane exceeded chronic-toxicity guidelines for the protection of freshwater aquatic life. Twenty pesticide compounds were detected during sampling in August 1999. Atrazine was the most widely detected herbicide, and azinphos-methyl was the most widely detected insecticide. The median number of sites at which a particular pesticide compound was detected was six. Pesticide compounds detected at more than six sites include atrazine, simazine, terbacil, trifluralin, deethylatrazine, azinphos-methyl, carbaryl, diazinon, malathion, and p,p'-DDE.

Because many factors affect the transport of pesticides from areas of application to surface water, there was not a simple correspondence between pesticide occurrence and use in the Yakima River Basin. For example, the high detection rates of atrazine, simazine, deethylatrazine, and p,p'-DDE are probably related more to their mobility and wide distribution in the hydrologic system than to their usage. Likewise, higher detection frequencies of the insecticides azinphos-methyl and carbaryl compared with chlorpyrifos appear to be related more to differences in their physical and chemical properties than to usage.

The highest detection frequencies and concentrations of pesticides generally occurred during irrigation season, which is from mid-March to mid-October. Pesticides are applied during irrigation season, and runoff of excess irrigation water from fields transports them to surface water.

Ground-water discharges also transport some pesticides to surface water. Atrazine, deethylatrazine, and simazine were frequently detected in samples collected after the irrigation season when there was little or no surface runoff and most of the flow in irrigation drains was derived from ground water.

Daily loads of atrazine, terbacil, azinphos-methyl, and carbaryl discharged to the Yakima River from inflows between river mile 103.7 and river mile 72 varied widely between sites. For example, East Toppenish Drain discharged over 50 percent of the total load of terbacil to this reach of the Yakima River, but none of the total load of carbaryl and only about 4 percent of the total load of atrazine. Pesticide loads from the wastewater treatment plants were relatively small compared with loads from other inflows because their discharges were small.

Pesticide losses, defined as the ratio of the amount discharged from a basin from May 1999 through January 2000 divided by the amount applied during 1999, were estimated for Moxee and Granger Drains and the Yakima River at Kiona. Losses ranged from less than 0.01 to 1.5 percent of pesticides applied and are comparable to those observed (0.01 to 2.2 percent) in irrigated agricultural basins in the Central Columbia Plateau of Washington State.