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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5180

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Study Design

The sampling network was designed to test two hypotheses about the movement of pesticides in the Yakima River Basin. First, that the use of sprinkler and drip irrigation reduces the transport of agricultural chemicals. Second, that there is a pattern in the occurrence of pesticides in streams and drains in the Yakima River Basin, and that pattern is a function of the type and timing of chemicals applied in the catchment, the physical properties of the chemicals, and the physical properties of the catchment.

The sampling network consists of 44 sites on streams or drains issuing from agricultural catchments, 2 sites on streams issuing from catchments with no canal water deliveries and no agricultural activities (except rangeland grazing), and 11 sites on irrigation-water delivery canals (table 1, pl. 1). All sites were sampled two times in 2000—once during the height of the irrigation season (July 10–July 20, hereinafter referred to as “July”) and once shortly after the end of the irrigation season (October 30–November 2, hereinafter referred to as “October”). The July sampling was timed to assess water-quality conditions during the months when chemical use, water application, and runoff are highest. The October sampling was timed to assess water-quality conditions in the drains and streams when they are fed entirely by ground-water discharge and chemical use is low.

Twenty-eight of the 44 agricultural catchments contained a single dominant irrigation method—either rill or a mixture of sprinkler and drip (table 2). The remaining 16 agricultural catchments contained a mix of irrigation methods. Catchments ranged in size from 3 to 112,264 ha, however, the agricultural area¹ of the catchments was much smaller—ranging from 3 to 8,168 ha. The difference between total catchment area and agricultural area was greatest in large catchments, and was due to the presence of rangeland and forests upslope from the agricultural activities. The 11 canal sites were selected to approximate the quality of the water being delivered to the agricultural catchments in which samples were collected.

¹ For the purposes of this study, the agricultural area was defined as the area influenced by agricultural activities such as farming, dairies, and other infrastructure. It may include canals, roads, farmhouses and other noncrop areas.

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