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Scientific Investigations Report 2007–5180

Scientific Investigations Report 2007–5180

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Summary and Conclusions

The effort to maximize production and protect agricultural commodities leads to high pesticide usage throughout the Yakima River Basin. Despite the massive amounts applied, instream concentrations of most chemicals are small compared to other intensely cultivated areas of the United States, for example the Midwest Corn Belt. Heat and aridity in the Yakima River Basin makes irrigation a necessity, and routine operation of the region’s irrigation system results in large amounts of relatively uncontaminated water being returned to drains and streams during the summer. This is not to imply that the system is wasteful (most farms and irrigation districts are conscientious in their water use), but rather that the volume of water being applied to farmland throughout the basin provides a large amount of dilution relative to the total mass of pesticides applied in the region.

Forty-seven pesticides and 14 pesticide degradates were detected in the streams, drains, and canals sampled during this study. The herbicide 2,4-D was the most frequently detected pesticide in this study, occurring in 82 percent of the samples collected in July 2000 and in 3 percent of the samples collected in October 2000 (based on concentration values screened at 0.02 µg/L). In the Yakima River Basin, it is used on a variety of crops, to control weeds along rights-of-way, and is an ingredient in several products sold for use by homeowners. In 2000, it was the 6th most heavily applied pesticide (by mass of active ingredient) to agricultural land and rights of way. The second most frequently detected pesticide was azinphos-methyl (based on concentration values screened at 0.02 µg/L). Like 2,4-D, azinphos-methyl is widely used in the Yakima River Basin, and was the most heavily applied pesticide in 2000. Despite greater use, the detection frequency of azinphos-methyl was less than half that of 2,4‑D. The disparity illustrates the importance of understanding the myriad factors that affect pesticide occurrence in the aquatic environment, including soil-half life, time of use, Koc, irrigation method, proximity of place of use to flowing water, and numerous others.

In this study, irrigation method and Koc were identified as the best predictors of the mass of pesticide in a stream or drain relative to the amount of pesticide applied in the catchment it drains. Pesticide loss was defined as the ratio of instream mass to applied mass. Among pesticides with large Koc values, pesticide loss was significantly correlated with irrigation method – large values of pesticide loss were associated with a large percentage of the agricultural land in the catchment using rill-irrigation. Properly functioning sprinkler and drip irrigation systems virtually eliminate irrigation-related soil erosion, and therefore, the mobilization of sediment-bound pesticides. Among pesticides with small Koc values, there was no significant correlation between pesticide loss and irrigation method, possibly due to the timing of sampling or the transport of pesticides through the ground water system.

Samples of waters collected during base flow conditions in October provide evidence of widespread, low-level contamination of shallow ground water with pesticides and pesticide degradates. Most of the samples collected in October contained at least one pesticide, and most compounds detected were herbicides or their degradates. Concentrations in October tended to be lower than concentrations in July. In general, the pesticides detected in October had small Koc values, consistent with interpretations of the pesticide loss relations. More work is needed to understand the extent to which the shallow ground water is contaminated with pesticides and the processes controlling the transport and attenuation of these chemicals.

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