Pesticide Fate Research Group

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FIELD STUDIES: Yolo Bypass (2004-2005)

Map of Field Study Area

Building on Yolo Bypass

Inputs to the Yolo Bypass are potential sources of pesticides that could impact critical life stages of native fish. To assess the direct inputs during inundation, pesticide concentrations were analyzed in water, in suspended and bed-sediment samples collected from six source watersheds to the Yolo Bypass, and from three sites within the Bypass in 2004 and 2005. To understand inputs from direct application on fields, pesticides were also measured in soils from several sites within the Bypass. Water samples were collected from the six input sites to the Bypass during the first flood event of the year representing pesticide inputs during high-flow events. Samples were also collected along a transect across the Bypass and from three sites within the Bypass under low-flow conditions. To assess loads of pesticides to the Bypass associated with suspended sediments, large-volume water samples were collected during high flows events from three sites, while bed sediments and soils were collected during the dry season.

Thirteen current-use pesticides were detected in surface water samples collected during the study. The highest pesticide concentrations detected at the input sites to the Bypass corresponded to the first high-flow event of the year. The highest pesticide concentrations at the two sites sampled within the Bypass during the early spring were detected in mid-April following a major flood event as the water began to subside. The pesticides detected and their concentrations in the surface waters varied by site; however, hexazinone and simazine were detected at all sites and at some of the highest concentrations.

Thirteen current-use pesticides and three organochlorine insecticides were detected in bed and suspended sediments collected during the study. The pesticides detected and their concentrations varied by site and sediment sample type. Trifluralin, DDE, and DDT were highest in the bed sediments, whereas oxyfluorfen and thiobencarb were highest in the suspended sediments. With the exception of the three organochlorine insecticides, suspended sediments had higher pesticide concentrations compared with bed sediments, indicating the potential for pesticide transport throughout the Bypass, especially during high-flow events. Soils samples were dominated by DDT and its degradates but also contained a variety of current-use pesticides typically at lower concentrations. The types of pesticides detected in water and sediments were correlated with agricultural applications in each watershed. Understanding the distribution of pesticides between the water and sediment is needed to assess fate and transport within the Bypass and to evaluate the potential effects on native fish.

Smalling, K.L., Orlando, J.L., and Kuivila, K.M., 2005, Analysis of Pesticides in Surface Water and Sediment from Yolo Bypass, California, 2004–2005: U.S. Geological Survey Scientific Investigations Report 2005-5220, 20 p.

Smalling, K.L., Orlando, J.L., and Kuivila, K.M., 2007, Occurrence of Pesticides in Water, Sediment, and Soil from the Yolo Bypass, California, San Francisco Estuary and Watershed Science. Vol. 5, Issue 1 (February 2007). Article 2. (online)

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