Pesticide Concentrations Measured in Zooplankton Samples Collected from the Sacramento River and Yolo Bypass, California, 2017-2020

Zooplankton samples were collected at a site in the Yolo Bypass in Northern California and at a comparison site on the Sacramento River below the city of Sacramento and analyzed for a large suite of current-use pesticides and degradates. Samples were collected biweekly in the summer/fall 2017 and 2018, and from May 2019-March 2020. Samples were collected by towing a 150-micron conical plankton net for 5 minutes by boat at each site. In the laboratory, zooplankton samples were filtered through a 63-micrometer sieve to separate the zooplankton and vegetation/detritus from the water. Large sticks, twigs, rocks, and leaves were rinsed with organic free water into the sieve and then discarded. The remaining mass in the sieve was transferred into 50 milliliter plastic centrifuge tubes, frozen overnight at -20 degrees celsius, dehydrated completely using a freeze dryer, and then stored at -20 degrees celsius. Immediately prior to analysis, each freeze-dried sample was homogenized and subsampled to a targeted final weight of 0.5 to 1.0 grams. Each subsample was further homogenized with sodium sulfate and extracted with dichloromethane at 100 degrees celsius and 1500 PSI using an ASE 200 solvent extraction system. Extracts were cleaned up using activated carbon, evaporated to 200 microliters in ethyl acetate, internal standards added, and analyzed by gas chromatography-tandem mass spectrometry for 86 pesticides. A total of 29 pesticides were detected in the zooplankton samples, and 70 of the 73 samples collected contained multiple pesticides (up to 17 per sample). The most frequently detected compounds in zooplankton were 3,4-dichloroanline, azoxystrobin, bifenthrin, p,p?-DDD, p,p?-DDE, pendimethalin, and thiobencarb.