Pesticide Occurrence in California – Yolo Bypass Pesticide Analyses

Science Center Objects

The project is part of an overarching Interagency Ecological Program (IEP)/Department of Water Resources (DWR) study that is focused on understanding the processes by which the Yolo Bypass may provide a fall food web supply for the Cache Slough Complex and downstream regions of the Sacramento/San Joaquin Delta.  Several studies conducted since 2011 have shown that phytoplankton blooms can be initiated in the Yolo Bypass/Toe Drain during seasonal flow pulses. During the fall of 2015 and summer of 2016 flow pulses were intentionally conducted in the Yolo Bypass/Toe Drain to stimulate phytoplankton blooms.

Map showing sampling site locations in the Yolo Bypass

Map showing sampling site locations in the Yolo Bypass


Current-use pesticides are known to be present in urban runoff and agricultural return flows entering the Yolo Bypass; therefore, their occurrence in pulse flows and their potential effects on the Yolo Bypass/Cache Slough ecosystem are of concern.  As part of a separate project funded by the State and Federal Contractors Water Agency (SFCWA), the USGS Pesticide Fate Research Group (PFRG) analyzed water samples collected before, during, and after the 2015 and 2016 pulse flows, for current-use pesticide levels. Results from these analyses (which are available in NWIS) showed that mixtures of multiple pesticides were present in pulse flow waters. Considering these results, DWR contracted with the USGS for continued analysis of water samples collected during pulse flow events in 2017 and 2018.


Results of this study will provide the Department of Water Resources with critical information regarding the occurrence of current-use pesticides in managed pulse flows in the Yolo Bypass. This information may be used to optimize the timing and source waters used for future pulse flow events.

Science Plan

Water samples were collected by DWR staff from five sites within the Yolo Bypass during the summer/fall of 2017 and 2018. Samples were collected before, during, and after flow pulses in the Bypass Toe Drain and Cache Slough. Samples were collected following Surface Water Ambient Monitoring (SWAMP) program protocols and were delivered to the USGS PRFG Organic Chemistry Research Laboratory (OCRL) within 24 hours of collection for analysis of a suite of 175 current-use pesticides in water and 140 current-use pesticides in suspended sediments. A total of 67 water samples (55 environmental and 12 quality control) were collected each year.

Results of all pesticide analyses will be entered into the USGS National Water Information System (NWIS) database and made publicly available. Results of all pesticide analyses will also be provided to DWR at the end of the project using California Environmental Data Exchange Network (CEDEN) compatible templates provided by DWR. Pesticide analytical results will be entered CEDEN by DWR.

A non-interpretive USGS data series report detailing the results of all environmental and QC pesticide analyses will be prepared for DWR.

As described in the 2016-12 Umbrella Proposal: data analyzed as part of this project will contribute to our overall knowledge of pesticide fate and transport in California; these data can lead to additional projects or be included in a future synthesis report.