Aquatic Risk Assessment: Organophosphate insecticide mixtures in Washington surface waters

water quality
Aquartic risk assessment of insecticide mixtures in WA streams

Jadey Ryan


November 9, 2021



📆 November 9, 2021 // 1:30 pm - 1:50 pm PT
🏨 Virtual
🌠 American Water Resources Association (AWRA)


Ecological risk assessments often do not consider potential additive, synergistic, or antagonistic effects from mixtures of chemicals and instead typically base risk on a single chemical. In the last decade, more tools and models have been developed to consider the interactive effects of chemicals within a mixture when conducting risk assessments. Therefore, this study uses actual environmental concentrations measured in 2018 and 2019 from the Washington State Department of Agriculture’s Surface Water Monitoring Program. Aquatic risk from exposure was assessed from chlorpyrifos, diazinon, and malathion (as individual chemicals and as binary and ternary mixtures) using the concentration addition model. These pesticides were selected because they have a common mechanism of toxicity, are frequently detected in surface waters in Washington, and were recently evaluated in a biological opinion by the National Marine Fisheries Service.

All detected concentrations of chlorpyrifos and malathion, assessed as individual chemicals, exceeded the predicted no effect concentration, indicating potential for adverse effects on aquatic life. Further, risk quotients for all binary and ternary mixtures were greater than one, also indicative of potential for adverse effects on aquatic life. In all samples containing a mixture, the maximum cumulative ratio suggested that a single insecticide contributed >50% of the overall toxicity of each mixture. Based on the individual and mixture risk quotients, chlorpyrifos and malathion were the primary drivers of the toxicity of each mixture.


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BibTeX citation:
  author = {Ryan, Jadey},
  title = {Aquatic {Risk} {Assessment:} {Organophosphate} Insecticide
    Mixtures in {Washington} Surface Waters},
  date = {2021-11-09},
  url = {},
  langid = {en}
For attribution, please cite this work as:
Ryan, Jadey. 2021. “Aquatic Risk Assessment: Organophosphate Insecticide Mixtures in Washington Surface Waters.” November 9, 2021.