Derivation of species sensitivity distributions and ecotoxicity threshold values for 66 pesticide active ingredients and the hazard and risk they pose to freshwater waterways that discharge to the Great Barrier Reef, Australia.

Pesticide active ingredients (PAIs) are one of the main contributors affecting water quality in the Great Barrier Reef Catchment Area (GBRCA). While an extensive list of pesticides is monitored in the GBRCA, only a limited number have water quality guideline values (WQGs), meaning it is not possible to know whether these PAIs are present at concentrations that may pose a hazard to the aquatic environment. In the current study, we derived 66 ecotoxicity threshold values (ETVs) for PAIs, the equivalent of WQGs, with a focus on PAIs applied to sugar cane. The hazard posed by PAIs monitored as part of the Great Barrier Reef Catchment Loads Monitoring Program (GBRCLMP) was assessed by comparing the derived ETVs with monitoring data from 2016/2017 to 2021/2022. The derived ETVs included herbicides, insecticides and fungicides, with the values that should protect 99 or 95 % of aquatic species (PC99 or PC95) spanning nine orders of magnitude. The concentrations of 10 PAIs exceeded their respective ETVs, giving a hazard quotient (HQ) >1. Of particular concern were insecticides chlorpyrifos, diazinon and methomyl, which have maximum HQ values >10. However, joint probability plots indicated that the PAIs generally pose a low risk to the aquatic environment, with most samples below the limit of reporting. As PAIs are predominantly found in mixtures in the GBRCA, the hazard posed by PAI mixtures was assessed by summing all individual HQ values in a sample for all PAIs with an ETV or WQG. On average, the insecticide active ingredient imidacloprid and herbicide active ingredients metolachlor, metsulfuron methyl, diuron and imazepic were the drivers of combined mixture hazard. Methomyl was an important contributor at some sites, suggesting that this pesticide should be considered for inclusion in any future PAI mixture hazard and/or risk assessment of the GBRCA.

Estimating the aquatic risk from exposure to up to twenty-two pesticide active ingredients in waterways discharging to the Great Barrier Reef.

Pesticides decrease the quality of water reaching the Great Barrier Reef (GBR), Australia. Up to 86 pesticide active ingredients (PAIs) were monitored between July 2015 to end of June 2018 at 28 sites in waterways that discharge to the GBR. Twenty-two frequently detected PAIs were selected to calculate their combined risk when they co-occur in water samples. Species sensitivity distributions (SSDs) for the 22 PAIs to fresh and marine species were developed. The SSDs, the multi-substance potentially affected fraction (msPAF) method, Independent Action model of joint toxicity and a Multiple Imputation method were combined to convert measured PAI concentration data to estimates of the Total Pesticide Risk for the 22 PAIs (TPR22) expressed as the average percentage of species affected during the wet season (i.e., 182 days). The TPR22 and percent contribution of active ingredients of Photosystem II inhibiting herbicides, Other Herbicides, and Insecticides to the TPR22 were estimated. The TPR22 ranged from <1 % to 42 % of aquatic species being affected. Approximately 85 % of the TPR22 estimates were >1 % - meaning they did not meet the Reef 2050 Water Quality Improvement Plan's pesticide target for waters entering the GBR. There were marked spatial differences in TPR22 estimates - regions dominated by grazing had lower estimates while those with sugar cane tended to have higher estimates. On average, active ingredients of PSII herbicides contributed 39 % of the TPR22, the active ingredients of Other Herbicides contributed ~36 % and of Insecticides contributed ~24 %. Nine PAIs (diuron, imidacloprid, metolachlor, atrazine, MCPA, imazapic, metsulfuron, triclopyr and ametryn) were responsible for >97 % of TPR22 across all the monitored waterways.