An improved method for calculating toxicity-based pollutant loads: Part 2. Application to contaminants discharged to the Great Barrier Reef, Queensland, Australia
Authors: Smith Rachael A , Warne Michael St J , Mengersen Kerrie , Turner Ryan DR 
Published: October 2016

Pollutant loads are widely used to set pollution reduction targets and assess regulatory compliance for the protection of receiving waterbodies. However, when a pollutant load consists of a mixture of chemicals, reducing the overall load (mass) will not necessarily reduce the toxicity by a similar amount. This can be overcome by setting targets based on toxicity-based loads (toxic loads, TLs), where the load is modified according to the relative toxicity (expressed as toxic equivalency factors [TEFs]) of each toxicant. Here, we present the second article of a 2-part series in which a case study is used to demonstrate the application of the toxic load method proposed in Part 1. The toxic load method converts a pollutant load, comprised of multiple chemicals, to a toxic load, using a modified TEF approach. The modified approach uses a cumulative distribution of relative potency (ReP) estimates of multiple species to determine a TEF. It further improves upon previously published methods by including two tests to select the optimal percentile of the ReP distribution to determine the TEF. The first test is a test for environmental relevance that compares results against an independent mixture method, identifying the percentile that produces the most environmentally relevant TEFs and TLs. The second is a test for robustness which ensures the results are independent of the ReP of the selected reference chemical. Here, the TL method is applied to mixtures of pesticides that are discharged from agricultural land to the Great Barrier Reef (GBR) to test its utility. In this case study, the most environmentally relevant and robust TLs were generated using the 75th percentile of the ReP cumulative distribution. The results demonstrate that it is essential to develop pollution reduction targets based on toxic loads and making progress to meeting them will lead to a commensurate reduction in toxic effects caused by toxicants in waters of the GBR. Integr Environ Assess Manag 2017;13:754–764. © 2016 SETAC

Key Points

Converting herbicide loads, discharged from catchments to the Great Barrier Reef, to toxic loads changed the contribution of each herbicide to the mixture load and the magnitude of the loads from each catchment. Converting herbicide loads to toxic loads improved their environmental relevance and will therefore better inform water quality managers on which chemicals and regions they should focus their actions, leading to a more environmentally beneficial allocation of resources. The percentile of the multispecies relative-potency cumulative frequency distribution, selected to calculate the toxic equivalency factor, can markedly affect environmental relevance, robustness and the relative magnitude of the toxic load, including the relative contribution of each constituent. The toxic equivalency factors that generated the most environmentally relevant and robust toxic loads were calculated from the 75th percentile of the multi-species relative-potency cumulative-frequency distribution.