An improved method for calculating toxicity-based pollutant loads: Part 2. Application to contaminants discharged to the Great Barrier Reef, Queensland, Australia

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 is comprised 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), where the load is modified according to the relative toxicity (expressed as toxic equivalency factors— TEFs) of each toxicant. Here we present the second paper of a two-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 toxicity-based load (toxic load), using a modified TEF approach. The modified approach is based upon the cumulative distribution of relative potency (ReP) estimates of multiple species, and is further improvedon previously published TEF methods with the inclusion of two tests to select the percentile of the cumulative ReP distribution which generate TLs: that align with an independent mixture method (test for environmental relevance); and are independent of the reference chemical used (test for robustness). Here, the TL method is applied to mixtures of pesticides that are discharged from agricultural land to the Great Barrier Reef (GBR) in order 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.