Water Quality Guidelines for Metal Contaminants in Stormwater Runoff: Insights and Considerations

Abstract

Stormwater runoff can result in significant temporal fluctuations in the concentration of metal contaminants in receiving waterways. The quality of stormwater runoff during rainfall events can be highly degraded with metal concentrations that may exceed toxicity thresholds. Metals in storm water can be present in a range of different forms, with the partitioning of these forms controlled by a variety of factors that display strong interstorm variability. Of particular importance are metals that exist in a bioavailable form, which is influenced by lability from dissolved complexes, colloidal and particulate forms. As a result, resident aquatic organisms in these environments are exposed to pulses of metal contaminants at high concentrations and short-term changes in physicochemical conditions.Water quality guidelines (WQGs) are often used to interpret the results of stormwater runoff quality analyses and identify key problematic pollutants. In most jurisdictions, WQGs are generated using species sensitivity distributions of toxic effect data obtained from laboratory-based tests (Australia and New Zealand Governments, 2018) where organisms are usually exposed to highly labile metals in solution (e.g., free metal ions or weakly bound complexes). When applying guideline values, it is appropriate to make comparisons to the measured concentration of labile metal forms. In the context of stormwater runoff, these WQGs may not provide environmentally relevant assessments. Specifically, WQGs will generally achieve overly conservative outcomes because organisms can typically tolerate high metal concentrations for shorter exposure durations, and only a fraction of the total metal concentration is available for organism uptake in natural systems because of ligand binding.The current need for WQGs for short-term "pulsed" contaminant discharges like stormwater runoff is an issue of concern. An approach is required that will provide an adequate level of protection and prediction of risk for a variety of