Risk assessment approaches for ecosystem responses to transient pollution events in urban receiving waters

Alternative risk assessment approaches are reviewed for the evaluation of the ecological status and health of urban receiving waters subject to intermittent pollution events. Performance-based criteria founded on exceedance probabilities and related to the end-of-pipe discharge of chemical-specific substances comprise the conventional basis for setting regulatory standards in both North America and Europe. The difficulties and limitations of this approach, particularly in identifying realistic chronic, sub-lethal toxic risks arising from complex effluents are discussed. The potential role of Toxicity Based Criteria (TBC) for setting ecological consent limits for stormwater effluents is considered and the capabilities and limitations of Direct Toxicity Assessment (DTA) are identified. The inability of DTA procedures to satisfactorily evaluate chronic, sub-lethal risks has led to increasing interest in the potential use of in-situ biomarker techniques for the fingerprinting of stress-response properties as a means of diagnosing risk assessment for integrated urban runoff management.