Tiered aquatic ecological risk assessment of organochlorine pesticides and their mixture in Jiangsu reach of Huaihe River, China

A tiered approach consisting of several probabilistic options was used to refine aquatic ecological risk assessment (ERA) of individuals and mixture of various Organochlorine Pesticides (OCPs) detected in Jiangsu reach of Huaihe River, China. The tiered approach ranged from determined Hazard Quotient (HQ) to Joint Probability Curve and Monte Carlo simulation based HQ-distribution. The results from all levels of ERA methods in the tiered framework are consistent with each other. The results show that Endrin, o,p'-Dichloro-Diphenyl-Trichloroethane (DDT), alpha-Endosulfan and beta-Endosulfan posed clear ecological risk; p,p'-DDT, p,p'-DDD, Aldrin, Heptachlorepoxide and Methoxychlor posed potential risk; while Hexachlorocyclohexanes, Heptachlor, Dieldrin and Hexachlorobenzene posed negligible risk. Further, based on the concept of total equivalent concentration, combined ecological risk caused by the mixture of all detected OCPs was calculated, and it proved to be significantly higher than the risk caused by any individual OCP. Despite inevitable uncertainties in current ERA, a comprehensive tiered approach can help to get a more credible result of risks of individuals and mixture of hazardous pollutants and screen the major risk pollutants contributing to the combined ecological risk.