Dilution or enrichment: the effects of flood on pollutants in urban rivers

Background Flood events increase the risk of sediment erosion and hence the release of particle-bound pollutants besides other processes that can be observed during such events like transportation, lateral distribution and other. Macropollutants, such as acids, salts, nutrients, and natural organic matter, are usually diluted by flooding, while the effect of floods on micropollutants is still unclear. To fill this gap, Qingshui Stream, a tributary of the Jialing River in the city of Chongqing that suffered 75,000 m(3)center dot s(-1) flood in August 2020 was selected in the current study to clarify effects of flood-induced pollution transportation. 14 surface water samples and 14 sediment samples were collected to analyze the occurrence of micropollutants (including 21 organochlorine pesticides (OCPs), 34 organophosphorus pesticides (OPPs) and 3 estrogenic compounds) before, during and after the flood. Finally, the environmental risks were evaluated by risk quotient (RQ). Results The concentrations of total phosphorus (TP), total nitrogen (TN), chemical oxygen demand (COD) in the surface water decreased from upstream to downstream, and the amounts were diluted by flooding from 0.08 to 0.05 mg center dot L-1 for TP, from 0.06 to 0.02 mg center dot L-1 for TN and from 132 to 27 mg center dot L-1 for COD, respectively. The concentration of estrogenic compound was up to 90 ng center dot g(-1), which was reduced to be lower than the limit of detection during flood. Alpha-endosulfan, delta-BHC, mirex, dichlorvos, phosdrin, thionazine, tetraethyl pyrophosphate, diazinon, methyl parathion, malathion, chlorpyrifos, famphur, and EPN were diluted by flooding, i.e., the concentration of delta-BHC reduced from 6.67 to 0.09 ng center dot g(-1); whereas, pp'-DDD, heptachlor epoxide, o,o,o-triethylphosphorothioate, dimethoate, rabon and fensulfothion were enriched after the flood. The environmental risk was observed to be mainly arise from the presence of OPPs, which increased after flooding. The RQ values of OPPs and OCPs increased after the flood, and the potential environmental risk of OPPs accounted for the majority portion of the risk. Conclusions The concentrations of macropollutants in surface water and sediments, and 14 micropollutants in sediments were diluted, while pesticides such as pp '-DDD, heptachlor epoxide, o,o,o-triethylphosphorothioate, dimethoate, rabon and fensulfothion were enriched after the flood. These results suggested management on urban river should focus on potential risk of OPPs. The current study therefore could provide scientific evidence and regulatory reference for urban river ecosystem protection.