Spatiotemporal distribution of mass flows of quaternary ammonium compounds in Japanese rivers.

Global demand for quaternary ammonium compounds (QACs)-biocides linked to multidrug resistance-increased in response to the recent outbreaks of covid-19 and African swine fever. To better understand their sources and dynamics in the aquatic environment, we monitored 13 QACs (benzalkonium chloride [BAC]-C8, C10, C12, C14, C16, C18; benzethonium chloride; cetylpyridinium chloride; dialkyldimethylammonium chloride [DDAC]-C8, C10, C12; and alkyltrimethylammonium chloride-C12, C16) in 14 rivers across Japan that drain catchments with various degrees of sewerage and intensities of swine farming, once a season for 3 years. Mass flows of BACs with high removal rates in sewage treatment plants (STPs) and DDAC-C10, a dominant QAC in Japanese animal husbandry, fluctuated widely in rivers with limited sewerage and active swine farming, respectively. Mean mass flows of BACC10, C12, and C14 in the rivers were (marginally) significantly correlated with human populations with untreated gray water, and those of DDAC-C10 with swine populations, indicating such populations to be key parameters for modeling their emissions. Per-capita mass flows of QACs labile in STPs and DDAC-C10 in the rivers often exceeded those in STP outlets, attributable to untreated gray water and swine wastewater, respectively. These findings advance our understanding of managing QACs.