Occurrence of diazepam and its metabolites in wastewater and surface waters in Beijing.

Occurrence of diazepam and its metabolites, nordiazepam, temazepam, and oxazepam in the water environment in Beijing was investigated. Samples were collected from four rivers flowing through the city and from all the thirteen sewage treatment plants in the urban area. Average influent concentrations of diazepman, temazepam, and oxazepam in 2013 summer ranged from 0.9 to 7.1, 1.5 to 3.4, and 2.9 to 12.4 ng L-1, respectively, whereas nordiazepam concentrations were below quantification limit on the majority of sampling dates. No significant seasonal variation in influent concentrations was observed. Removal during treatment was low for diazepman (<50%), temazepam (<20%), and oxazepam (<20%), consistent with previous findings reported in the literature. Wastewater-based epidemiology approach was applied to back-calculate population size-normalized diazepam consumption (using temazepam as biomarker) in Beijing, which was found to be at least 3.8 times more of the national average. Diazepam, temazepam, and oxazepam were widely detected in surface waters, with concentrations greater than concentrations in sewage influents at many sampling points, strongly indicating direct discharge of wastewater of high diazepam concentrations into the surface waters in the city.