Combined sewer overflows attributable to wet weather season contributing to pharmaceutical pollution: a case study in northern Spain

The use of combined sewer systems is associated with frequent overflow events and discharges of untreated wastewater containing micropollutants. The aim of this study was to evaluate the influence of weather-related factors, concretely the contribution of combined sewer overflows, on the presence of pharmaceuticals and illicit substances, and assess the related environmental risk by measuring the risk quotients. To address this goal, three sites in the surroundings of the city of Vitoria-Gasteiz were sampled in April (dry week) and November (wet week): Salburua Wetlands and Zadorra River before and after receiving the wastewater treatment plant’s (WWTP) effluent. An enantioselective risk assessment has been performed for environmentally relevant concentrations of illicit drugs. A total of 33 out of 102 substances were detected in at least one of the samples and a minimum of 6 active pharmaceutical ingredients (APIs) in all sample sites. Post-WWTP samples taken in autumn presented the highest cumulative concentration (7739 ng L-1). Risk quotients of 12 APIs in the most contaminated site in the rainy weather sampling period reached values >1. The current study reveals that normally unpolluted stages of the river are contaminated due to rainfall events and associated combined sewer overflows.