Spatial and seasonal trends of organic micropollutants in Sweden's most important drinking water reservoir.

The main objective of this study was to comprehensively investigate the occurrence of organic micropollutants (OMPs) in the surface water in Lake Mälaren over a period of one year and assess the spatial (horizontal and vertical) distribution and seasonal trends of OMPs. The applied multi-residue method comprised 111 OMPs covering compounds with wide range of physicalal-chemical properties. In total, 46 OMPs were detected at least once above limit of quantification (LOQ). DEET (diethyltoluamide), lamotrigine, bicalutamide, tolyltriazole, caffeine, carbamazepine, metoprolol, oxazepam, cetirizine, fexofenadine, lidocaine and tramadol were detected in more than 75% of the sampling points. The highest detected concentration was found for lamotrigine with 140 ng/L. The locations Ekoln and Västeråsfjärden were identified to be most affected by OMPs pollution. In the surface water (0.5 m depth) most highest concentrations of OMPs were observed in spring and lower concentrations in summer. Antihistamine demonstrated a seasonal trend that can be related to seasonal consumption patterns. The investigation of the vertical distribution of OMPs demonstrated that thermal lake stratification can lead to different concentrations within the water column. For instance, in February the highest concentration for most OMPs was observed in the deepest sampled water depth (30 m). Two industrial chemicals, tolyltriazole and tris(2-butoxylethyl)phosphate were detected frequently and it is recommended to incorporate these compounds more regularly in future analysis. To the best of our knowledge, this study is the first one to report the occurrence and distribution of OMPs representing such wide physico-chemical properties, including industrial chemicals, in a Swedish lake.