Many components of complex organic pollutant mixtures in waters are often present at very low levels, posing a challenge for current analytical methods. Passive sampling is one of the perspective methods that could address this problem. In our work, we used a mobile "active" passive sampling system with two types of sorbent (EMPORE discs, silicone rubbers), which preconcentrates a wide spectrum of polar and non-polar organic pollutants from the water column. Eight Danube river stretches were sampled and spatial distribution profiles were described for a comprehensive range of organic pollutants including pesticides, pharmaceuticals and industrial chemicals. Sampling rate and the original concentrations of the pollutants in water were estimated for pollutants with different physicochemical properties by a set of reference compounds. The employed mode of passive sampling with an active exchange of water in the sampling chamber proved to be about five-times more effective than static exposure and in relatively short sampling times allowed quantification of chemicals in pg.L-1 levels. Beside chemical analyses, samples were characterized with a battery of in vitro bioassays sensitive to chemicals with selected modes of action. Toxic potentials for causing endocrine disruption (anti/estrogenic, anti/androgenic), dioxin-like activity, pregnane X receptor (PXR)-mediated activity and oxidative stress-associated response showed spatial variation along the river. Polarity of chemicals played an important role, while AhR-mediated effects were associated mostly with non-polar compounds (sampled by the silicone rubber sampler) the rest of toxic potentials was associated mainly with more polar chemicals sampled by the EMPORE disc samplers. To link the toxic potentials with chemical analysis data, the detected pollutant levels and their relative effect potencies were used for calculation of bioanalytical equivalent concentrations (BEQs) using the concentration addition concept, with effect concentrations obtained from literature or US EPA ToxCast database. BEQs served for explaining the observed bioassay-derived toxic potentials and identification of the main drivers of toxicity. Our work demonstrates the utility of passive sampling for analysis of trace contaminants in river water as well as effect-based monitoring. The SOLUTIONS Project is supported by the Seventh Framework Programme (FP7-ENV-2013) of the European Union under grant agreement no. 603437
