Amultitude of pharmaceuticals enter surface waters via discharges of wastewater treatment plants (WWTPs), and many raise environmental and health concerns. Chemical fate models predict their concentrations using estimates of mass loading, dilution and in-stream attenuation. However, current comprehension of the attenuation rates remains a limiting factor for predictive models.We assessed in-stream attenuation of 75 pharmaceuticals in 4 river segments, aiming to characterize in-stream attenuation variability among different pharmaceutical compounds, as well as among river segments differing in environmental conditions. Our study revealed that in-streamattenuation was highly variable among pharmaceuticals and river segments and that none of the considered pharmaceutical physicochemical and molecular properties proved to be relevant in determining the mean attenuation rates. Instead, the octanol–water partition coefficient (Kow) influenced the variability of rates among river segments, likely due to its effect on sorption to sediments and suspended particles, and therefore influencing the balance between the different attenuation mechanisms (biotransformation, photolysis, sorption, and volatilization). The magnitude of themeasured attenuation rates urges scientists to consider themas important as dilutionwhen aiming to predict concentrations in freshwater ecosystems

This research was supported by aMarie Curie European Reintegration Grant (PERG07-GA-2010-259219) and by a Marie Curie Career Integration Grant (PCIG9-GA-2011-293535) within the 7th European Community Framework Programme, aswell as by the SpanishMinistry of Economy and Competitiveness through the projects SCARCE (Consolider-Ingenio 2010 CSD2009-00065) and ENDERUS (CTM-2009-13018), and the postdoctoral grants “Juan de la Cierva” (jci-2009-05604 and jci-2010- 06397), and by the European Union through the European Regional Development Fund (FEDER). This work was partly supported by the Generalitat de Catalunya (Consolidated Research Group: Water and Soil Quality Unit 2009-SGR-965)