Waters released from wastewater treatment plants (WWTPs) are a relevant source of pharmaceuticals and personal care products to the aquatic environment, since many of them are not effectively removed by the treatment system. The consumption of these products increased in the last decades and concerns have consequently risen about their possible adverse effects on the freshwater ecosystem. In this study, we present a simple, yet effective, analytical model of transport of contaminants released in surface waters by WWTPs. Transport of dissolved species is modeled by solving the Advection-Dispersion-Reaction Equation (ADRE) along the river network by using a Lagrangian approach. We applied this model to concentration data of five pharmaceuticals: diclofenac, ketoprofen, clarithromycin, sulfamethoxazole and irbesartan collected in two field campaigns, conducted in February and July 2015, in the Adige river, North-East of Italy. The model showed a good agreement with measurements and the successive application at the monthly time scale highlighted significant variations of the load due to the interplay between streamflow seasonality and variation of the anthropogenic pressure, chiefly due to the variability of touristic fluxes. Since the data required by the model are widely available, our model is suitable to large-scale applications.