Many engineered nanoparticles (ENPs) are functionalized with different types of surface coatings to suit specific applications. The functionalization affects the fate and behavior of these ENPs in aquatic environments. In this study, gold nanoparticles (GNPs) coated with either citrate or 11-mercaptoundecanoic acid (MUA) are used as examples of functionalized ENPs. A method has been developed to assess the colloidal stability of functionalized ENPs under complex hydrochemical conditions, using their aggregation rates as indicators. The spatial distributions of stream-water chemistry data fromacross Europewere combinedwith the results of in-vitro colloidal stability testing. Aggregation rateswere extracted for each stream-water sample and stability maps for Europewere plotted. The tendency of the tested GNPs to be dispersed or aggregated is described for water bodies of the respective region. Natural organic matter was identified as the predominant factor controlling the stability of the GNPs tested. The properties of surface coatings also affect aggregation rates as a result of differences in their hydrochemical parameters. The developed method can be used as a template for a stability assessment, and the results of this study provide a basis for exposure modeling and precautionary decision making.