The presentation will provide preliminary findings from several research projects aimed at defining highway runoff, which is a major pollution source for receiving water bodies. The growth of new pollutants, such as microplastics, forces us to increase our understanding of their fate, loading, and movement by sampling and monitoring treatment facilities. The monitoring and sampling plan applied consider the chronic impacts caused by the complex matrix of contaminants carried by runoff in a plant with filtration treatment and a system able to detect and prevent accidental spills. Real-time monitoring and new sampling techniques have been applied and all the obtained data combined to understand the behavior of heavy metals and emerging contaminants such as microplastics and pesticides in a different matrix. Given the intrinsic heterogeneity of stormwater pollutant release and transport mechanisms, a mass balance of various contaminants and accurate characterization of pollutant loads have been investigated.