Roadside filters can be used to treat polluted stormwater. In this study, sand and sand-biochar filters were investigated in full-scale field conditions during three rain events. Stormwater influent and effluent of the filters were sampled with high frequency and up to 29 water quality parameters were determined. The obtained field data was analyzed and used as an input for geochemical modelling. The study showed promising results for both filter types with respect to retention of metals, total suspended solids and total phosphorus. Biochar amendment had clear improvement only for nitrogen removal; some release of organic carbon from biochar filter material was observed. Concentrations of most measured parameters varied considerably during and between the studied events. Geochemical modelling showed that precipitation is one possible removal mechanisms for many pollutants along with sorption and physical filtration.