Stormwater treatment in infiltration swales with an engineered and planted soil media – Results of a pilot-scale approach

Infiltration swales are well-established stormwater control measures. To meet the future multi-coded requirements, i.e. drainage safety, pollutant retention, and the promotion of biodiversity, we identified a research need regarding the soil layer and its simultaneous adaption of technical and soil functions. In particular, we aimed for higher water storage capacity and increased pollutant retention by developing engineered soil media based on natural topsoil or subsoil with amendments such as compost and brick sand. The media was flushed with real road runoff and monitored over more than two years, focusing on external influencing factors (e.g., antecedent dry-weather period, inflow rate, and de-icing salts) on pollutant retention. We found comparable water retention rates between both engineered soil media and the control but concluded that the amendment of brick sand had a beneficial impact given that the control had a considerably higher fine soil and humus content. An improvement of the retention efficiency was not achieved through the amendments compared to the control and retention efficiencies were >89 %, >97 %, and >94 % for Cu, Zn, and fine suspended solids. External factors had no dominant effect on the retention efficiency, a significant influence could only be demonstrated for organic matter. Additionally, we evaluated a minor but constant leaching for V and DOC. We concluded that engineered media may be suitable in swales through its potential higher water retention capacity with a simultaneous good retention of pollutants. However, attention must be paid to the leaching behavior of the soil amendments.