Transport and Removal of Polar Organic Contaminants in Stormwater Biofilters with and without a Submerged Zone under Simulated Extreme Events

Next-generation stormwater drainage systems, such as green infrastructure, are increasingly implemented to promote runoff infiltration and aquifer recharge in water-scarce cities. Unfortunately, urban runoff can act as an efficient transport vector for polar (mobile) organic contaminants that are poorly removed by conventional biofilter designs, allowing them to reach subsurface water reserves. While significant efforts have been made to optimize biofilter treatment performance, contaminant attenuation remains highly sensitive to operational conditions, which are expected to be further stressed by climate change effects. We studied the transport and removal of polar stormwater vehicle-related organic contaminants of emerging concern in 1-m sand biofilters amended with granular activated carbon (GAC), operated under variably saturated conditions, with and without a submerged zone (SZ), during an 8-week intermittent-flow regime at a constant infiltration rate (15 cm h-1). Contaminant transport was subject to kinetic limitations, making removal highly sensitive to infiltration rates and hydraulic retention times. The presence of a SZ enhanced the removal of weakly sorbed compounds, likely by improving pore connectivity and increasing solute–sorbent contact time, thereby reducing rate-limiting sorption processes. Dry periods further increased attenuation, with a single day being sufficient to achieve near-complete sorption of contaminants in the SZ. Results suggested that steric effects and molecular interactions governed the sorption behavior of target pollutants. A one-dimensional kinetic transport model assuming sorption-site saturation effectively reproduced breakthrough behaviour. As a continuation, we are currently assessing the performance of SZ-enhanced biofilters under future climate scenarios, including different extreme rainfall patterns (based on peak-flow timing) and projected temperature increases for the Mediterranean region. To complement the assessment, we will evaluate the potential formation of transformation by-products using a combination of non-target chemical analysis and targeted suspect screening, followed by a preliminary toxicity screening. Overall, our research highlights the role of biofilter operational conditions and contributes to a better understanding of the fate and transport of emerging organic pollutants during stormwater infiltration. Acknowledgments: This work was supported by the predoctoral program FI ajuts Joan Oró (2023 FI-1 01092) funded by AGAUR, and by the Ramón y Cajal contract (RYC2022-037083-I) and project PID2021-124008NB-I00, both funded by the AEI.