Field performance of biochar-amended topsoil for heavy metal removal from urban stormwater runoff

• Field-scale evaluation confirms biochar as a viable heavy metal treatment strategy. • Biochar increased infiltration and porosity, enhancing capture and sorption. • Copper and Zinc show consistently strong attenuation; Lead exhibits variable behavior. • Effluent pH and EC shifts govern ion exchange and surface complexation. • Aging and vegetation reduce performance; Fe/Al/Mn-oxide co-media advised for Pb. Urban stormwater runoff is a major pathway for heavy metal pollution in watersheds. However, the field-scale performance and durability of biochar-amended media treatment remain underexplored. This study evaluated biochar-amended topsoil as a stormwater BMP, comparing heavy metal removal performance against unamended media and assessing treatment durability under natural field conditions. Building on our prior bench-scale experiments, we conducted a year-long, paired field trial at an urban parking lot to assess heavy metal (Cu, Zn, and Pb) removal and hydrological performance. Two identical surface infiltration cells (each 23.23 m² × 0.30 m depth) were constructed: a Control Cell (topsoil only) and a Test Cell (5% (wt%) biochar-amended topsoil). Over the monitoring period (Summer 2023 to Summer 2024), 1,999 L of stormwater were treated per cell across 23 rainfall events. Influent concentrations varied considerably across storm events, ranging from 1.06–4.61 µg/L for total Zn, 0.12–0.27 µg/L for total Pb, and 0.09–1.56 µg/L for total Cu. The Test Cell demonstrated significantly greater removal than the Control Cell (p < 0.001), achieving removal efficiencies of 58.88–97.18% for total Cu, 40.37–97.43% for total Zn, and 2.38–84.25% for total Pb. Dissolved Cu (75.90%) and Zn (74.00%) were effectively removed, whereas dissolved Pb exhibited persistent desorption. Ion exchange and surface complexation were identified as dominant removal mechanisms. However, treatment performance declined over time with biochar aging and vegetation growth. These findings provide rare field-scale validation of biochar-amended topsoil as a sustainable green infrastructure strategy, while highlighting the importance of long-term monitoring and targeted design refinements for dissolved Pb attenuation.