Biochar-Amended topsoil for highway runoff treatment: Unintended consequences of heavy metal release amid success in solids and oil & grease removal

The United States Environmental Protection Agency (U.S. EPA) rigorously enforces water quality guidelines, mostly successfully controlling point source pollution. However, non-point source pollution, particularly highway stormwater runoff, remains a persistent challenge. Civil engineers typically implement stormwater Best Management Practices (BMPs), often focusing on stormwater quantity rather than quality to mitigate pollutants. Firstly, a spatial analysis approach was utilized to innovatively determine high-risk impervious areas affecting stormwater quality. Then, we simplified the Georgia Department of Transportation's (GDOT) bioslope BMP design by substituting costly materials with sustainable biochar and four onsite topsoils (Tifton, Cecil, Pacolet, and Cowarts). Biochar-amended topsoils, with biochar additions of 0%, 5%, 7%, and 10%, consistently achieved over 60% removal efficiencies for physical pollutants, including total suspended solids (TSS), total dissolved solids (TDS), and total solids (TS) from real sampled highway stormwater runoff. Organic pollutants such as chemical oxygen demand (COD) and oil & grease exhibited removal efficiencies exceeding 80% and 20%, respectively. However, the study also revealed the inherent difficulties in treating heavy metals such as lead (Pb), zinc (Zn), and copper (Cu), where the removal efficiency was highly variable due to biochar saturation and competitive ion exchange, particularly for the dissolved metals. Overall, 5% biochar amendment of Tifton and Cecil topsoils provided the optimum balance between pollutant removal efficiency and minimizing adverse effects such as increased pH, conductivity, and dissolved metal mobilization. This research underscores the potential of biochar-amended topsoils as a cost-effective solution for physical and organic pollutant removal while highlighting the complex challenges of heavy metal treatment in highway stormwater runoff.