In a recent study published in the Journal of Environmental Management , Ahmed I. Yunus, George Yuzhu Fu, Yongsheng Chen, and Joe F. Bozeman III explored an innovative approach to treating highway stormwater runoff using bioslopes with biochar-amended topsoils. Highway stormwater runoff is a major contributor to nonpoint source pollution in U.S. water bodies, carrying pollutants such as nutrients, heavy metals, and total suspended solids. Traditional treatment facilities like bioslopes, while effective, often rely on costly engineered media mixes. This research investigated a more sustainable and economical alternative: integrating readily available and cost-effective biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More into topsoil for bioslope media.

The study involved sampling four common topsoil series across Georgia—Tifton, Cecil, Pacolet, and Cowarts—and amending them with varying percentages (0%, 5%, 7%, and 10% by weight) of commercial wood-derived biochar. These amended topsoils were then tested in laboratory-scale bioslopes to evaluate their efficiency in removing nutrient pollutants from actual highway stormwater runoff. A key finding was the impressive pollutant removal performance, with bioslopes containing only a 5% biochar amendment achieving over 60% removal for total nitrogen (TN), including ammonia nitrogen, nitrate nitrogen, total Kjeldahl nitrogen (TKN), and total phosphorus (TP). For instance, Tifton and Pacolet topsoils amended with biochar showcased impressive removal efficiencies for TN, ranging from 97% to 100% across all biochar combinations.

While Cecil topsoil with a 0% biochar amendment recorded a 68% increase in TN concentration, introducing a 5% biochar amendment in Cecil topsoil achieved a 68% removal efficiency for TN. Cowarts topsoil with a 10% biochar amendment displayed removal efficiency ranging from 84% to 100% for TN, with 5% and 7% amendments still demonstrating 92% removal efficiencies. Ammonia nitrogen removal consistently reached 100% across all biochar amended topsoils, except for Cecil topsoil with 0% and 5% biochar additions, where the removal efficiencies were both 86% in the second infiltration treatment. TKN removal consistently showcased 100% removal efficiencies across all biochar amended soils except for Cecil topsoil with 0 and 5% biochar, displaying a range of 77%-89% removal efficiencies. In the treatment of phosphorus, Cecil topsoil exhibited the highest TP removal (100%) with a 7% biochar amendment. Similarly, in Pacolet topsoil, higher biochar additions correlated with increased TP removal efficiency. Conversely, Cowarts topsoil achieved a 97% TP removal with a 5% biochar amendment.

Beyond pollutant removal, the study also observed significant changes in the hydraulic properties of the amended topsoils. The addition of biochar drastically reduced the topsoil’s saturated hydraulic conductivity and increased its total porosityPorosity of biochar is a key factor in its effectiveness as a soil amendment and its ability to retain water and nutrients. Biochar’s porosity is influenced by feedstock type and pyrolysis temperature, and it plays a crucial role in microbial activity and overall soil health. Biochar More. This reduction in hydraulic conductivity exceeded 50% in all four topsoil samples when amended with 5% biochar. This phenomenon, while potentially increasing surface runoff volume due to lower infiltration, also means a larger initial capture of stormwater runoff, effectively treating flush flood pollutants. Biochar generally has a lower bulk density than topsoils, and its addition reduces the amended topsoil’s bulk density, creating more soil pore volume to hold water. This change in topsoil particle interpores and the biochar’s high surface areas and intraporosity promotes high water retention in amended topsoils.

However, the study also highlighted some nuances, particularly concerning nitrate nitrogen removal. Nitrate nitrogen removal showed variations, with Cecil topsoil demonstrating a significant increase in concentration without biochar and even with higher biochar percentages in some instances during repeated infiltration cycles. This suggests that factors like biochar residence timeResidence time refers to the duration that the biomass is heated during the pyrolysis process. The residence time can influence the properties of the biochar produced.   More and the growth of heterotrophic bacteria for denitrification are crucial for consistent nitrate removal. The researchers noted that the laboratory setting, lacking adequate biotic (biofilm formation for biofiltration) and other environmental influences (climate variability, including rainfall, sunlight, humidity, and temperature variations), might have contributed to a breakthrough in pollutant removal efficiencies during a third infiltration treatment after 14 days. This led to observed reductions in TN, TKN, ammonia nitrogen, and TP removal efficiencies in the third infiltration. Overall, the findings demonstrate that bioslopes utilizing biochar-amended topsoil offer a promising, green, and sustainable alternative for treating highway stormwater runoff, while also minimizing material costs in construction.

Source: Yunus, A. I., Fu, G. Y., Chen, Y., & Bozeman III, J. F. (2025). Removal of nutrient pollutants from highway stormwater runoff using bioslope with new media of biochar amended topsoils. Journal of Environmental Management, 390, 126259.