Removal of nutrient pollutants from highway stormwater runoff using bioslope with new media of biochar amended topsoils

United States (US) Department of Transportation (DOT) has constructed various treatment facilities on its right-of-way (ROW) to collect and treat highway stormwater runoff. Highway stormwater best management practice (BMP) treatment facilities include infrastructure such as bioslopes, sand filters, infiltration trenches, and grass channels. Among these BMPs, bioslopes have become more popular due to their applicability for roadway embankments and areas with limited ROW constraints. They remove pollutants effectively through physical filtration and chemical adsorption, facilitated by the engineered topsoil, which features a media mix of crushed rock, perlite, dolomite, and gypsum, albeit at a high cost. Biochar, typically made from wood biomass, is readily available and more cost-effective. This study, therefore, explores a new media mixture of biochar and topsoil for bioslopes. Four topsoil series (Tifton, Cecil, Pacolet, and Cowarts) were sampled across Georgia, analyzed, and amended with 0 %, 5 %, 7 %, and 10 % (weight percent, wt. %) biochar to treat highway stormwater runoff through infiltration. By utilizing biochar amended topsoil as a new bioslope media, the removal performances exceeded 60 % for total nitrogen (TN), including ammonia nitrogen, nitrate nitrogen, total Kjeldahl nitrogen (TKN), and total phosphorus (TP) with only a 5 % biochar amendment to the topsoils. The water holding capacity increased due to reduced hydraulic conductivity (>50 %) in the biochar amended topsoils, potentially increasing surface runoff volume due to low infiltration. The results from this study demonstrate that bioslopes of biochar amended topsoil can potentially minimize material costs in construction while providing a green and sustainable alternative to treat highway stormwater runoff.