Highway Stormwater Runoff On-site Treatment Using Bioslope With New Media Of Biochar Amended Topsoils

Georgia Department of Transportation (GDOT) has constructed various treatment facilities on its right-of-way (ROW) to collect and treat highway stormwater runoff, emphasizing total suspended solids (TSS) removal. The permit performance goal set by the Georgia Environmental Protection Division (EPD) is 80% TSS removal. Current stormwater best management practice (BMP) treatment facilities include complex and permanent infrastructures such as bioslope, bioretention basins, sand filters, infiltration trenches, and grass channels. Among these BMPs, bioslope has been becoming more popular due to its applicability for roadway embankments and areas with limited ROW constraints. It removes pollutants effectively by physical filtration and chemical adsorption through the engineered topsoil and media mix of crushed rock, perlite, dolomite, and gypsum, which are expensive. Biochar is typically fabricated from wood biomass, which is readily available and cheaper to obtain in Georgia. This study explored a new media of mixture of biochar and topsoil for bioslope. In this study, four (4) topsoil series (Tifton, Cecil, Pacolet, and Cowarts) were sampled across Georgia, analyzed, and amended with 5, 7, and 10% (weight percent, wt %) biochar to treat highway stormwater runoff through infiltration. Three (3) biochar products from the established manufacturers were selected and screened based on their properties and treatment efficiencies. By utilizing biochar amended topsoil as a new bioslope media, the removal performances exceeded 80% for TSS, total dissolved solids (TDS), total solids (TS), and 60% for oil and grease, ammonia nitrogen, nitrate nitrogen, total Kjeldahl nitrogen (TKN), total nitrogen (TN), and phosphorus with only 5% biochar amendment to the topsoils. The water holding capacity was increased due to reduced hydraulic conductivity in the biochar amended topsoils. For a three (3) yd3 installation volume, 5% biochar amended topsoil was 60% less costly in terms of materials than the current GDOT engineered topsoil for bioslope. Bioslope of biochar amended topsoil will minimize the material cost in construction while providing a green and sustainable alternative compared to the current GDOT bioslope. The biochar improved topsoil properties such as hydraulic conductivity, which was reduced by 50% plus in all four (4) topsoil samples. Therefore, the highway stormwater runoff volume is reduced and filtered as it passes through biochar amended topsoils.