Bioswales and Bone char: Improving Infiltration Rates and Contaminant Sorption Properties of Green Stormwater Infrastructure in Detroit, MI

Controlling stormwater runoff from impervious surfaces in urban areas such as Detroit, MI presents two main challenges: (1) handling a large volume of water in a short period of time and (2) removing contaminants. The solution in the early 1900's was to combine stormwater and sanitary wastewater in an underground network to be treated in a centralized treatment plant. Although Detroit has one of the largest wastewater treatment plants in the United States, large storm events can cause untreated sewage to be discharged to nearby water bodies in events known as combined sewer overflows (CSOs). As many abandoned structures in Detroit are demolished and replaced with vacant lots, Detroit has revisited the possibility for a more sustainable approach to stormwater management with the introduction of many tools such as raingardens, bioswales, and rain barrel storage. Bioswales are being constructed all over the city in an attempt to increase infiltration while providing a place to concentrate and/or remove suspended materials (e.g., silt, dirt) and pollution (e.g., automotive fluids, heavy metals) through mechanisms such as physical deposition, chemical sorption, and biogeochemical transformation. We investigated the potential to increase contaminant removal by amending bioswale soil with novel and inexpensive sorbents such as bone char (the product of heating animal bones to ~500 °C in the absence of oxygen). We collected cores from various bioswales, measured soil parameters (pH, organic matter, electric conductivity, and texture), and quantified hydraulic conductivity using a constant head permeameter and Darcy's Law. We also analyzed copper transport through undisturbed soil cores with and without a surface addition of bonechar. Copper concentrations were measured by colorimetry and were confirmed with ICP-MS analysis. The effect of temperature on copper sorption kinetics was also tested in batch experiments where after 10 days, 5 °C, 25 °C, 40 °C samples had 21%, 28%, and 79% sorption, respectively (with mass loading of 64, 176, and 200 mg Cu/g bonechar, respectively). Bioswales with bone char amendments show promising results and better quantification of these mechanisms will enable improved designs to ensure economical, predictable, and reliable treatment of runoff from urban areas.