as quoted from the American Biochar Institute.
Biochar is incorporated into stormwater BMPs to improve infiltration, water holding, and pollutant removal thereby enhancing flow mitigation and pollutant reduction performance. Biochar can be effective for removing heavy metals, nitrogen, phosphorus, bacteria, trace organic compounds (TrOCs), and emerging toxic contaminants (ETC). Abundant research on biochar performance suggests several key benefits that support its use in stormwater BMPs, such that, despite the challenges in developing standardized sourcing and implementation guidance, it should be considered as an amendment to stormwater BMPs in many areas.
One major benefit of biochar amendment is pollutant removal, which is achieved through multiple mechanisms. For heavy metals, removal occurs primarily through adsorption, facilitated by cation exchange or surface complexation with functional groups on the biochar surface. Biochar amendment has been consistently shown to reduce copper and zinc from stormwater. Nitrogen removal is also enhanced, with biochar-amended media tending to outperform sandy media mixes. Ammonium can be removed via adsorption to negative functional groups, while nitrates and nitrites can be reduced through denitrification occurring in the micro-anoxic zones within biochar pores. For biological contaminants, such as Escherichia coli (E. coli) and Enterococci, the increased porosity and surface area of biochar, combined with the hydrophobicity of some biochar, increases the potential for trapping bacteria within the media. High production temperature wood-based biochar, in particular, has shown efficacy in reducing E. coli concentrations. Trace organic compounds (TrOCs), which include various categories of contaminants like pesticides and poly aromatic hydrocarbons (PAHs), are initially removed by adsorption, but biochar also enhances long-term removal by increasing water retention, which provides more time for microbial degradation to occur in pore spaces.
Beyond pollutant removal, biochar offers significant hydrologic benefits, critical for runoff reduction. Biochar amendment increases the water holding capacity (WHC) in soils, particularly in sandy soils such as bioretention soil media (BSM), which is important for detaining or retaining runoff for extended periods, allowing natural removal processes (like denitrification) to occur. This increased WHC is attributed to the porosity of biochar but also its ability to enhance soil agglomeration, leading to increased pore space and pore size diversity.
Biochar amended soils also tend to have lower bulk densities, reducing compaction. Furthermore, biochar amendment can increase infiltration properties in fine and medium-grained soils and reduce infiltration rates in sandy soils, by shifting the particle size distribution (PSD) and improving soil structure via macropores in biochar and soil agglomeration.
Another crucial benefit of biochar is enhanced plant survivability. The majority of biochar research, and indeed the majority of biochar use globally is as a soil amendment to improve plant health and crop yields. In BMPs, plants are essential, supporting evapotranspiration and pollutant removal through uptake, hosting and supporting beneficial microbial communities in root zones, and maintaining infiltration rates via rooting action.
Biochar improves soil structure and captures and stores runoff and associated nutrients in pores, improving nutrient uptake and making water available to plants during drought conditions. This leads to increased plant vigor, health, and survivability, which can improve public perception of stormwater BMPs and reduce long-term operation and maintenance costs associated with plant replacement. One way that biochar can be used effectively in stormwater BMPs is as a partial replacement for compost, providing long-lived organic matter that exports only limited amounts of nutrients.
