Finding the Right Bioretention Soil Media

The successful functioning of a bioretention system depends on the use of suitable soil media. Predictive computer modelling of a bioretention system (e.g. using the MUSIC software) assumes specific characteristics of the soil media to achieve a certain level of stormwater treatment. Therefore, the soil media used in the construction of a bioretention system must exhibit the required characteristics. The use of inappropriate soil media can result in failure of a bioretention system, ranging from a reduced level of stormwater treatment through to a total lack of infiltration through the soil media and the associated ponding of water on the surface of the system. This paper describes the soil media characteristics that are critical to the successful functioning of a bioretention system and outlines the methodology behind the development of the Guideline Specifications for Soil Media in Bioretention Systems (FAWB, 2006). Some of the challenges associated with obtaining soil media that meet the specifications are addressed, including: (i) determining the acceptable range of saturated hydraulic conductivity for the filter media; (ii) assessing the suitability of on-site topsoil for use as filter media; (iii) accounting for the variability in laboratory soil test results; (iv) relating laboratory soil test results to likely performance in a constructed system; (v) assigning responsibility for obtaining and verifying soil media, and; (vi) sourcing appropriate soil media from suppliers. The importance of good soil media design specifications, quality controls for the supply of soil media, and detailed construction procedures are stressed. Bioretention systems for stormwater treatment Pollutants are removed from stormwater both on the surface of the bioretention system and within the filter media. When large storm inflows cause temporary ponding on the surface of the system, pollutants are removed from the stormwater through sedimentation and particulate adhesion onto the stems and leaves of the vegetation. As stormwater percolates down through the filter media, fine particulates and some soluble pollutants are removed through processes such as adhesion on to the surface of the filter media particles, biological transformation of pollutants by biofilms growing on the surface of the filter media particles, and biomass uptake through the root systems of the vegetation. Key aspects of bioretention soil media Bioretention systems are typically comprised of three soil layers (see Figure 1): (i) filter layer; (ii) transition layer; and, (iii) drainage layer. All three layers must exhibit structural integrity and must not be prone to structural collapse. These soil layers are critical to the successful functioning of a bioretention system: Filter Media The filter media must be capable of sustaining healthy vegetation, whilst also exhibiting a suitably high permeability (saturated hydraulic conductivity) to ensure a significant proportion of the incoming stormwater runoff infiltrates through the soil layers. Transition Media The transition media needs to have a particle size distribution that prevents the downward migration of filter media particles. Drainage Media The drainage media must have a sufficiently high saturated hydraulic conductivity to allow infiltrated water to flow horizontally across the base of the bioretention system towards the under-drainage pipes. The drainage media must also have a particle size distribution that prevents the downward migration of transition media particles.