This will ultimately control the lifetime of bioretention systems, especially in areas with high heavy metal loads. While this is a great benefit for water quality improvement, bioretention systems have a finite capacity for heavy metal removal. In laboratory experiments, bioretention cells removed 94%, 88%, 95%, and >95% of zinc, copper, lead, and cadmium, respectively from water with metal concentrations typical of stormwater runoff. Additionally, heavy metals may adsorb to soil particles in the bioretention media as the runoff filters through. Dissolved forms of heavy metals may bind to sediment particles in the roadway that are then captured by the bioretention system. Treatment systems such as rain gardens and stormwater planters utilize a bioretention layer to remove heavy metals in stormwater runoff. Heavy metal remediation Ĭontaminant trace metals such as zinc, lead, and copper are found in stormwater runoff from impervious surfaces (e.g. An unstable surrounding soil stratum and soils with a clay content greater than 25 percent may preclude the use of bioretention, as would a site with slopes greater than 20 percent or a site with mature trees that would be removed during construction of the best management practices. Sites with loamy sand soils are especially appropriate for bioretention because the excavated soil can be backfilled and used as the planting soil, thus eliminating the cost of importing planting soil. The layout of the bioretention area is determined after site constraints such as location of utilities, underlying soils, existing vegetation, and drainage are considered. The stored water and nutrients in the water and soil are then available to the plants for uptake. Stormwater storage is also provided by the voids in the planting soil. The clay in the planting soil provides adsorption sites for hydrocarbons, heavy metals, nutrients and other pollutants. The maximum sheet flow velocity prior to erosive conditions is 0.3 meters per second (1 foot per second) for planted groundcover and 0.9 meters per second (3 feet per second) for mulch. Planted groundcover reduces the potential for erosion as well, slightly more effectively than mulch. This layer acts in a similar way to the leaf litter in a forest and prevents the erosion and drying of underlying soils. The organic or mulch layer also filters pollutants and provides an environment conducive to the growth of microorganisms, which degrade petroleum-based products and other organic material. Some particulates not filtered out by the grass filter strip or the sand bed settle within the ponding area. The ponding area provides a temporary storage location for runoff prior to its evaporation or infiltration. Aeration and drainage of the planting soil are provided by the 0.5 m (20 in) deep sand bed. The sand bed also reduces the velocity, filters particulates, and spreads flow over the length of the bioretention area. The grass buffer strip reduces incoming runoff velocity and filters particulates from the runoff. Filtration Įach of the components of the bioretention area is designed to perform a specific function. Stored water in the bioretention area planting soil exfiltrates over a period of days into the underlying soils. Runoff passes first over or through a sand bed, which slows the runoff's velocity, distributes it evenly along the length of the ponding area, which consists of a surface organic layer and/or groundcover and the underlying planting soil. These materials were reported to have enhanced performance in terms of pollutant removal. Various soil amendment such as water treatment residue (WTR), Coconut husk, biochar etc have been proposed over the years. Stormwater is firstly directed into the designed treatment area, which conventionally consists of a sand bed (which serves as a transition to the actual soil), a filter media layer (which consists of layered materials of various composition), and plants atop the filter media. The main objective of the bioretention cell is to attenuate peak runoff as well as to remove stormwater runoff pollutants.Ĭonstruction of a bioretention area Plants are in winter dormancy.īioretention is the process in which contaminants and sedimentation are removed from stormwater runoff. It is designed to treat polluted stormwater runoff from an adjacent parking lot. A bioretention cell, also called a rain garden, in the United States.
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