It’s hard to tell just by looking at a tree-lined street, but soil and its microbes, in combination with trees, work together as a powerful system to improve the quality of stormwater that is filtered through it. Some pollutants are held or filtered by soil, others are taken up or transformed by plants or microbes, and still others are first held by soil and then taken up by vegetation or degraded by bacteria, “recharging” the soil’s sorption capacity in between rain events.
|Pollutant||Bioretention Cleansing Mechanism|
|TSS||Sedimentation and filtration (e.g. Davis et al 2009)|
|Metals||Filtration of particulate metals, sorption of dissolved metals onto mulch layer (e.g. Davis et al, 2009), plant uptake (e.g. Toronto and Region Conservation, 2009)|
|Nitrogen||Sorption; uptake by microbes and plant material, uptake into recalcitrant soil organic matter (e.g. Henderson, 2008)|
|Phosphorus||Sorption, precipitation, plant uptake, uptake into recalcitrant soil organic matter (e.g. Henderson, 2008)|
|Pathogens||Filtration, UV light, competition for limited nutrients, predation by protozoa and bacterial predators (e.g. Zhang et al 2010)|
|Hydrocarbons||Filtration and sorption to organic matter and humic acids, then degraded by soil microbes (e.g. Hong et al 2006)|
This table summarizes some of the main bioretention pollutant cleansing mechanisms.
Several literature reviews of lab and field studies of bioretention pollutant removal have concluded that bioretention systems have the potential to be one of the most effective BMP’s for pollutant removal. High concentration and load reductions are consistently found for suspended solids, metals, polycyclic aromatic hydrocarbons (PAH), and other organic compounds. Nutrient (dissolved nitrogen and phosphorus) removal has been more variable. Healthy vegetation has been found to be especially crucial for removal of dissolved nitrogen and phosphorus.
Similarly, several studies that have compared vegetated media to unvegetated media have found that the presence of vegetation substantially improves TP and TN retention, as vegetated media is much more effective than unvegetated media at removing PO4 from solution and preventing NO3 leaching from media (e.g. Henderson et al 2007, Lucas and Greenway 2007a, 2007b, 2008, May et al 2006). Not only has vegetation been shown to significantly improve nutrient removal, trees also seem to benefit from the nutrients in the stormwater, as a study that compared growth of trees irrigated with stormwater to trees irrigation with tapwater found that the trees irrigated with stormwater had greater height growth and root density compared with those irrigated with tap water (May et al 2006).
For a summary of some of the research on bioretention and water quality, see Davis et al 2010, Davis et al 2009, Table 1.1 in Henderson 2008, and the BMP database.
For more on how vegetation improves bioretention nutrient removal, see (for example) Henderson et al 2007, Lucas and Greenway 2007a, 2007b, 2008, May et al 2006.
In addition to beautifying streets and generally making people happier, street trees growing in large soil volumes are capable of providing significant stormwater quality benefits. That is why you may have heard them described on this blog and elsewhere as green infrastructure. Stormwater pipes alone, by contrast, do not provide any stormwater quality benefits. For this reason, installing bigger pipes is not always the best solution for addressing non-point source pollution and flooding problems in cities.