In a follow up to his Society for Ecological Restoration presentation on The Challenges of Maintaining Living Systems, FTLA Ecology Expert Steve Allison shares his thoughts on maintaining bioretention facilities.
Bioretention facilities have fast become an answer for problematic stormwater runoff in our cities locally and nationally. These living systems are an opportunity to create a clean chemical free way of treating stormwater at the source. Inspired by nature’s terrestrial forest ecosystems and their filtering process, bioretention is in effect a biological system.
But how do they actually work? The answer is chemistry.
At first glance, many see a water loving landscape planting bed along the street or parking lot, but these facilities are more than simple planting beds. It’s what we don’t see that is most important. Within bioretention systems the top layers of soil completes a natural pollution removing process that rivals any fabricated chemical process we have created.
These initial organic layers often referred to as the metabolic horizon, is where most of the pollution removal takes place. Within these soil layers, plants’ roots and naturally occurring mycorrhizal fungi filter the water by working together in a symbiotic relationship to extract and metabolize pollutants. This relationship between plant and fungi greatly improves the success rate for each bioretention system. In the metabolic horizon there is a microscopic world of interactions taking place: converting, dispersing, and adsorbing valuable nutrients, including the ones we are trying to remove from our waters.
Maintaining a rich organic layer within our bioretention systems is critical for the health and function of the system. When we lose an organic layer of soil through the lack of timely maintenance, erosion, and degradation, we lose the biological environment, thus reducing the pollution removing capacity of the facility. Each element of the system is essential for the facility to function at its greatest potential.
How do we ensure bioretention facilities sustain target pollutant removing capacity over time?
Establishing essential maintenance is a high priority for each facility. The first year is the most critical for these systems. Inspecting new facilities after the first two rain events over an inch is critical for identifying potential needed repairs and evaluating the design’s threshold to the velocity of incoming water. Equally important is continued inspection until plants become established and roots can stabilize the organic soil layers. The first storm events can disable the pollution removal process by eroding the organic soil layer and removing the moisture capturing mulch layer which fosters biomicrobial activity. Creating a maintenance program with regular inspections will identify necessary maintenance needed in a timely fashion helping to prevent those living systems from failing.
Generally small in size, these facilities are intended to be dispersed throughout the urban landscape, inserting biological diversity into our communities. As landscape architects, we envision bioretention facilities as high performing landscapes. The planting design must incorporate plant species that are tolerant to salt, drought, and flood inundation. Using our keen understanding of plant community composition and incorporating combinations of native plants we can build a food web that supports insects and butterflies above ground and fungi and bacteria below ground creating a healthier and more diverse system.