Bioretention
Bioretention is a primary soil and plant-based tool employed in the practice of stormwater harvesting and management. Bioretention systems typically begin with filtration, which may be accomplished with layers of gravel and sand or other filtration media. Bioretention features are often fully exposed to the underlying soil in order to promote infiltration. In other cases, where the plan is to collect the water for use elsewhere, or when nearby infrastructure is vulnerable to excessive infiltration, the basin or swale is typically lined with an impermeable geomembrane liner.
Filtration
Hidden from view, many large features in the urban and suburban landscape incorporate full underground stormwater detention systems. Athletic fields, parks, large lawn areas, greenspaces, and even some industrial structures save considerable space by capturing and storing stormwater in large underground vaults until it’s used to keep the greenery just overhead healthy and happy. In these cases, infiltration into the soil is not part of the goal, so an impermeable geomembrane liner lines and underlays each element in the stormwater system to prevent escape of captured stormwater. In this storage space, sedimentation processes can take place and non-woven geotextiles may be employed to filter out larger debris as well.
Stormwater Hotspots and Contamination Concerns
The use of bioretention features like swales and basins provide a means for captured stormwater to undergo the first steps in treatment as soon as it’s collected. But each stormwater flow produces a different profile of contaminants, and some flows will emerge from pollution hotspots that are so heavily contaminated they cannot be safely managed in typical WSUD treatment chains. Surfaces in junkyards, auto repair lots, gas stations, aircraft deicing stations, and wood preserving facilities all leave contaminants that must not be exposed to the environment lest they contaminate vulnerable groundwater.
In these cases, stormwater emerging from these surfaces need to be conveyed and stored using materials that offer complete impermeability. From a gas station facility, the lot may be graded with a gentle slope toward corners where stormwater is expected to run off and travel downhill until it reaches a river or lake. Since the pollution potential in this case is so high, that outflow must be captured in a manner that doesn’t permit any spill or overflow, such as with a closed or underground bioretention basin lined with an impermeable geomembrane, like RPE. The time spent in the bioretention basin still allows suspended sediments to settle out before the remaining water is conveyed to a fully appointed wastewater treatment facility. It should be noted that each step, from collection to conveyance to delivery, must employ 100% reliable containment and secondary containment protocols.
Soil and Structure Protection
In some cases, such as instances where the nearby soil is already contaminated, it’s not advisable to use typical stormwater management features like bioswales, storage ponds, or similar elements. Allowing stormwater to infiltrate these areas will only help existing soil contaminants to travel further through the soil, extending the polluted area or entering the water table.
In these circumstances, the goal is to capture and detain stormwater in leak-proof underground basins, backed up with a secondary containment system in the form of an impermeable geomembrane liner made from a durable, chemically resistant material like reinforced polyethylene.
Impermeable liners are also used in stormwater planters and other practices where infiltration into the surrounding soil is not desired or practical. In certain instances, permeable pavements may be employed to immediately manage stormwater runoff even when it’s not appropriate for it to immediately enter the local soil and groundwater. In this case, a drainage layer is installed underneath the permeable paving, which captures the stormwater and quickly drains it to a more suitable area.
Impermeable liners are also useful in instances where the flow of stormwater needs to be blocked because steep slopes or nearby structures could be weakened by infiltration, or alternatively, when water needs to be retained to maintain a permanent pool in a wetlands system.
