Green Stormwater Infrastructure: Does It Work?

Green Stormwater Infrastructure (GSI) is typically built for multi-purposes: stormwater treatment and green space. Vegetation is used in GSIs to achieve these functions. However, plants often fail before established because there are too many stressors in GSIs, e.g. extreme soil moisture fluctuations (underwatering or overwatering), deicing salt and other pollutants, excess heat, pollutants, limited soil volume, and soil compaction. For the example of soil moisture fluctuation, GSIs are either too wet or too dry for most of the time. By design, GSIs are flooded (too wet) during storms by taking in the stormwater runoff from impervious surfaces and dry out (too dry) quickly in between storms by using very porous growth media. Since GSIs are designed to take large amounts of stormwater runoff, the real problem with soil moisture fluctuation is the low Plant Available Water (PAW) in porous growth media. This talk is to discuss novel technologies that can stabilize soil moisture by using the abundant stormwater in GSIs. During the storms, it absorbs water and reduces runoff. When the soil dries, the water is released to keep the soil moist.

We will also review and discuss solutions to resolve the other stressors such as deicing salt, excess heat, and soil compaction.

Full Abstract: The growth of urban areas is accompanied by impervious surfaces and the environmental impacts of stormwater. Some efforts have been directed at sustainable and distributed stormwater treatment systems to achieve low-impact developments or Green Stormwater Infrastructure (GSI). Different from the centralized wastewater treatment--also known as gray infrastructure, a distributed GSI stormwater onsite. Vegetation that includes grasses, trees, shrubs, and other types of plants is often used to achieve the goal is to mimic what happens in natural environments.
GSI is a great concept. However, plants are often under stress and consequently die. The main reason is that plants undergo constant stresses like flooding and drought conditions. By design, green infrastructures are flooded during storms. Also by design, the growth media drains and dries out quickly between precipitation events to make room for the next storm and to prevent overwatering issues. To achieve this, very porous growth media is used in these systems. As a result, soil moisture changes quickly from soaking wet to bone-dry in a short period. The extreme variation in soil moisture makes it very hard for any plants to survive. Large woody plants like trees are often used in these systems for their large capacity of retaining, reducing, and consuming stormwater compared to smaller plants like grasses and shrubs. However, there are not many trees can tolerate the moisture variation.
There are many other stressors in GSIs such as deicing salt and other pollutants, excess heat, pollutants, limited soil volume, and soil compaction. Although deicing salt may be just an issue in the northern climates, the other stressors are common in all regions, and it is impossible to avoid. The places where deicing salts are heavily used, i.e. roads, sidewalks, and parking lots are also the common locations where GSI are designed to collect stormwater from. These impervious surfaces quickly warm up during summer, reduce the soil volume for the plants, accumulate pollutants, and require soil compaction during construction.
Frequent replacement of dead plants has been the only standard operation procedure to keep some living plants in the system. Even when the right species are used, it is difficult to keep them in the optimal state for the designed purpose of removing pollutants from the stormwater. After a certain number of replacement of dead plants, some agencies have called it enough is enough and abandoned the GSI concept. It raises the question: Does green stormwater infrastructure work?. More importantly, is there any solution to this problem?

By looking at the fundamental aspects of the system, the real limitation is the plant available water (PAW), which is known to be very low in porous growth media used in GSI systems. Because PAW is an intrinsic property of a certain soil composition and structure, it cannot be changed. So although there is a large amount of stormwater entering the system, the low holding capacity allows the soil dry too quick. One solution is to increase the PAW by storing some stormwater GSIs collect and by releasing as soil dries. When the soil is too wet, the system would absorb the excess water up to reduce the overwatering problem. This two-way moisture movement is driven by osmosis. By keeping the water in the system and allow plants to use it, this technology also reduces stormwater runoff. A field trial of the system in a roadside bioretention system in central Virginia showed that it can successfully stabilize the soil moisture by reducing the soil moisture during rainy season and increasing soil moisture during droughts. This system also mitigates the other stresses in GSIs.