Wetlands
Natural wetlands fill specific functions in the ecosystem. They collect runoff and flood waters, and filter sediment, nitrates and other nutrients. They also provide habitat for an extremely rich and diverse population ranging from pollinators to fish, migratory birds, and other wildlife. Unfortunately, over 85% of natural wetlands in the US and Canada have been destroyed in the interests of expanding agriculture, leading to devastating pollution that has damaged the health of our surface waters and even created a recurring dead zone in the Gulf of Mexico.
Dead zones, like the one in the Gulf of Mexico, exist all over the world now, and there has been a recent slew of interest in replacing lost wetlands. Constructed wetlands are particularly well suited to agricultural areas, not least because they can be customized to accept unique configurations like drainage tiles. When tile drainage is directed to a man-made wetland, hazardous levels of nitrates are naturally treated before the water is ultimately released into a stream, ditch, or even back into the tile line.
Most agricultural wetlands are designed for subsurface flow, where water passes horizontally through shallow gravel beds. The gravel and roots of wetland plants support microbial colonies that are responsible for reducing many common pollutants in addition to nitrates, such as bacteria and some toxic metals. This type of wetland is constructed with an impermeable liner to prevent incoming waters from leaching into and contaminating groundwater before it’s been fully treated.
Subsurface flow wetlands are usually shallow gravel beds through which water passes horizontally. The extensive root system of the wetland plants growing in the gravel media provides substrate for the microbial communities responsible for the reduction of common pollutants. These systems use impermeable liners to prevent groundwater contamination. Since wetlands are expected to have a long lifetime, designers should specify high quality, extremely durable liners that are resistant to damage from temperature extremes, punctures, tears, and other environmental stresses. Reinforced polyethylene is an excellent, affordable material for this purpose.
Man-made wetlands remove, on average, 52% of nitrate pollution in agricultural waters. Even better, they have a long lifespan, require little maintenance, and offer valuable habitat, feeding grounds, and migratory stopovers to support vital ecosystems. Subsurface flow designs are highly efficient and require much less land than surface flow designs, making their capital cost substantially lower.
