When Green Complements Gray.

• Position: Green infrastructure and wastewater and stormwater management

Gray infrastructure is the traditional approach to stormwater and wastewater management that involves impervious surfaces to redirect water flow away from the built environment. These impervious surfaces, as defined by the Environmental Protection Agency, include "rooftops, parking lots, streets, sidewalks, driveways, and surfaces that are impermeable to infiltration of rainfall into underlying soils/groundwater."

Much of the U.S.'s gray infrastructure with respect to stormwater and wastewater is nearing or at the end of its life span and is in need of costly maintenance or replacement. That infrastructure is stressed by increasingly frequent and intense wet weather events as a consequence of climate change. While repairs and replacements to gray infrastructure systems are necessary, implementing complementary green infrastructure alongside them can be a more cost-efficient option for designing municipal systems that also improves the overall environmental health of communities.

Broadly speaking, green infrastructure is an approach and set of practices for water management--primarily stormwater management--that "are designed to mimic the natural ways water flows over and absorbs into land to reduce stormwater pollution." It includes permeable pavements that allow rainwater and melting snow to pass through and be absorbed into the ground below, as well as rain gardens and bioswales that use certain plants and soil mixtures to capture, treat, and filter stormwater runoff.

In general, these practices help absorb and redirect excess precipitation that otherwise might overwhelm stormwater and wastewater systems, resulting in flooding and the runoff or overflow of pollution into local bodies of water. More technically speaking, the Federal Water Infrastructure Improvement Act of 2019 defines green infrastructure as "the range of measures that use plant or soil systems, permeable pavement or other permeable surfaces or substrates, stormwater harvest and reuse, or landscaping to store, infiltrate, or evapotranspirate stormwater and reduce flows to sewer systems or to surface waters."

Permeable pavements are designed to allow rainwater and snow to pass through them, rather than to cause runoff and flooding. The permeable or porous surface allows for rain and melting snow to seep down to the layers of soil and gravel underneath, helping to filter out water pollutants. This method can be cost-effective in areas with flooding concerns, and already has...