The Brentwood Home Page recently featured a story on one of the largest rain gardens to be built in the state.  This rain garden will reduce the environmental impact of run-off from surrounding impervious surfaces. 

The Harpeth River Watershed Association and the City of Brentwood have built one of the area's largest rain gardens at the Concord Road YMCA, hoping to absorb polluted rainwater before it enters the neighboring river.

The city's engineering department and storm water coordinator Matt White played a role in designing and overseeing the project, which will serve as an example in coming years as new Tennessee Department of Environment and Conservation storm water requirements hit Brentwood.

The 4,000 square foot rain garden is designed to absorb the first inch of rainfall that falls on the YMCA property in a normal weather event-a day or so of average rainfall, not a heavy deluge or unusual rain pattern.

Because the Concord Road property is almost all building and parking lot, there was very little grass to absorb any of the property's rainfall, which can collect sediments, pesticides and other pollutants on its way to the river.

"It's hammered with a lot of runoff," said project manager Michelle Barbero with the HRWA. "There's massive erosion happening because of the volume of water, but the idea is to allow nature the chance to infiltrate that water, and it soaks into the ground instead of going straight to the river."

The project was completed in part with a $5,000 grant from the Maddox Charitable Foundation, a necessary contribution, Barbero said, because rain garden projects are very expensive.

The YMCA rain garden is composed of sand and topsoil tilled into the existing soil to create an absorbent base. Ten different species of shrubs and flowers were planted into the soil, several with particularly long root systems that drive the water into the soil.

The idea, Barbero said, is that the rainwater can sit in the garden for between 24 to 36 hours before it's completely absorbed by the Virginia sweetspire, pussy willows and oak leaf hydrangeas.

"This is green infrastructure, not just a garden," Barbero said. "Brentwood is older and developed, and you can't make people change everything. But as we learn more about how development affects water and air quality, you can find a way to limit the amount of pollutants coming of the property and that will make a difference."

Brentwood will have to limit those pollutants in 2015 as part of state environmental requirements affecting cities with populations over 10,000.

Storm water regulations require those cities to keep the first inch of rainfall on a newly developed site. That requirement is included in Brentwood's current storm water permit, which became effective in February 2011. The city has 48 months to implement the new requirement.

"The bottom line," said city engineer Mike Harris, "is that on a newly or redeveloped site, the first inch of rainfall will be held on the site and disposed of using one or more techniques."

Those techniques include reusing for irrigation, infiltration into the ground, consumed by vegetation or evaporation.

"The rain garden recently constructed at the Concord Road YMCA is an example of how this can be done," Harris said. "The water discharged with rain gardens will contain considerably fewer contaminants than a normal open detention area."

Developers will face these requirements all over the state, but Harris said the real challenge is maintaining the facilities, as the city will be required to regularly inspect them.

Interested or environmentally minded citizens can personally inspect the YMCA's purple coneflowers and black-eyed susans, and can expect to see more-but possibly smaller-rain gardens as the city implements the new regulations.

"This is one of the biggest rain gardens I've ever seen anywhere," Barbero said. "This is definitely a showpiece."