Bio-retention Gardens Help Reduce Water Pollution

LACONIA — Recognizing that the city of Laconia has taken strong efforts over the last 5 years to protect regional water quality, the Lakes Region Planning Commission (LRPC) recently acquired funds from the NH Department of Environmental Services (DES) to support the city's Department of Public Works (DPW) with installing a bio-retention basin at the foot of White Oaks Road at Paugus Bay.

City officials recognize that, due to climate change and intense development over the last 10 years, its aging stormwater infrastructure is no longer meeting the needs of the community. Several areas are experiencing flooding due to undersized culverts and piping networks. At White Oaks Road, the older system was designed with the intent to move stormwater off the roads and into the lake as fast as possible. Unfortunately, this design method also moves the pollution into the lake.

As part of a Comprehensive Drainage Study started in 2009, the DPW identified several areas where simple changes to the stormwater system could help improve stormwater management and water quality protection. The most recent project is a bio retention garden located at the intersection of White Oaks Road and Weirs Boulevard. Stormwater in this area enters a series of catch basins that is channeled onto the lawn of a private residential property located along the shoreline. Recognizing the stormwater quantity and quality issues this discharge creates, the city is constructing an 850 square-foot bio-retention garden to capture stormwater and clean it before it goes into the lake.

The bio-retention garden, engineered by Michael Redding of Loureiro Engineering Associates, is designed using the powerful properties of compost and plants. Compost is very effective in removing stormwater pollutants such as petroleum hydrocarbons, bacteria, nutrients, and metals. The bio-retention garden on White Oaks Road will have the capability of treating and infiltrating the majority of rain storms, while safely bypassing overflows associated with very large storms.

As the rain enters the basin it will infiltrate through the open rock surface and pass through a two and a half foot layer of compost. The compost will absorb the stormwater like a sponge and begin to remove the pollutants from the water. The treated stormwater will also slowly drain into the surrounding soils making its way into the groundwater, reducing the discharges to the lake. When storms are larger and the stormwater begins to pond in the basin, plants and amended soils located on the slopes will begin absorbing the water and removing the pollutants.

According to the U.S. Environmental Protection Agency (EPA) bio-retention gardens are capable of removing between 80% to 90% of petroleum hydrocarbons, sediment, and bacteria and 70% to 80% of nutrients like nitrogen and metals like copper and zinc which can affect the health of fish.

Improving water quality in the Lakes Region requires cooperation from all communities within a watershed. For this project it was the collaboration between the city and an adjacent condominium complex that made this effort possible. The Paugus View Condo Association recognizes the importance of protecting the lake and, when approached by Luke Powell of the DPW, was willing to allow a drainage easement on their property to accommodate the size of thes bio-retention garden.

While the working simplicity of this system will not be as evident until spring when buds start sprouting on the diverse species of plantings, the roots of the now-dormant plants and the soil around them are already working hard. The plantings include silky Dogwood shrubs, and variety of shapes and colors represented in perennials, such as Bee Balm, Meadow Sweet, Caesar's Brother Iris, Black Eyed Susan, and Feather Reed Grass. They will also provide habitat for birds and insects and will remind visitors entering Weirs Boulevard of Laconia's commitment to water quality protection.