Stormwater Treatment on Lake Garfield

JULY 2021 Lake Garfield Update Proposed Stormwater Treatment Back in 2017, lake specialist Dr. Ken Wagner studied potential phosphorus pollutant loading throughout Lake Garfield. Phosphorus is an important nutrient for growth in aquatic plants such as the invasive Eurasian water milfoil, which is an impediment to boating and kayaking on the lake if left unchecked. The town, with input from the Lake Garfield Working Group and financial contributions from the Friends of Lake Garfield, has been controlling this invasive plant species in recent years by hiring scuba divers to remove it from the lake bottom with suction harvesting.

According to the various sampling results Dr. Wagner looked at, he posited that the highest pollutant loading comes from stormwater runoff in three sections along Lake Garfield’s north shore which includes Hupi Road. A site visit with the Monterey Department of Public Works narrowed down these three areas to one site location based on space and the feasibility of installing structures to help treat stormwater. The site is a thousand-foot stretch of road starting at a seasonal stream just past Hupi Woods Circle to Elephant Rock Road, and treats stormwater pollution from Hupi Road, Mountain Laurel Way, and surrounding properties. The picture (above right) shows the area looking east down Hupi Road.

With funds obtained through the state from a Federal Clean Water Act Section 604b grant, the Town of Monterey hired the engineering firm Foresight Land Services to create conceptual designs for a stormwater treatment and drainage system which are now complete. Designs include a new drainage system of catch-basins, swales, and proprietary stormwater treatment system such as a hydrodynamic separator that will work in concert to improve road drainage and remove sediment, road salts, and road debris. (The image at right can give you an idea how the separator works.)

If you imagine the topography of that area, you’ll realize that rain currently flows from the hills northeast of Hupi Road down toward the lake, over Hupi Road. To capture this water and redirect it, the designs propose stormwater improvements sometimes referred to as “best management practices or BMPs” to be installed along both sides of Hupi Road. Some of the BMPs are also considered green infrastructure, utilizing plants and soil to filter, treat, and infiltrate stormwater in the process.

Here are the design details: Along the northeast side of Hupi Road, a vegetated swale—that is a linear depression along the road with plantings and soil substrate—will be installed to catch stormwater coming off the hills where the existing grass/gravel swale is located. Rocks within the new swale will slow the flow of water during peak storm conditions and allow for more water to seep into the ground. Oversized or “deep sump” catch basins would be installed along both sides of Hupi Road at driveway locations, and at Mountain Laurel Way. This design of catch basin allows heavier solids such as gravel, larger sediments, and undissolved road salts to settle to the bottom, for later collection and proper disposal. These catch basins would be connected by a closed drainage system that will pipe stormwater downhill and into a proprietary stormwater treatment structure such as a hydrodynamic separator (see drawing below left). Hydrodynamic separators like this one remove all remaining sediment that hasn’t settled out in the deep sump catch basins as well as remove additional pollutants including oil, small debris, and other floatables. After exiting this BMP, stormwater will drain into a final vegetated swale or bio-garden (much like a rain garden), where there is one more opportunity to infiltrate into the ground. Excess water will drain into the seasonal stream that flows to Lake Garfield. In effect, what the design calls for is a “train” of treatment and best management practices—structures that remove sediment, debris, oils, and other pollutants stormwater carries as it flows over the landscape around Hupi Road and Mountain Laurel Way. By removed sediment, or “total suspended solids” as we sometimes refer to them, the treatment train will in effect also remove attached phosphorus, the key pollutant impairing Lake Garfield. Moreover, improved drainage will decrease the need for sand and salting during the winter and reduce erosion and sediments from washing off the road, thereby reducing the town’s need to maintain and repair the gravel road so often.

The designs here, along with the Lake Garfield Watershed-Based Plan, and estimated phosphorus load reductions, allow the Town of Monterey to apply for Clean Water Act Section 319 funding from Massachusetts Department of Environmental Protection, which will fund full engineering designs and installation, and work toward removing Lake Garfield from the Massachusetts List of Impaired Waters. Stay tuned for more project updates.

— Courteny Morehouse, Senior Planner, Berkshire Regional Planning Commission