Site Stormwater Management, Falmouth, MA
Long Pond Water Treatment Plant, Falmouth, MA
The Long Pond Water Treatment Plant project consists of an 8.4 mgd DAF treatment plant, ozone contactors, and raw water pump station with intakes. The site stormwater management part of the project was separated into two designs: the parts of the site under the Conservation Commission’s jurisdiction and those that are not.
The majority of the site from the entrance along the access road and around the main treatment plant building did not fall under the Conservation Commission’s jurisdiction, and therefore did not require stormwater treatment. However, we did not want to ignore the potential impacts of stormwater, so this portion of the site was designed to allow for 100 percent infiltration. Along the access road, grading allowed for water to discharge and infiltrate in undeveloped areas where it will not impact the site. The area around the main building flows over land to an infiltration basin that is designed to collect, store, and infiltrate most storm events. There is an overflow in the case that the infiltration basin is overwhelmed during a large storm event.
The construction of the raw water pump station and intakes fell within the Conservation Commission’s and other agencies’ jurisdiction. Since the only impervious area was the building, and the building did not have a metal roof, MassDEP guidelines allowed for direct infiltration. The guidelines usually require the infiltration to occur in drywells in this situation, but the local Conservation Commission agreed that having an infiltration basin with an overflow would require fewer man-made structures and therefore have less impact on the surrounding environment. The infiltration was designed to store and infiltrate the raw water pump station roof runoff, and included an overflow for large storms.