Stormwater Engineering: Whitin Wellfield Water Treatment Facility, Whitinsville, MA
The Whitin Wellfield Water Treatment Facility design consisted of a pressure filtration vessel housed within a new pre-fabricated metal building, and upgrades to the chemical feed system housed within an existing building on site. The project also included associated electrical and instrumentation, as well as site stormwater management. The design called for replacement of the existing paved driveway, but since no new impervious pavement was added, the only stormwater runoff that required treatment was the rooftop runoff from the new building. MassDEP guidelines require treatment of runoff from metal roofed buildings within the Zone II radius of a wellfield. The necessary treatment was achieved by discharging the roof runoff into a grass-lined channel. The grass-lined channel provided pre-treatment for a bioretention area, which consisted of a shallow excavation in which specially selected metal-resistant grasses and shrubs were planted within a specific soil to provide treatment. This treatment train provided the necessary total suspended solids removal but did not receive credit for infiltration, which was required by MassDEP. The biofiltration area was designed to collect, store, and infiltrate a 100-year storm event, so although the BMP train did not receive credit by MassDEP standards, the local Conservation Commission agreed that runoff would be infiltrated and therefore approved the design.