Early in 2019, Boston College built a road salt storage facility on the rear parking lot of its land in Webster Woods.
Drainage System
This drawing from the Boston College building permit application shows the drainage facilities at the northern end of the parking lot:
- The grid in the center is described as follows: “SUBSURFACE INFILTRATION SYSTEM (INFILTRATES STORMWATER WITH OVERFLOW TO OUTLET PIPE)”
- The outlet pipe is shown at the upper left corner of the plan, and in a photograph, above. Presumably, if a storm exceeds the capacity of the infiltration system, unfiltered water (including any substances washed off of the parking lot) will flow directly into Webster Woods through the outlet pipe shown in one of the photos above.
- The catch basin (pictured in a photo, above) collects most of the runoff from the parking lot and sends it into the infiltration system. A “VERTICAL GRANITE CURBING TO PREVENT STORMWATER RUNOFF OVERLAND” was built along part of the western and northern edges of the parking lot. However, this curbing does not extend the full length of the northern edge of the parking lot, as shown in the photos above. Water flowing to this edge of the parking lot goes directly onto the adjacent land, which slopes steeply downhill (as shown by the closely spaced dotted elevation lines on the plan) into the woods. According to the site plan, the distance between the edge of the parking lot and the edge of the city-owned Webster Conservation Area is 20 feet. (The line at top right that is mostly solid, but with occasional dashes, shows the boundary between BC land and City of Newton conservation land.)
Questions
- Does the “subsurface infiltration system” remove dissolved salt from water that enters the system from the parking lot, or does the outflow from the system contain dissolved salt?
- According to the building permit application materials filed by BC, the “design storm” used to determine the capacity of the system is “8.78 inches in 24 hours.” Does this mean that a bigger storm will cause water to leave the system via “overflow to outlet pipe” without passing through the filtering materials? According to the Weather Channel, the largest 24-hour rainfall experienced in Massachusetts was 18.2 inches.
- What percentage of runoff from the parking lot will flow off the northeast edge of the lot into Webster Conservation Area, instead of flowing into the infiltration system?
- During the winter, when road salt is present in the storage facility, what is the saline concentration of parking lot runoff?
