Where Does Our Drinking Water Come From?

USGS watershed image


Water Supply

The drinking water supply system (United States Geological Survey above figure) consists of the Hobbs Brook Reservoir (upper reservoir) and Stony Brook Reservoir (middle reservoir) which drains a 24 square mile basin in Lexington, Lincoln Waltham and Weston, MA, and Fresh Pond (terminal reservoir), a glacial kettle-hole lake located in Cambridge.  The drainage basin for the Hobbs Brook Reservoir include Hobbs Brook and three unnamed  tributaries that discharge directly into the reservoir.  Additional water enters the reservoir through other unnamed tributaries, storm drains associated with State Routes 2, 2A and Interstate 95, secondary roads, and commercial parking lots.  Water is discharged from the dam at Winter Street, Waltham,  which joins Stony Brook about 1.5 miles downstream.  Stony Brook Reservoir is fed by Stony Brook, a small tributary from Weston, and by storm drains from  Interstate 95  and State Routes 20 and 117.  The water is then piped through an aqueduct from the Stony Brook Reservoir to Fresh Pond where it is stored prior to treatment.  Fresh Pond raw water is purified at the Walter Sullivan Water Treatment Plant (WTP) and pumped to Payson Park Reservoir, which is two 16 million gallon covered storage tanks located in Belmont at a maximum elevation of ~172 feet, where it is further disinfected with chloramines and distributed to the city by gravity.

The largest of the reservoirs, Hobbs Brook, reaches its maximum elevation at ~169.6 feet (NAVD88), its maximum depth at approximately 25 feet, and at full capacity, holds approximately 2.5 billion gallons of water.  Stony Brook Reservoir reaches its maximum elevation at ~68.9 feet, its deepest point is approximately 30 feet, and at full capacity contains roughly 425 million gallons of water.  Fresh Pond Reservoir reaches its maximum elevation at ~5.3 feet, its maximum depth is 50 feet, and at full capacity, holds roughly 1.5 billion gallons.   

While the Watershed's primary storage reservoir is Hobbs Brook, in the winter and spring months it is largely unused.  Due to its vast size, and relatively small watershed, Hobbs Brook is slow to fill up.  This winter hiatus is necessary for Hobbs to regain the water transferred in the summer months.  Conversely, Stony Brook is relatively small compared to its large watershed and fills much faster than Hobbs Brook.  Due to this condition, water from Stony Brook is transferred during the winter and fall months.  During times of high water flow, excess water is diverted to the Charles River at Stony Brook.  The modeled source yield (average reservoir inflow over the period of record) for this system is 24.6 million gallons per day (MGD).

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