WATER QUALITY AT THE CAMBRIDGE, MASSACHUSETTS DRINKING-WATER SUPPLY SYSTEM

The Cambridge, Massachusetts, drinking-water supply system is noted for its elevated chloride concentrations (100 to 150 mg/L dissolved chloride) in the distributed residential water. The system consists of two primary storage reservoirs, namely Hobbs Brook reservoir and Stony Brook reservoir, both located in close proximity to I95/Rt.128, a multilane highway. Fresh Pond reservoir is an intermediate transfer reservoir with no surface water inlet or outlet (kettle pond). Surface water stored at Hobbs and Stony Brook reservoirs is first pumped into Fresh Pond reservoir, then conditioned in the Fresh Pond treatment plant followed by distribution into the public water supply network. This study evaluates the impact of the winter application of road salt on water quality around the source area for the Cambridge drinking-water supply system. Chloride responses, calculated from the specific conductance data, water temperature variation and precipitation events were evaluated for signs of road salt migration patters. A particular emphasis is placed on the most recent two years of specific conductance, stream-flow, and water temperature data. Utilization of data from six real-time monitoring stations located around Hobbs Brook and Stony Brook reservoirs allows for 15 minute examination of the specific conductance which relates to the amount of sodium and chloride, two main constituents of road salt that are responsible for the deterioration of water quality. The data indicate that salt-bearing highway runoff is responsible for the observed high chloride concentrations that range from 500-2500 mg/L at Cambridge Reservoir Unnamed Tributary 2, near Lexington, MA. Furthermore, the persistent high concentrations of chloride measured at the stations over the course of an entire year suggests that the the road salt contamination is extensive through out the region.