ANALYSIS OF SEVENTEEN CATCHMENTS IN A SUBURBAN WATERSHED FOR CONTAMINATION BY ROAD SALT

Road salt contaminates many water supplies where below freezing temperatures and snow are common in the winter. Increases of four to five times higher concentrations in 15 years suggest both greater salt use and basin retention of applied road salt raising sodium and chloride concentrations. Seventeen sampling locations in the drainage leading to Lake Waban in Wellesley, Massachusetts were analyzed for sodium by ion chromatography (IC) or inductively couple plasma emission spectroscopy (ICP) and their catchment areas delineated with StreamStats 4.0. Samples were taken during a summer dry spell, three days later after 2cm of rainfall, and after an extended drought period in September 2016.

The Massachusetts Turnpike (I-90) crosses some of the stream catchments, while others are entirely upstream. Stream sodium correlates with the number of lane meters of Turnpike (R squared =0.84) but much less well with total road surfaces within the catchments, showing only a weak positive trend. In addition, some catchments show significant decreases in sodium during summer, while others do not, suggesting most salt goes downstream in those areas. Correlation with the 6-lane Turnpike might indicate higher concentrations of salt application giving higher density brines which are partially retained year-round. During the 2016 drought, some stream segments became isolated pools and sodium is apparently reflective of groundwater chemistry and/or rainwater as it shows changes from 50 to 150 to 60 to 117 mg/L in one stream.

Measurements of Na concentration change in stream segments between sampling stations in mg/L-m stream segments showed increases (saline groundwater) and decreases (dilution), with the largest increases (0.1 mg/L sodium increase per meter of stream channel) opposite a highway maintenance facility with year-round salt storage and winter salt loading.