ROAD SALT PATHWAYS IN URBAN WATERSHEDS WITHIN EASTERN MASSACHUSETTS

Chloride-based deicers (NaCl, CaCl₂, MgCl₂), commonly referred to as road salt, are the most frequent substances used in maintaining safe roadway surfaces during the winter months. Upon application, road salt establishes equilibrium with the accumulated snow or ice along the liquidus curve in the salt-water binary system, leading to increased concentrations of the respective ions in nearby soils, surface water, and groundwater. Chloride ions behave as conservative tracers in the subsurface environment unaffected by any ionic exchange interferences. In this paper we present data on the mechanisms of chloride return flow during different seasons over the course of four years (2003 – 2007) in the Saugus River, located in the greater Boston area. The Saugus River watershed is highly urbanized with a population density of 2,291 people per square mile, has 369 total miles of roadway, and approximately 626 total lane miles of roadway within its borders.

The chloride tracking technique used in this study uses calibrated chloride concentrations obtained from specific conductance signals recorded every 15 minutes at the USGS gauging station on the Saugus River located near the Saugus Iron Works National Historic Site. The dissolved chloride load carried by the Saugus River is calculated for each single 15-minute interval by multiplying water volume (recorded simultaneously with specific conductance) with the corresponding chloride concentration, yielding a total of over 34,000 data points per annum. Hydrograph separation techniques were used to separate dissolved chloride load into two separate flow components, event flow resulting from precipitation events, and baseflow resulting from groundwater discharge. Baseflow supported periods of stream flow yield consistent chloride concentration of 140 +/- 40 mg/l, independent of a season. On average over 4,000 metric tons of dissolved chloride per year is transported from the watershed; 1,000 metric tons of which is transported during the months of January and February and approximately 220 metric tons of dissolved chloride during the months of August and September. The average yearly baseflow contribution of dissolved chloride discharged by the Saugus River is over 2,500 metric tons, with the remaining 1,500 metric tons removed by the event flows.