DISSOLVED CHLORIDE TRANSPORT PATHWAYS IN A SMALL WATERSHED IN EASTERN MASSACHUSETTS

Chloride based deicers, commonly referred to as road salt are the most common substances used for the purpose of roadway ice and snow removal. Upon application, road salt readily dissolves and dissociates, causing increased concentrations of the respective ions in nearby soils, surface water and groundwater. Of the ions present in road salt, chloride is the most conservative and is the focus of this study.

For this study, we use a robust, four year dataset consisting of continuous (15 minute interval) specific conductance and discharge records collected by the USGS for the Saugus River site from 2003 to 2007. Specific conductance values were empirically calibrated to represent chloride concentrations which provided ways to indirectly monitor chloride variations in the stream channel. The real time chloride concentration dataset, coupled with the real time stream flow rates can be combined to calculate total dissolved chloride loads discharging from the Saugus River. The dissolved chloride loads exhibit seasonal fluctuations in flow rate and dissolved chloride load. During the winter months of December and January, when road salt use is at its peak, the Saugus River averaged a flow rate of 55 CFS transporting an average of 790 kg of chloride per hour (a total of 1,280 tons for 2 months). In contrast, during the summer months of July and August the Saugus River averaged a flow rate of 17 CFS transporting an average of 228 kg of chloride per hour (a total of 340 tons for the 2 month period). From hydrograph separation of precipitation events, groundwater base flow discharge contributes the bulk of the dissolved chloride load in the summer months. We present an analysis of the dissolved chloride load distribution during precipitation events in the summer and winter seasons. The results of analysis on Cl transport mechanisms in the environment show that the migration pathways of salt are complex involving at different times and in different proportions aquifer flow, baseflow, interflow, overland flow, evaporation, and transpiration processes.