FLUCTUATIONS IN CHLORIDE LOADING DURING THREE PRECIPITATION EVENTS IN SAUGUS RIVER, MA

Detailed review of simultaneous and continuous records of specific conductance and discharge data for recent precipitation events recorded at a USGS monitoring station (near Saugus Iron Works National Historic Site on Saugus River) indicates that specific conductance changes during precipitation do not represent a simple binary mixture of overland runoff and base flow discharge. In this study, stream chemistry changes are evaluated using high-frequency water sampling near the Saugus Iron Works USGS gauging station in Saugus, MA, a suburban area north of Boston. At the sampling site, the Saugus river drains an area of 20.8 square miles. In the Saugus River and other urbanized streams in Massachusetts, where road salt is regularly applied during winter, there is a strong empirically determined linear relationship between specific conductance and chloride concentration. Chloride is stored in aquifers throughout the year and discharged as a dissolved component of baseflow. During autumn, when road salt is not applied to the surface, groundwater baseflow is the only source of dissolved chloride to the stream. Precipitation events during that time of year cause temporary drops in conductance during the surface runoff and interflow regimes of the hydrograph, indicating dilution. Using measured chloride concentrations from field sampling with a 15-minute interval and continuously averaged instantaneous discharge data, dissolved chloride load in Kg/15 minutes was obtained. Analyzing these changes over the course of a precipitation event indicates that while concentration (mg/L) of chloride decreases, the dissolved chloride load increases. This indicates a possible contribution during precipitation events of deeper groundwater circulation. Three precipitation events occurring in the months of October, November, and December were intensely sampled. A robust dataset of continuous (15-minute interval) specific conductance and discharge measurements was collected during the time of the events by the USGS (Station number 01102345). I will present data from this dataset and a total of 95 samples taken during the events.