A STUDY OF DISSOLVED CHLORIDE PATHWAYS IN AN UNCONFINED AQUIFER IN SE MASSACHUSETTS

Road de-icers, predominantly composed of sodium chloride or calcium chloride salts, are applied during the winter to ensure safe driving conditions during snow storms. Over time, dissolved chloride is retained in subsurface soils and groundwater which can result in negative health impacts with reduced quality in drinking water. For 20 years, the increasing chloride concentration in the Old Pond Meadow aquifer has become a concern for the local population. The Old Pond Meadow aquifer is an unconfined aquifer that covers 28.9 square kilometers that provides the majority of the public water supply to the towns of Norwell (pop 10, 506) and Hanover, MA (pop 13,879). The range of salt application is between 582 and 1784 tons per year, with additional unquantified leakage from the two large salt storage facilities.

Dissolved chloride is eventually removed from the drainage area by two return flow pathways. The first pathway is direct runoff, which bypasses the subsurface, and the second is storage in the subsurface with eventual discharge to streams via baseflow. The objective of this study is to quantify the amount of dissolved chloride removed from the Old Pond Meadow aquifer by these two return flow pathways, and to track the movement of the salt plume(s) through the surface water drainage -- Third Herring Brook (THB) -- for the 2013/2014 winter season. We will continually measure water quality paramenters using Iin-Situ, Inc. AquaTroll-200 probes which will measure specific conductance (as a means to measure chloride, a proxy for NaCl & CaCl), temperature, and water column depth every 15 minutes simulaneously at 5 selected intervals along THB. To quantify the total dissolved load of chloride in Third Herring Brook, streamflow is measured via Sontek ultrasonic device calculations. This information facilitates more accurate projections of future contamination, locates possible point sources of road salt, and will aid in remediation efforts.