CONTINUOUS MONITORING OF INCREASING ROAD SALT CONCENTRATION IN GROUND WATER IN AN ACTIVE PUMPING FIELD IN MASSACHUSETTS

In March 2011, 4 AquaTroll 200 probes were installed across a vertical profile of an aquifer in Norwell, MA to monitor time variations of water quality (temperature, depth, and specific conductance). A twenty month record of data acquired every 15 minutes is further augmented by additionally installed probes along two local streams to provide data on the time variations of water quality at several locations. At these locations we anticipate a significant contribution of dissolved de-icing chemicals in baseflow discharge such as salt plume originating at a salt storage shed.

Several important trends of de-icing contaminations can be presently identified: (1) lasting aquifer stratification; (2) seasonal variations; and (3) continually increasing concentration from year to year.

The data record show vertical stratification of the aquifer where the upper zone has larger amounts of dissolved deicing chemicals (250 ppm) than the lower zone (150 ppm). This superposition does not change over the study period (Mar ’11 – Aug ’12).

The data show an increase in specific conductance of 25% during the late summer and a minimum during early spring. Although this pattern appears in all sensors, it is particularly noted for the upper part of the aquifer which is likely due to increased evapotranspiration.

One of the most critical observations is the continual rise of the salinity in the lower zone of the Norwell aquifer. In March 2011 the chloride concentrations (calculated from calibrated chloride vs. specific conductance measurements) were 155 ppm of Cl and in the summer of 2012 the concentration of chloride for the first time exceeded 180 ppm. The 12 month increase equals to 11 ppm of Cl/y. At this rate the entire aquifer will eclipse the secondary drinking water standard for chloride in just 6 years!