ASSESSMENT OF VERTICAL PROFILES OF DISSOLVED ROAD SALT IN SUBURBAN AQUIFERS OF EASTERN MASSACHUSETTS

In this project, we investigate the transport of road salt in a typical aquifer in southeastern Massachusetts. The study area is a glacial basin that includes 5 public water supply wells for the Towns of Norwell and Hanover, MA. Water quality data collected over the last several decades show that the concentration of chloride in one of the wells has increased from around 30 ppm in 1986 to slightly over 250 ppm in 2012. The EPA designates 250 ppm to be the Secondary Drinking Water Standard for chloride. Upstream from this well are two major highways, large area of an impervious surface part of a shopping mall, and a salt storage facility.

In March 2011, four multi-parameter sensors were installed at different locations and depths within the aquifer. Based on higher specific conductance measurements over the past year at 3 different horizon depths (shallow, medium and deep aquifer zones), ground water with a higher chloride concentration of 255 ppm (and therefore higher density) lies above deeper water with lower chloride concentration of 160 ppm. Since these density differences have been observed for over a year it is suggested they do not force free convection in this environment. Advection and dispersion are likely the dominant modes of solute transport. Data from ion chromatography analysis of ground water and surface water samples show that although chloride and sodium are the most abundant electrolytes, there are also significant concentrations of magnesium and calcium. Unlike chloride, which is assumed to be a conservative tracer, the transport of major cations may be influenced by adsorption and cation exchange. A secondary but still significant environmental concern of road deicer runoff is the potential for mobilization of heavy metals from cation exchange of road salt cations with clays and organic matter. Other water quality data besides those collected by the sensors and ion chromatography analysis include stable isotope analysis (D & 18O) and inductively coupled plasma optical emission spectroscopy.