INVERSION OF CONTINUOUS SPECIFIC CONDUCTANCE DATA: EVIDENCE FOR ROAD SALT CONTAMINATION OF AQUIFERS IN EASTERN MASSACHUSETTS

Recent deployments of continuous (USGS real time) specific conductance sensors at a number of monitoring sites in Eastern Massachusetts yield robust datasets of specific conductance, streamflow, and water temperature for every 15 min time interval. This paper presents findings of a study for a site located near an interstate multilane highway (I95/RT128 corridor), exposed in winter months to a heavy road salt loading that has a small perennial discharge flow from a small drainage area. Baseflow values are typically less than 1 cfs. The dataset include 15 min readouts of specific conductance and flow rates for 2005 and 2006 water years. The specific conductance data were inverted to chloride concentrations using an empirical equation (regression of regional data of Cl concentrations and sp.cond.) and a semiempirical equation of Granato and Smith (1999). Dissolved chloride loads, chloride concentrations, and precipitation data were recalculated for 30 min intervals for each water year. Chloride concentration show a uniform pattern/range for baseflow dominated intervals throughout the year usually between 500 and 1000 mg/L. Peak flows in winter have chloride levels in excess of 2,000 mg/L often exceeding 5,000 and occasionally reaching up to 10,000 mg/L. In summer, chloride levels for peak flows fall sharply to 20-50 mg/L range (dilution by overland flows). However, chloride concentrations always return to approximately the same values during the baseflow dominated discharge intervals. No broad scale variations for baseflow are observed. Dissolved chloride loads patterns reflect local hydrological regime, i.e. time intervals of higher dissolved chloride loads coincide with higher flow discharge rates (higher hydraulic gradient). The observed patterns suggest that during the deeper aquifer discharge periods the dissolved chloride load is higher and vice versa. We attribute the observed pattern to sinking of denser saltier water toward the lower aquifer zones during the melting of snow banks containing embedded dissolved road salt. Such ground water recharge process will inevitably result in a density stratified aquifer and will have long term consequences on mitigations of road salt contaminated aquifers.