LAND USE PRACTICES AND THE EXTENT OF ROAD SALT CONTAMINATION IN A SEMI-URBAN WATERSHED, EASTERN MASSACHUSETTS

To maintain safe winter driving conditions in Massachusetts, an average of 19.4 tons of road salt is applied per roadway lane-mile each year. Road salt, predominately sodium chloride (NaCl) contaminates surface water, groundwater, and soils. Road salt contamination is correlated with the depletion of soil nutrients, the release of contaminants, damage to infrastructure, decreases in native animal, plant, and bacteria populations, the proliferation of invasive species, water body stratification and stagnation, and a decrease in water quality. This study evaluates the relationship between land use and the extent of road salt contamination within a 23.3 square mile semi-urban watershed in eastern Massachusetts with particularly diverse land use characteristics within its subcatchments. 29 surface water sampling sites were chosen and categorized based on the dominant land use characteristics in their drainage areas: 1) pristine lakes, 2) public water reservoirs, 3) residential neighborhoods, 4) areas with major highways, 5) ocean influenced areas, and 6) areas with mixed land use characteristics. Surface water samples were collected during periods when stream flow was dominated by groundwater baseflow for duration of 15 months. All samples were analyzed for concentrations of major ions (F^-, Cl^-, NO₂^-, Br^-, NO₃^-, PO₄^-3, SO₄^-4, Li⁺, Na⁺, NH₄⁺, K⁺, Mg⁺², and Ca⁺²) using ion chromatography. Results show that the application of road salt has increased the background Cl- concentration in surface and groundwater throughout the watershed from natural levels below 5 mg/L to as high as 250 mg/L. The highest levels of contamination are observed year-round near major highways and dense residential neighborhoods. In the most urbanized areas, the concentration of Cl- in discharging groundwater can exceed 840 mg/L. These extreme concentrations are only observed during the late summer and early fall when the groundwater reservoir is at its lowest levels. The results suggest the existence of a Cl- concentration-stratified aquifer with denser, more concentrated water being overlaid by less dense and less concentrated water above it. Road salt budget models show that only about 50 % of road salt applied each year exits the watershed as dissolved load in streamflow.