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 (Massachusetts Highway Department, 2009). Road salt, predominately sodium chloride (NaCl) contaminates surface water, groundwater, and soils. Road salt contamination is correlated with the depletion of soil nutrients (Environment Canada, 2001), the release of other contaminants (Kelly et al., 2008), damage to infrastructure (Kelly et al., 2008), decreases in native animal, plant, and bacteria populations, the proliferation of invasive species (Kelly et al., 2010; Environment Canada, 2001), water body stratification and stagnation (Hon et al., 2010), and a decrease in water quality (Kelly et al., 2010). This study evaluates the relationship between land use and the extent of road salt contamination within a 23.31 square mile semi-urban watershed in eastern Massachusetts with particularly diverse land use characteristics within its subcatchments. The watershed has been divided into 6 different land use groups, 1) a pristine lake, 2) a public water reservoir, 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 discharge at 29 selected sites for duration of 1 year. 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 Na⁺ and Cl^- concentrations in groundwater throughout the watershed from natural levels below 5 mg/L to as high as 150 mg/L and 433 mg/L, respectively. The highest levels of contamination are observed year-round near major highways and dense residential neighborhoods. Road salt budget models show that only about 50 % of road salt applied each year exits the watershed as dissolved load in stream discharge. The rest remains in the aquifer system. From the hydrograph separation calculations groundwater (baseflow) contributes approximately 75 - 85 % of the total road salt removal from the watershed each year. The remaining 15 - 25 % is removed from the watershed during storm event flows.