TRACING ROAD SALT APPLICATION THROUGH SURFACE WATER IN THE NASHUA RIVER WATERSHED, MASSACHUSETTS

Previous studies have demonstrated that the application of road salt during winter months can result in high concentrations of dissolved chloride in surface water due to direct runoff of chloride bearing melt water. Moreover, elevated chloride concentrations can continue into summer months due to subsurface salt storage and base flow contributions to the surface water. The elevated salt concentrations negatively impact flora and fauna, cause nutrient depletion in soils through ion exchange, and can mobilize potentially toxic metals. Based on USGS water quality data, it can be shown that there is a strong correlation between chloride concentration and specific conductance in rivers and streams throughout Massachusetts. Thus specific conductance can be used as a proxy for dissolved chloride. In Massachusetts, as well as in other heavily salted regions, the data indicates that dissolved chloride concentrations have been steadily increasing in surface and shallow groundwater for decades.

To further our understanding of the effects of road salt application on increases in dissolved chloride concentrations during summer months, beginning in the summer of 2010, several surface water bodies in the southern region of the Nashua River watershed, within Worcester County, Massachusetts, were sampled for specific conductance. Using the correlation model developed from the USGS historic data, dissolved chloride concentrations were calculated and show that sample locations within heavily forested areas have very low dissolved chloride, while sample sites located around the major highways yield the highest dissolved chloride concentrations. During rainfall events, the chloride concentration decreases in streams consistent with the dilution of the stored salt load by fresh rainwater. The concentration of dissolved chloride increases with time after the rainfall events have ceased reinforcing the importance of base flow as a carrier of dissolved salt to the surface water.

The strong correlation between specific conductance and dissolved chloride is evident in other regions of the US where road salting is common. These models have similar slopes to the model produced for this study suggesting that the models could be combined to produce a master curve suitable for use in all regions.