Study of Environmental Effects of De-icing Salt on Water Quality in Minnesota

Project summary:

The use of sodium chloride as a de-icing chemical has increased dramatically in northern areas of the United States over the last 50 years. Leading states in highway salt use are Minnesota, Michigan, New York, Ohio, and Pennsylvania. About 10 million tons of salt are added to the runoff in the United States each year, which eventually finds its way into storm sewers, streams, rivers, lakes, and groundwater. Chloride concentrations of 30 mg/L in soils have been found to be lethal to some land plants, and chloride concentrations of 1000 mg/L have been shown to have sub-lethal effects on aquatic plants and invertebrates. In Minnesota, water quality standards of 280 mg/L and 800 mg/L are exceeded in some fresh water bodies. The objectives of this project were: 1) to assemble existing data on the use of salt in every form and application (from the de-icing salt to water softener salt) and on the salinity/sodium chloride concentrations measured in streams, rivers, lakes, and groundwater in Minnesota, especially in the Twin Cities area; and 2) to interpret these data by statistical analysis to establish typical mean values, standard deviations, and trends in two ways: by regression analysis to establish correlations (e.g. with population density, road lengths, and snowfall) and by development of chloride budgets. The results of this analysis provided baseline information on the long-term effects of salt applications on water resources at a regional (metropolitan) and a local watershed (lake) scale. In addition, several specific lakes that are prone to receiving de-icing salt during snowmelt runoff were monitored monthly for approximately two years. The data provided information on salt use and environmental effects of de-icing salt on water resources at a seasonal time scale, including salt residence times and trends in chloride concentrations of lakes and rivers. Results produced by the study included: 1) about 350,000 short tons of road salt (NaCl) are applied in the Twin Cities metro area every year; 2) the salt load of the Mississippi River increases substantially as it passes through this area; 3) many Twin Cities metro area lakes have unnaturally high sodium and chloride concentrations, and these chloride concentrations have a rising trend; 4) area lakes have increasing salinity in winter and decreasing salinity in summer, which corresponds to seasonal road salt applications; 4) about 70% of the road salt applied in this area is not collected and carried away by the Mississippi River; and 5) shallow groundwater aquifers in urban areas or near major roadways have started to show increasing chloride concentrations.