Substantial amounts of road salt are spread annually on highways, streets, sidewalks, and
parking lots in the northern regions of the U.S. Road salt application is considered an
economic necessity to keep roads free of ice for safe winter travel in northern climate
zones. Commonly used road salts to deice roads are sodium chloride (NaCl) and calcium
chloride (CaCl2). Because of a large difference in cost, NaCl is applied in much larger
quantities. Snowmelt runoff containing dissolved road salt feeds many Twin Cities Metro
Area lakes, but chloride levels in these lakes have not been studied explicitly. Little is
known about the fate of NaCl entering these lakes.
Eight urban lakes in the Minnepolis/St.Paul metropolitan area were therefore studied for
14 months during two winters and the summer in between. Specific conductance profiles
were measured at roughly bimonthly intervals in each of the eight lakes. Variations in
specific conductance with season and with depth were found in each of the lakes.
Specific conductance values varied from 400 to 1800 μS/cm in seven of the lakes and
reached a maximum of 3500 μS/cm in the eighth lake which was meromictic. The largest
specific conductance values were in late winter and at the bottom of the lakes, and the
lowest in late summer near the surface of the lakes.
Chloride concentration was linearly related to specific conductance. Chloride
concentration profiles were calculated from specific conductance, and integrated with
depth to give the total chloride and the total NaCl content. There was clearly a seasonal
pattern in all eight lakes, i.e. an accumulation of NaCl during the winter months and a
decrease in total salt content during the summer. Chloride concentrations near the lake
bottom in Brownie Lake and Ryan Lake exceeded the chronic MPCA chloride standard
of 230 mg/L during the entire 14 months of the field study. Chloride concentrations
exceeded the chronic standard near the lake bottom in McCarrons Lake during late spring
in 2004 and Medicine Lake during late spring in 2005.
Minnesota Local Roads Research Board, Center for Transportation Studies, University of Minnesota: Twin Cities
Murphy, Dan; Stefan, Heinz.
Seasonal Salinity Cycles in Eight Lakes of the Minneapolis/St. Paul Metropolitan Area.
St. Anthony Falls Laboratory.
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