Seasonal Salinity Cycles in Eight Lakes of the Minneapolis/St. Paul Metropolitan Area

Abstract

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.