Road Salt Effects on the Water Quality of Lakes in the Twin Cities Metropolitan Area

Abstract

Approximately 349,000 tons of road salts (NaCl) are applied annually for road de-icing in the Twin Cities Metropolitan Area (TCMA) of Minnesota. To determine if and how 13 lakes in the TCMA respond to seasonal applications of road salt, the ionic composition of lake water samples was analyzed, and specific conductance and temperature profiles in the lakes were measured over a 45-month period. The lakes were selected based on four criteria one of which was their proximity to roadways. Natural lakes in Minnesota are dominated by calcium, sulfate and carbonate ions. Sodium and chloride are, however, the dominant ions in the TCMA lakes studied. The one to one stoichiometry of sodium and chloride in the lakes suggests that NaCl is the source of both ions. Concentrations of the two ions were linearly related to specific conductance. Chloride concentrations in TCMA lakes have been increasing over several decades since the 1950 when road salt applications started to become popular. Long-term increasing trends in specific conductance in 39 lakes in the TCMA mimic amounts of rock salt purchases by the state of Minnesota. If current trends continue the chloride content of TCMA lake waters will continue to rise. Chloride concentrations in TCMA lakes have a seasonal cycle; concentrations are highest during the winter and early spring months and lowest during the late summer and fall. This cycle matches road salt applications and snowmelt runoff in winter/spring, and flushing of lakes by rainfall runoff during the summer.