Road deicing salt impacts on urban water quality.

Abstract

Salt is widely used for ice and snow control on roads in the US, Canada and other parts of the world affected by adverse winter driving conditions. The primary product applied in North America for deicing of roads is sodium chloride (NaCl), a readily available and inexpensive material. The use of sodium chloride as a road deicing chemical has increased dramatically in the northern areas of the United States over the last 50 years with about 23 million tons used in 2005. The primary objective of my research was to determine how road salt applications influenced the water quality in a major metropolitan area (Twin Cities metropolitan area (TCMA) of Minneapolis/St Paul, Minnesota, USA). This objective was met by collecting and analyzing data from area lakes, streams and rivers and through the development 0 Dimensional (0D) and 1 Dimensional (1D) models. It was determined that on average over 70% of the chloride applied annually in the TCMA was retained in the watershed instead of entering the Mississippi River and eventually exported to the Gulf of Mexico. Salinity cycles were observed in area lakes with high concentration in the winter followed by lower concentrations in the spring and summer. Mean annual concentrations in 38 lakes in the TCMA rose on average 1.4 mg/L per year over 22-years matching a similar trend in the amount of rock salt the state of Minnesota purchased over the same time period. Salt water inflows changed the natural mixing behavior of area lakes. In some lakes monomictic behavior developed with mixing events only occuring in the fall. The presence of a saline layer at the bottom of the lake prohibited dissolved oxygen from reaching the benthic water layer in the spring extending the anoxic period of this water layer by 6 months. Simulations conducted without the presence of a saline layer showed complete mixing and oxygenation of the benthic layers in the spring and fall. Rising Cl concentrations in lakes are expected to continue. If the annual inputs of salt to the lakes were stopped it would take 10 to 30 years to reach chloride concentrations equal to predevelopment concentration. Chloride retention in urban areas where road salt is applied should cause much concern. Mitigation measures, best management practices (BMPs) for road salt application and alternatives to NaCl need to be examined.