Inventory of Road Salt Use in the Minneapolis/St.Paul Metropolitan Area

Abstract

In the northern regions of the U.S. and in Canada a water contaminant of increasing concern is chloride. The source of chloride contamination is mostly sodium chloride (NaCl) which is used in large quantities for road de-icing and for water softening. Chloride enters into the environment easily in snowmelt runoff and in wastewater treatment plant discharges. Because it is a fairly conservative material it can travel long distances without assimilation. It affects the quality of aquatic ecosystems. Because of its adverse effects on plants and aquatic animals, MPCA standards of 230mg/L and 800 mg/L have been set for chronic and acute exposure. The Twin Cities Metropolitan Area (TCMA) is one of the regions where salt is used for winter road maintenance and for water softening. The TCMA is an urbanized area with a population of about 2.7 million people. It is located at about 45oN latitude and has a reputation of cold and long winters. The TCMA is traversed by the Mississippi River and several of its tributaries, and claims to have 949 lakes within its seven counties and over 186 cities and townships. There are over 26,000 lane miles of roadways with impervious surfaces in the TCMA. In this report an inventory of the amount of road salt (NaCl) applied in the TCMA each year is given. This study was not aimed at evaluating the effectiveness, suitability or cost of using NaCl as a de-icer. In addition to the total amounts of salt used, it is of interest to identify and quantify the metrics that determine the rates of road salt application. The spatial and temporal scales will be large enough so that the results of this study can be compared to other regions in northern climates. Many sources of information regarding the use of road salt in the TCMA and the state of Minnesota were consulted. In the TCMA, most municipalities and counties purchase salt through contracts offered by the Material Management Division (MMD) of the state of Minnesota. The average amount of reported salt use per winter season (2001 to 2006) was 236,800 tons per year and varied between 243,000 and 274,000 tons per year. Relative to the sum of the contract amounts, the actual reported use was on average 93% for the TCMA. Not all municipalities participate in the MMD contract or keep track of the actual road salt applications. The amount of road salt used by municipalities without information was estimated based on a per capita road salt use rate of 0.045 tons per season per person. The rate was determined from the reported road salt use in the TCMA covering about 90% of the population.