Potential for Groundwater Contamination by Road Salt in Minnesota

Abstract

In northern states of the U.S. and in virtually all provinces of Canada sodium chloride is becoming a water contaminant of increasing concern. The source of contamination is mostly anthropogenic: Salt (NaCl) is used in large amounts for road deicing and water softening. Sodium chloride enters into the environment easily in snowmelt runoff and from wastewater treatment plant discharges. Sodium chloride is not removed by conventional waste water treatment. It is a fairly conservative material and can travel long distances without assimilation. It affects terrestrial and aquatic ecosystems. Because of its adverse effects on plants and aquatic animals, water quality standards for Cl have been set, e.g. by the Minnesota Pollution Control Agency (MPCA) at 230mg/L and 800 mg/L for chronic and acute exposure, respectively. The Minneapolis/St. Paul (Twin Cities) Metropolitan Area (TCMA) is one of the regions where sodium chloride 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 in seven counties. 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 188 cities and townships. There are over 26,000 lane miles of roadways with impervious surfaces in the TCMA. In previous reports of this study we have given a budget for road salt applications in the TCMA and field observations of salinity in lakes of the TCMA. The goal of this study is to give an overview of measured chloride and sodium concentrations in groundwater (wells) of the TCMA. The information will be compared to statewide observations in Minnesota, and to other regions in northern climates. Changes in groundwater salinity in the TCMA and Minnesota over time, particularly the last 50 years are of particular interest, because it is suspected that aquifers are the ultimate recipients (sinks) of some of the road salt applied at the ground surface. This study is not aimed at evaluating the effectiveness, suitability or cost of using NaCl as a deicer.