Chloride ion transport and mass balance in a metropolitan area using road salt

Abstract

In the Twin Cities metropolitan area (TCMA) of Minneapolis/St. Paul, Minnesota, an estimated 317,000 metric tons (t) of road salt were used annually for road deicing between 2000 and 2005. To determine the annual retention of road salt, a chloride budget was conducted for a 4150 km2 watershed encompassing the populated areas of the TCMA. In addition to inflows and outflows in the major rivers of the TCMA, multiple sources of chloride were examined, but only road salt and wastewater treatment plant (WWTP) effluents were large enough to be included in the analysis. According to the chloride budget, 235,000 t of chloride entered the TCMA annually with the Mississippi and Minnesota rivers, and 355,000 t exited through the Mississippi River. Of the 120,000 t of chloride added annually to the rivers inside the TCMA watershed boundaries, 87,000 t came from WWTPs and 33,000 t came from road salt. Of the 142,000 t of chloride applied annually in the TCMA watershed as road salt (241,000 t NaCl), only 23% (33,000 t) were exported through the Mississippi River and 109,000 t or 77% were retained in the TCMA watershed. Chloride budgets for 10 subwatersheds within the TCMA analyzed in a similar way, gave an average chloride retention rate of 72%. The retention is occurring in the soils, surface waters (numerous lakes, wetlands, and ponds) and in the groundwater. Chloride concentrations in many of these urban water bodies are now considerably higher than the presettlement background levels of less than 3 mg/L with concentrations as high as 2000 mg/L in shallow groundwater wells. The continued accumulation of chloride in the groundwater and surface waters is a cause for concern.