Metro Area Chloride Project history

The Minnesota Pollution Control Agency (MPCA) recently completed a Metro Chloride Feasibility study to obtain a better understanding of the extent, magnitude, and causes of chloride contamination to surface waters in the seven county Twin Cities Metropolitan Area (TCMA) and to explore options and strategies for addressing chloride impairments and other impacts to water resources. This project included extensive data analysis, a literature review, a telephone survey, and analysis of potential strategies for further research, public education, and potential regulation.

A telephone survey of local municipalities was also conducted as part of this study to better understand current chloride reduction practices undertaken by county and local road authorities. Survey results indicate counties and many larger cities are beginning some chloride reduction activities. Cost restrictions of new technologies and alternative products can be barriers to their wider implementation. A lack of data on cost effectiveness and load reduction rates has also resulted in some reluctance to more widely adopt these practices.

A multi-agency team led by the MPCA was put together to provide guidance to the project team.  This team consisted of partners from:

• MPCA
• Minnesota Department of Transportation (Mn/DOT)
• Metropolitan Council
• Board of Water and Soil Resources
• University of Minnesota, St. Anthony Falls Laboratory
• Wenck Associates, Inc.

The full report can be found in the Project Reports section below.

Technical Information

• Four primary sources of chloride are Mn/DOT, counties, municipalities, and private applicators.
• Data was retrieved from STORET (MPCA data storage), the U.S. Geological Survey and Metropolitan Council.
• There are roughly 35,700 chloride data points from lakes, streams and groundwater.
• Toxicity-based standard: chronic standard is 230 mg/L and the maximum standard is 860 mg/L.
• The majority of data values greater than 230 mg/L (217/295) occur during the winter (November through March).
• Chloride data is available from only about half of the Metro watersheds.
• Twenty percent of the chloride data was collected during the winter period.
• Chloride levels are typically higher in deeper portions of the lake.
• Combined average annual application mass (estimated using purchasing records) with lane mile information was provided by Mn/DOT (see graph below).
• Typically, higher chloride concentrations were found in lakes in the more developed core of the TCMA.
• De-icing material impacts on lake biota and lake mixing were analyzed.
• Proportions of road salt applied by major sources including municipal, Mn/DOT, county and commercial application for TCMA watersheds were estimated.

This graph shows where the largest amount of road salt is being applied (based on purchasing records) in the TCMA. The watersheds with the highest application rates are the most likely to have chloride exceedances in their waters now or in the near future if current rates continue.

Road salt application rates in the TCMA

Tons of road salt applied per square mile in the TCMA*
*application rates are based on salt purchasing records                                      

Project Reports

• Chloride Feasibility Study for the Metropolitan Twin Cities Area - Phase 1 report
  • Appendix A - Stream Monitoring Locations by Watershed and AIUD
  • Appendix B - Stream Chloride Data by Site Year
  • Appendix C - Stream Conductivity Data by Site Year
  • Appendix D - Chloride-Conductivity Regression Graphs Stats
  • Appendix E - Lake Chloride Data by Site Year
  • Appendix F - Lake Chloride by Site Depth
  • Appendix G - Annotated Bibliography
  • Appendix H - BMPs
  • Appendix I - Telephone Survey Results
  • Appendix J - Maps

Fact sheets and guidance documents

• Phase 1 - Feasibility Study Summary Fact Sheet
• Phase 1 Project Reports

Next steps

In the next phase of the project, the MPCA will work with the multi-agency team and local stakeholders to develop a chloride restoration and protection plan which will satisfy total maximum daily load requirements for impaired waters, address waters not yet listed, and protect waters that are not yet impaired. This plan will also include implementation activities for road salt and chloride load reductions in the TCMA.

For more information

For more information, go to the Road salt and water quality webpage or or contact the MPCA at 800-657-3864 or 651-296-6300 and ask for the Metro Watershed Unit.