TCMA Chloride Management Plan - Background and Description
Background and Description
The TCMA includes 186 cities and townships and a population of approximately 3,000,000 people. It covers approximately 3,000 square miles with about one-third in urbanized areas. It is a vibrant and growing community. The area is fortunate to be home to nearly 1,000 lakes and wetlands, small streams and large rivers, as well as shallow and deep groundwater aquifers. These water resources hold high value to the community and visitors to the area.
The Twin Cities receives approximately 54 inches of snow each year on average. The thousands of miles of streets and highways in the TCMA, along with parking lots and sidewalks, must be maintained to provide safe conditions throughout the winter. Winter maintenance of these surfaces currently relies heavily on the use of salt, primarily sodium chloride (NaCl), to prevent ice build-up and remove ice where it has formed. The chemical properties of NaCl make it effective at melting ice, but these properties also result in the chloride dissolving in water and persisting in the environment. The dissolved chloride moves with the melted snow and ice, largely during warm-up events, and ends up in the water resources. Salt applied in winter for deicing in urban areas is a major source of chloride to Minnesota surface waters and groundwater.
Residential water softener use is also a significant source of chloride. Residential water softeners use chloride to remove hardness, which is typically caused by high levels of calcium and/or magnesium. In areas with hard water, residential water softeners which use salt are common. The chloride from water softeners makes its way to the environment either through discharge to a septic system or by delivery to a municipal WWTP. Chloride is not removed from wastewater using conventional treatment methods. However, chloride can be removed from wastewater by using reverse osmosis (RO) technology, which is considered cost-prohibitive for an issue of this scale.
Elevated chloride concentrations have been found in waterbodies throughout the TCMA. At levels exceeding the WQS, chloride is toxic to aquatic life. Water quality samples from lakes, wetlands, streams and groundwater show increased chloride levels in urban areas across the state. While monitoring has only been conducted for about 10% of all the surface waterbodies in the TCMA, the available data indicates 39 waterbodies in the TCMA currently exceed chloride levels protective of the aquatic community. Two of these impaired waterbodies have approved TMDLs (Shingle Creek and Nine Mile Creek). These high concentrations call for immediate attention to the issue, the development of a plan to restore waters already impaired, and for protection of waters at risk of further degradation.