TCMA Chloride Management Plan - Background and Description

Background and Description

Twin Cities 7-County Metropolitan Area (TCMA)

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.

A Chloride Feasibility Study for the TCMA (Phase 1) was completed in December 2009. This study improved understanding of the extent, magnitude, and causes of chloride contamination of surface waters and explored options and strategies for addressing impacts. 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.

In 2010, the MPCA initiated the TCMA Chloride Project. It built on the previous work to improve and maintain water quality with respect to chloride for the TCMA. A robust stakeholder involvement process was undertaken to develop partnerships and gain insight into winter maintenance activities and other sources of chloride. This process allowed the stakeholders to assist in the development of the CMP and has generated the support of local partners. This effort consisted of over 115 participating stakeholders on seven teams; an inter-agency team (IAT) made up of state government agencies, a technical advisory committee (TAC) consisting of local stakeholders, a monitoring advisory group (MSG) with local and state water quality monitoring experts, an Education and Outreach Committee (EOC) that included local education specialists throughout the TCMA, a technical expert group (TechEx) which was comprised of winter maintenance professionals, and an implementation plan committee (IPC), which was a combination of all the teams.

The Problem with Too Much Chloride

Low levels of chloride can be found naturally in the TCMA lakes and streams and is essential for aquatic life to carry out a range of biological functions. However, high concentrations of chloride in the surrounding water harm aquatic life as a result of a disruption in the cellular process called osmosis which moves molecules, such as water, through cell membranes. Too much chloride in the surrounding water can cause water to leave the cell and also prohibit the transport of needed molecules into the cell. If elevated concentrations of chloride persist in the water, aquatic life such as fish, invertebrates, and even some plant species become stressed and/or die. The MPCA has adopted the United States Environmental Protection Agency’s (EPA) recommended water quality criteria for chloride, which is designed to protect aquatic life from the harmful effects of excessive chloride. The allowable chloride concentration to protect for acute (short-term) exposure is 860 mg/L. The allowable chloride concentration to protect for chronic (long-term) exposure is 230 mg/L. These values were developed based on toxicity test results for a range of freshwater aquatic organisms. Short-term exposure (one hour or more) to concentrations greater than 860 mg/L or continued exposure (four days or more) to chloride concentrations greater than 230 mg/L can be expected to have detrimental effects on community structure, diversity, and productivity of aquatic life.

Increased chloride concentrations due to salt applied to paved surfaces in winter can also have indirect effects on biota. Additives and contaminants such as phosphorus, cyanide containing compounds, copper, and zinc may cause additional stress or accumulate to a potentially toxic level (Wenck 2009). Impacts on water quality in lakes, wetlands and streams are not the only concern related to high levels of chloride in the environment. Chloride can affect groundwater and drinking water supplies, infrastructure, vehicles, plants, soil, pets, and wildlife. The Phase 1 Feasibility Study documented the results of a literature review on the impacts of chloride from salt. Research identifies the negative impacts that chloride has on the environment, whether from pavement salt sources or water softeners, but there are still many unknowns. Continued research will help us understand how chloride interacts with the environment and therefore, how to protect our water resources. Additional concerns associated with chloride in the environment, including an analysis of the estimated cost of those impacts, are discussed below.