TCMA Chloride Management Plan - Background and Description
Contents
Background and Description
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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.
Chloride is persistent in the environment
Once chloride is in water, the only known technology for its removal is RO through massive filtration plants, which is not economically feasible. This means that chloride will continue to accumulate in the environment over time. A study by the University of Minnesota (UMN) found that about 78% of salt applied in the TCMA for winter maintenance is either transported to groundwater or remains in the local lakes, and wetlands (Stefan et al. 2008).
Surface Water
Chloride concentrations in lakes, wetlands and streams in the TCMA, as well as in many cold climate states have been increasing (Novotny et al. 2007; Novotny at al. 2008). Thirty-nine lakes, wetlands, and stream reaches are impaired for aquatic life due to high concentrations of chloride in the TCMA according to the MPCA’s 2014 Draft 303(d) List of Impaired Waters (MPCA 2014). Impacts on lakes include toxicity to aquatic life as well as the potential interruption of the vertical mixing (turnover) process.
It is difficult to put a financial value on the impacts of chloride impairments. However, the Adirondack Watershed Institute (Kelting and Laxson 2010) did a simulation of road salt impacts on surface waters and forests and showed a $2,320 per lane mile per year reduction in environmental value. If this value is applied to the 26,000 lane miles of roadways in the TCMA (Sander et al. 2007), the resulting estimate of economic impacts on surface waters and forests in the TCMA is roughly $60 million per year. On a cost per ton of salt basis, using 349,000 tons per year applied in the TCMA (Sander et al. 2007), the resulting reduction in environmental value is $172 per ton of salt. These are not actual out-of-pocket costs, but indicate the cost of the loss of environmental value.
Groundwater and drinking water
Groundwater is another important resource in Minnesota; about 75% of Minnesotans rely on groundwater for their drinking water supply (MPCA 2013). Groundwater also contributes flow to lakes, wetlands, and streams. Deicing salt application is resulting in higher chloride concentrations in groundwater. A recent MPCA study found that 30% of monitoring wells tested in shallow sand and gravel aquifers in the TCMA exceeded the state chronic standard for surface waters of 230 mg/L for chloride (MPCA 2013). The amount of sodium (a common component of salt) in drinking water is a human health concern, particularly for individuals on sodium restricted diets (EPA 2003; EPA 2014).
The cost of mitigating groundwater contamination is substantial. The EPA has set a Secondary Maximum Contaminant Level of 250 mg/L for chloride in drinking water, which is a guideline for protection based on taste (EPA 2014). According to a 1991 report, $10 million is spent nationally each year on mitigating impacts to groundwater from salt (Transportation Research Board 1991). The United States uses approximately 20 million tons of deicing salt per year (Anning and Flynn 2014). This equates to a cost of about $0.50/ton for mitigating groundwater impacts. A 1976 estimate (Murray and Ernst) was much higher, with a figure of $150 million per year for damages due to contamination of water supplies by deicing salt. This estimate includes more direct and indirect costs such as treating water, replacing wells, supplying bottled water, adding practices to prevent additional contamination, human health concerns, and property value damage. Using an estimate of 9 million tons of salt used in 1976, this equates to $16.67 per ton for damages to water supplies.
Lake Ecosystems
Chloride changes the density of water, which can negatively affect the seasonal mixing of lake waters (Novotny et al. 2008). Mixing increases oxygen levels required by aquatic life. Changes in mixing can also affect nutrient cycling processes, phytoplankton community composition and productivity, zooplankton community composition and phenology, and fish.
No value has been assigned to impacts on aquatic life due to chloride toxicity or impacts on lake ecosystems whose natural turnover is disrupted due to formation of a chemocline caused by salt. Prevention of turnover can result in anoxia in the bottom of lakes and potential death of aquatic biota (Michigan DOT 1993). Increased salinity can result in a loss of native plant species and invasion by invasive salt tolerant species such as Common Reed (Phragmites australis), Narrowleaf cattail (Typha angustifolia) and Eurasian watermilfoil (Myriophyllum spicatum) (Kelting and Laxson 2010). Salt can be toxic to fish at fairly high concentrations (Evans and Frick 2001).
Plants
Direct deicing salt splash can kill plants and trees along roadsides and plants can also be harmed by taking up salty water directly through their roots. When chloride flows into lakes, wetlands, and streams, it harms aquatic vegetation and can change the plant community structure.
Vitaliano estimated that the aesthetic damage to trees in the Adirondacks due to road salt was $75 per ton (1992). Research in the Adirondacks has shown that the application of deicers and abrasives on roads has severely changed the chemical and physical structure of soil along roads (Langen et al. 2006). The New York State Department of Transportation spent $10,000 per mile to replant and reestablish natural vegetation along a two-mile stretch of highway in the Adirondacks (NYSDOT Press Release 2008).
Soil
Soil along roadsides can be impacted by road salt (primarily the sodium) in a number of ways, including change in soil structure, effects on the nutrient balance, accelerated colloidal transport, mobilization of heavy metals, reduced hydraulic conductivity and permeability (Amundsen 2010; Michigan DOT 1993). These changes can lead to reduced plant growth. Soil structure changes also may result in increased erosion and sediment transport to surface waters (Kelting and Laxson 2010).
Pets and Wildlife
Pets may consume deicing materials by eating them directly, licking their paws, or by drinking snow melt and runoff, which can be harmful to pets. Exposure to deicing salt can cause pets to experience painful irritation, inflammation, and cracking of their feet pads. Some birds, like finches and house sparrows, have an increased risk of death if they ingest deicing salt. Deer and other large mammals consume the salt on roadsides and roadside ponds to fulfill their sodium needs, resulting in increased traffic incidents (Environment Canada 2001; Amundsen 2010). Exposure of amphibians to road salt can result in abnormalities, reduced growth, behavior changes, lower survival rates, and changes in community structure (Environment Canada 2001; Denoël et al. 2010; Karraker 2008; Collins and Russel 2009). Deicing salt may also cause a decline among populations of salt sensitive species, reducing natural diversity.
Infrastructure
Chloride corrodes road surfaces and bridges and damages reinforcing rods, increasing maintenance and repair costs. The costs associated with infrastructure are based on damage to infrastructure and maintenance and replacement costs associated with this damage. A study by economist Vitaliano, included an estimate of expenditures of an additional $332 per ton of salt per season for bridge maintenance (1992). One ton of road salt results in $1,460 in corrosion damage to bridges, and indirect costs may be much higher (Sohanghpurwala 2008). The total annual cost of bridge decks damages due to road salt was estimated at greater than $500 million nationwide (Murray and Ernst 1976). Costs would be substantially higher now.
In addition to damage to bridges, chloride deicers also damage concrete pavement, requiring higher maintenance costs. Vitaliano (1992) estimates an overall increase in roadway maintenance costs of over $600 per ton. This figure is believed to include the extra cost due to damage to bridges. Salt applied to pavement is also damaging to parking garages and underground utilities (Michigan DOT 1993).
Vehicle Corrosion
Deicing salt also accelerates rusting, causing damage to vehicle parts such as brake linings, frames, bumpers. Vitaliano (1992) estimated that vehicle depreciation due to corrosion from road salt results in a cost of $113 per ton of salt. Automobile manufacturers have improved corrosion resistance in cars since the 1992 study; however, measures to protect vehicles against corrosion cost auto manufacturers an estimated $4 billion each year, which is passed on to consumers (Adirondack Council 2009).
Cumulative Costs
Estimates of damage to infrastructure, automobiles, vegetation, human health and the environment due to road salt were found in the literature from several sources. They ranged from $803 to $3,341 per ton of road salt used.
Table 1 shows the overall range of the cost estimate with a low and high range as well as the estimated associated cost for the TCMA based on 349,000 tons of salt applied per season.