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New Hampshire Department of Environmental Services
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Environmental, Health and Economic Impacts of Road Salt
Overview

NHDOT snow plowNew Hampshire winters demand an effective and affordable means of de-icing roadways. The primary agent used for this purpose is sodium chloride (road salt), which is composed of 40 percent sodium ions (Na+) and 60 percent chloride ions (Cl-). Other components in salt like ferrocyanide, which is used for anti-caking, and impurities like phosphorus and iron, can represent up to 5 percent of the total weight. The sodium, chloride, ferrocyanide and impurities make their way into our environment through the runoff from rain, melting snow and ice, as well as through splash and spray by vehicles and by wind. They find their way onto vegetation and into the soil, groundwater, stormdrains, and surface waters causing significant impact to the environment.

Chloride (Cl-) is completely soluble and very mobile. Chloride is toxic to aquatic life and impacts vegetation and wildlife. There is no natural process by which chlorides are broken down, metabolized, taken up, or removed from the environment. In 2008, New Hampshire listed 19 water bodies impaired by chloride; in 2010 that number increased to 40. Trends show that chloride levels continue to rise with increasing use of road salt.

saltThe transport of sodium (Na+) in the environment is not as prominent as chloride due to ion exchange; however, this exchange can alter the soil chemistry by replacing and releasing nutrients into the groundwater and surface water changing soil structure and impacting the aquatic environment. Contamination of sodium in drinking water is a concern for individuals restricted to low-sodium diets due to hypertension (high blood pressure). Wildlife is also prone to high sodium levels by ingesting salt or drinking water runoff from snow and ice melt.

Additives to road salt like ferrocyanide, which is used as an anti-caking compound in large salt supplies, can have impacts on both the environment and human health due to cyanide ions being released by certain types of bacteria as well as from exposure to sunlight. The USEPA in 2003 added this compound to its list of toxic pollutants under section 307(a) of the Clean Water Act. Other potential components and impurities of road salt can include calcium, potassium, iron, magnesium, aluminum, lead, phosphorus, manganese, copper, zinc, nickel, chromium, and cadmium.

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Water Quality Impacts

lakeContaminates from road salt enter water resources by infiltration to groundwater, runoff to surface water and through stormdrains. The chloride discharged into these waters remains in solution and is not subject to any significant natural removal methods; only dilution can reduce its concentration. During winter and spring and during times of low flow in the summer and fall, chloride levels can exceed 800mg/L, while natural background levels fall within the range of 1-10mg/L. The accumulation and persistence of chloride poses a risk to the water quality and the plants, animals, and humans who depend upon it.

Water contaminated with NaCl creates a higher water density and will settle at the deepest part of the water body where current velocities are low such as in ponds and lakes. This can lead to a chemical stratification which can impede turnover and mixing, preventing the dissolved oxygen within the upper layers of the water from reaching the bottom layers and nutrients within the bottom layers from reaching the top layers. This leads to the bottom layer of the water body becoming void of oxygen and unable to support aquatic life.

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The concentration of chloride found in surface water correlates with the proportion of impervious surfaces in the watershed. Chloride cannot be treated or filtered with BMPs, so once salt is applied, chloride remains in the watershed until it is flushed downstream. Given that groundwater residence time is so much longer, contaminated wells often must be replaced.

The following table summarizes various water quality standards and guidelines with respect to human health, wildlife, aquatic species, and vegetation. Note that the human health standards are based on drinking water at the tap while the other standards and guidelines are for surface waters.

Table 1 Summary of NaCl, Sodium and Chloride Guidelines
  Na+ (mg/L) Cl- (mg/L) NaCl (mg/L) Comments
Human health 20 250

 

EPA drinking water quality standards
Wildlife   600 1000 Nagpal et al (2003)1
Aquatic organisms   860 – 1-hour average
230 – 4-day average
  DES water quality standard
Terrestrial and emergent plants   300 800 Groundwater source2
Aquatic plants   200 – 36,400   USEPA, 19883

Additional information regarding the impacts of road salt Adobe Acrobat Reader Symbol

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Human Health Impacts

water glassSodium in drinking water is a health concern for individuals restricted to low-sodium diets due to hypertension (high blood pressure). Therefore, the US Environmental Protection Agency (EPA) now requires drinking water to be monitored for sodium and public water suppliers to report to local health authorities any concentration above 20 mg sodium per liter of water (20mg/l)4. Chloride is not toxic to human health at low levels but does pose taste and odor issues at concentrations exceeding 250 mg/l. In New Hampshire from 1983 to 2003 the NHDOT replaced more than 424 private wells contaminated by road salt at a cost of $3.2 million. Several public water supply wells have also been abandoned due to contamination.

Additional information on the New Hampshire Well Replacement Program Adobe Acrobat Reader Symbol

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Pet Impacts

SPCA dogThe two most important concerns for pet owners regarding road salt are ingestion and paw health. According to the ASPCA’s Animal Poison Control Center, ingestion of road salt by eating salt directly, licking salty paws, and by drinking snow melt and runoff “can potentially produce effects such as drooling, vomiting, diarrhea, loss of appetite, volcalizing/crying, excessive thirst, depression, weakness, low blood pressure, disorientation, decreased muscle function and in severe cases, cardiac abnormalities, seizure, coma, and even death (www.aspca.org).” Exposure of your pet’s paws to road salt can produce painful irritations, inflammation, and cracking of the feet pads that can be prone to infection and are slow to heal.

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Wildlife Impacts

white-tail deerbirdRoad salt in the environment affects the health of wildlife, including birds and mammals. Birds, the most sensitive wildlife species to salt, often mistake road salt crystals for seeds or grit. Consumption of very small amounts of salt can result in toxicosis and death within the bird population. Wildlife such as deer and moose are also attracted to the roadway to ingest salt crystals, which leads to higher incidents of vehicular accidents and wildlife kills. Particularly high concentrations of sodium and chloride can be found in snow melt, which many animals drink to relieve thirst and potentially can cause salt toxicity including dehydration, confusion and weakness, among other symptoms. Road salt can cause a decline among populations of salt sensitive species reducing natural diversity. Damage to vegetation can have significant impact on wildlife habitat by destroying food resources, shelter and breeding and nesting sites, and by creating a favorable environment for non-native invasive species. Additional information

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Aquatic Life Impacts

AquaticsAquatics fishChloride in surface waters can be toxic to many forms of aquatic life. Aquatic species of concern include fish, macroinvertebrates, insects, and amphibians. Elevated chloride levels can threaten the health of food sources and pose a risk to species survival, growth, and/or reproduction. Chloride toxicity increases when it is associated with other cations, such as potassium or magnesium, which may occur once the ions of road salt have dissolved and migrated at potentially different rates. Salinity stress on sensitive aquatic communities can impact species diversity. The presence of salt also releases toxic metals from sediment and when released into the water can inhibit nutrients and dissolved oxygen within the water that aquatic species rely on. The acute (1-hour average) standard is 860 mg Cl-/L; the chronic (four-day average) standard is 230 mg/l.

For additional information regarding toxicological impacts please refer to Hazard Identification for Human and Ecological Effects of Sodium Chloride Road Salt Adobe Acrobat Reader Symbolon RebuildingI93.com and EPA's Office of Water Science & Technology Adobe Acrobat Reader Symbol.

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Vegetation Impacts

cattailsshrubsThe most visible impact of road salt on our environment is in the grass, shrubs, and foliage along the roadside. Not only does salt effect the terrestrial roadside vegetation it also has an impact on emergent and submerged aquatic plants. Salt leaves the road and enters the environment by splash and spray from vehicles, transportation by wind, snow melt into the soil and as runoff to surface waters. Salt primarily causes dehydration which leads to foliage damage but also causes osmotic stress that harms root growth. Salt can disrupt nutrient uptake and cause injury to seed germination, stems, leaves, and flowering ability. Salt can lead to plant death and can also cause a colonization of salt tolerant species, such as cattails, thereby reducing species diversity. Vegetation along roadways is a natural buffer area between pollutants and the waters. With salt damage and vegetation degradation it compromises the retention and processing of pollutants in stormwater runoff to the groundwater source. The University of New Hampshire suggests planting salt tolerant tree species such as horsechestnut, black locust, honey locust, red oak and white oak in areas of high salt concentrations. Refer to the following link for additional information Adobe Acrobat Reader Symbol. PennState cooperative extension also provides a list of trees, shrubs, groundcover, and perennials that are salt tolerate.

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Soil Impactssoil

Salt influences the chemistry of the soil in which it infiltrates. Through ion exchange the Na ion stays within the soil and releases other ions such as Calcium, Magnesium, and Potassium into the groundwater as well as increasing metal mobilization. This causes depletion in the soil as well as changes the soil permeability causing the soil to become impervious which blocks water infiltration, reduces soil stability, and decreases the soil pH and overall fertility. Salt can have impacts on soil biota, soil welling and crusting, soil electrical conductivity, soil osmotic potential, soil dispersion, and structural stability. Salt can inhibit some soil bacteria compromising soil structure and inhibiting erosion control mechanisms and increasing sediment in runoff.

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Infrastructure ImpactsUNH bridge

Chloride ions increase the conductivity of water and accelerate corrosion. Chloride can penetrate and deteriorate concrete on bridge decking and parking garage structures, and damage reinforcing rods, compromising structural integrity. It damages vehicle parts such as brake linings, frames, bumpers, and other areas of body corrosion. It impacts railroad crossing warning equipment and power line utilities by conducting electrical current leaks across the insulator that may lead to loss of current, shorting of transmission lines, and wooden pole fires. The cost of corrosion damage and corrosion protection practices for highways and the automobile industry have been reported to cost a staggering 16-19 billion dollars a year.5

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1 Napgal, N.K.,D.A.Levy, and D.D MacDonald. Ambient Water Quality Guidelines for Chloride, 2003.

2 NH Department of Environmental Services, Hazard Identification for Human and Ecological Effects of Sodium Chloride Road Salt, 2007

3 USEPA Ambient Aquatic Life Water Quality Criteria for Chloride. EPA 440/5-88-001, 1988

1 USEPA Drinking Water Advisory: Consumer Acceptability Advice and Health Effects Analysis on Sodium, EPA-822-R-03-006, 2003a.

5 City of Madison Wisconsin, Report to The Salt Use Subcommittee for the Commission on the Environment, 2006

 

 

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