Winter is in full force and with it comes ice and snow-laden roadways that create hazardous travel conditions. For many years, U.S. municipalities have turned to use of “road salt” (sodium chloride) as a cost-effective solution that provides a first line of defense to minimize vehicular and pedestrian accidents. More than 20 million tons of road salt were reportedly spread on U.S. roads, parking lots, sidewalks, and driveways last year – attempting to lower the melting point of ice. Three times as much salt is used on roadways as is consumed through food. Unfortunately, road salt usage has spiraled out of control. Usage has increased dramatically to almost “unlimited” use – with the mantra, ‘the more the better”. But with overuse comes unintended damages and potential long-term environmental impacts – requiring balancing the economic and social benefits and short-term safety risks with long-term acute and permanent consequences to the environment.
What’s the Big Deal About Salt?
For one, those crystals can begin to break down the very pavement under your feet, dissolve the matrices of concrete, asphalt, and roadway stripes, and speed up the corrosion of rebar in concrete, weakening its structure. Salt corrosion has been cited time and again as a major factor in road and bridge collapses in the U.S. and Canada, including Montreal’s Champlain Bridge and Toronto’s Gardiner Expressway.
Road salt can migrate through runoff, splash, wind and storm drains into nearby waterways, and infiltrate into groundwater, causing spikes in sodium and chloride concentrations. When salt settles in deep waters of lakes and streams with low flow velocities, it can cause chemical stratification and prevent dissolved oxygen from reaching the bottom, leading to anoxic conditions.
For human health and ecological risks, the news is equally bad. Excess sodium in potable water supplies can cause hypertension in individuals. And for ecological populations, even small amounts of salt can be toxic. Birds often mistake the crystals for seeds; deer and moose are attracted to salt crystals along roadways; and high concentrations are found in snow melt that animals drink causing toxic effects. For pets, the ASPCA’s Animal Poison Control Center warns that dogs will eat salt directly and lick their salty paws or drink snow melt which can cause painful irritations, harmful effects, and inflammation to their paws.
But the damages from road salt may go deeper and are less understood. Pure road salt is comprised of an equal number of sodium ions (Na+) and chloride ions (Cl-). However road salt typically contains additives and impurities, such as ferrocyanide (used for anti-caking), phosphorus, and iron. When salt moves through the soil column, Na can dislocate other cations (Al, K, Mg) through ion exchange, and cause clays to swell and soil to lose its structure. Resulting effects can include lower permeability, slope failure, creation of hard pan, and havoc for crops and vegetation.
There are currently no federally-mandated maximum contaminant levels (MCLs) for sodium or chloride under the Safe Drinking Water Act. However, the Environmental Protection Agency (EPA) has established a health advisory level for sodium of 20 mg/L for people on salt-restricted diets and a secondary (aesthetic) drinking water standard of 250 mg/L for chloride and sulfate ions, for salty taste. In addition, EPA has added ferrocyanide to its list of toxic pollutants under Section 307(a) of the Clean Water Act.
Several states have soil standards, including New Mexico’s (NMEMDRD) land farming regulation of 500 mg/Kg in groundwater between 50-100 feet below ground surface (Bgs) and 1,000 mg/Kg if groundwater exceeds 100 Bgs − directed at “brine” cleanups from exploration and petroleum or produced water wastes.
Cleanups for road salt impacts are not common. Sodium and chloride are very soluble and mobile in groundwater, and impacts are difficult to remediate after contamination has occurred. Impacts are typically managed by replacement of private wells, replanting of vegetation, drainage modifications, and localized alterations to salt application protocols. Physical first-line-of-defense cleanups for excess salt have evolved; and new power and regenerative air power sweepers, including vacuum filter power sweepers, have been shown to be effective for cleanup of smaller fined-grained particles and crystals.
Salt cleanups are more commonly performed for releases of oil field brines, which are exploration and production wastes. In arid environments, infiltration and evapotranspiration result in net seasonal movement of water up through the soil column, which can result in sodium re-appearing over time in shallow soils even after shallow remediation is performed. Alternative approaches to remediation, which may be coupled with excavation of shallow or hot-spot soils, include soil flushing and application of calcium-rich products such as gypsum. Soil flushing can occur in-situ or ex-situ, and involves application of water to the soil to flush the salt, and collection of the resulting brine for disposal. Calcium amendments displace sodium in the soil, restoring soil structure and permitting re-vegetation. However, the displaced sodium may create groundwater quality impacts.
Are There Alternatives?
Creative alternatives for road salt are coming to market that are more sensitive to the environment. Beet 55 – a mixture of beet juice, sugar and salt – has become popular, and when a thin layer is applied to roadways can help salt work in temperatures near -30˚C, reduce runoff, and keep ice from bonding to a road’s surface. In Wisconsin, dairy companies apply a mixture of cheese brine and byproducts of mozzarella and provolone to roadways. The freezing point of water is lowered more by addition of this mixture than by addition of salt. In Ankeny, Iowa, a local spice factory donated a mixture of dried garlic and garlic salt that the city used on its 400 miles of roadway. Moreover, New Jersey has turned to using pickle juice, reporting that the salty green liquid melts snow as efficiently as salt and is cost-effective and better for the environment. Natural chloride compounds can coat the porous parts of the pavement so that snow and ice can’t affect the pavement matrix. Finally, in the Netherlands, teams of scientists have been working on using biomass sources such as roadside grass or kitchen waste to produce a powerful green deicing salt (calcium magnesium acetate). Full-scale implementation is coming. The Dutch report that their special mixture will be ready for large scale use by 2020.
HETI − Environmental Services and Solutions
HETI believes we are beginning to turn the corner in awareness of the overuse of road salt and its potential to create long-term scars on the environment. Economic and social benefits including short-term safety risks are becoming more balanced with finding creative solutions to lessen the use of road salt worldwide. And many scientists and researchers are working around the clock to bring less harmful products to large-scale use. HETI can help respond, investigate, and mitigate salt issues using the latest scientific and cost-effective approaches – blending soil science, geochemistry, and natural in-situ approaches that match or exceed the regulatory requirements and restore sites to pre-loss conditions – to support productive crop and vegetation cycles and avoid diminution of land value or use.
To find out more information about HETI’s environmental services, please contact us.
Joseph A. Guerriero, PG
Senior Professional Geologist