By Mark Jones
Ten inches of snow debilitated the Washington area in early January, closing a section of I-95. Snow closed roads, both in the mountains and in flatter geographies, are a relatively common occurrence. Closing of an interstate highway, even one of the nation’s busiest, alone is likely not particularly newsworthy. Motorists trapped for up to 30 hours, including a U.S. Senator, made the I-95 closure national newsworthy. It also caused no end of finger-pointing with a chemical twist.
Some fault the I-95 response for inadequately preparing the road with deicing salt. No technology, chemical or otherwise, allows control of the weather. Our response to wintry weather is decidedly chemical. We throw salt on the roads when it snows.
We harness a colligative property, a property of a solution that depends on how many particles are dissolved, to melt ice and snow. The particular colligative property is freezing point depression. To first approximation, any substance capable of dissolving in water will lower the freezing point and keep our roads and sidewalks clear of solid ice. Ionic compounds do this about twice as effectively as non-ionic solutes. More solute particles formed, whether ions or molecules, means more effective deicing.
Deicing salts spread on roads are almost exclusively ionic chlorides of sodium, calcium, or magnesium. Sodium chloride, rock salt or halite, is the cheapest and most widely used. It is mostly mined from geologic formations, layers deposited by repeated evaporation of primordial seas. It is abundant in the ocean. Evaporation of sea water and mining are both sources, though most deicing salt is mined. Calcium chloride and magnesium chloride are recovered from brines. Some naturally occurring deposits are enriched in calcium making them particularly attractive sources. Calcium chloride can also be produced from limestone by reaction with hydrochloric acid or sodium chloride. It is both an on-purpose product and a byproduct.
Faced with snow, our generally anti-chemical society calls for broadcast spreading of chemicals, both mined and synthesized, with little thought about the long-term environmental impacts. Needs of future generations take a back seat when there is snow on the road.
Chemical road deicing is a relatively new technology, dating only back to the 1940s. For most of human history, salt was highly valued, as valuable as gold at times, too valuable to be tossed on the ground. Some 70% of the U.S. population lives in areas that use salt for deicing. Deicing compounds, for those in climates with winter, are something we purchase directly for home use and with our tax dollars.
Thoughts about deicing led me to confront my salt footprint. We all have one. The U.S. salt consumption is about 160 kg per person. Road deicing and use as a chemical feedstock account for 80% of salt consumption. Much of the chemical production makes organochlorine compounds, but about 25% of chemical production ultimately releases salt. About 110 kg per person is deposited directly into the environment during deicing and in wastewater. Salt in our urine accounts for 2-3 kg per year. Just like with individual CO2 emissions, metabolism is a small part of my footprint.
The normally black asphalt roads here in Michigan are white from applied salt, applied at about 100 kg per resident. Spring rains will take the salt-laden roads back to black, out of sight, out of mind. The salt ends up in lakes and rivers. Rock salt’s damage to vegetation, metals, infrastructure, and the environment are largely overlooked, but have been the subject of research dating back decades. Salinity in the lower Great Lakes has increased 8 to 10 times historic levels and continues to rise about a milligram per liter per year. From a base of 1-2 mg/L in the 1800s, salinity is now at 15 mg/L in Lakes Erie, Ontario, and Michigan. The Great Lakes are still very fresh, well less than the 35,000 mg/L salinity of the ocean, because they are very large. It is the tributaries to the Great Lakes and smaller lakes where the problem is particularly acute. A recent study of randomly selected lakes found 44% experienced long-term salinization and estimates nearly 8000 lakes may be at risk from salinization. Other studies place the numbers even higher. Many lakes are in peril. Many tributaries now experience seasonal excursions that threaten species that live and breed there.
There is also research on the positive benefits of applying salt to roads. Deicing reduces accidents, estimated by over 78% on average roads and up to 93% on highways, saving lives and preventing injuries. Consciously or unconsciously, we’ve determined the immediate safety benefits are worth the deferred costs. We’ve also determined those immediate safety benefits are not worth the considerably higher cost of other deicing options. In deicing, the cheap solution wins.
There is cost on the economic and environmental cost of salt application. These studies estimate the peripheral cost to equipment, infrastructure, and the environment. All place the so-called true cost of our deicing addiction much higher than the cost of the salt. Some implicate road salt in the Flint water crisis.
We aren’t lacking for research on the impacts of deicing salt, but there is not much development. Recent reports are more disheartening than uplifting, concluding there is little to be done to stem the environmental damage caused by keeping our roads ice free. Salt is very cheap and quite effective. There are no alternatives that are as cheap and surprisingly few alternatives even at higher cost. Other ionic solids aren’t as plentiful and share many of the same issues. Calcium and magnesium chloride are both used. Calcium chloride is both recovered from brines and manufactured synthetically. Magnesium chloride is frequently present with calcium chloride in brines. Mixed chloride salts are by-products of other production. Pure magnesium chloride is produced for magnesium production through multistep processes. Calcium chloride and mixed calcium-magnesium chloride are used more often in deicing around commercial buildings — but are occasionally used on roads. Organic compounds are not as effective as deicers and come with additional burdens.
One very promising avenue is research into reducing the impact involves simply using less. Spreading salt on roads is often done in a ham-handed way. In my area of Michigan, it is by broadcast spreading of crystalline salt. Salt is commonly found up to 15 feed off the road on my street, the result of bouncing and poorly calibrated spreaders. Application of liquid brines rather than crystalline salt reduces use by 40%. Prewetting the salt as it is applied also leads to more targeted application, reducing the amount used. Both require upgrades to equipment. Research and development into better monitoring and applying deicing is one area sure to see activity. Studies over the last 40 years conclude we can’t afford to move away from salt, but we can use it smarter.
Recent studies also conclude salt storage is a problem. Leakage from storage facilities is largely preventable. Infrastructure upgrades can eliminate contaminated runoff from these facilities.
Alternatives are known, but not widely used. Salt of organic acids are proven solutions, but better production methods are needed. Acetate salts of calcium and magnesium are used available, costing about 30 times more than rock salt. They are available only in limited supply and are used on few roads in certain areas. Both avoid chloride runoff but are burdened by larger CO2 footprints. Studies show these can still have deleterious impacts. Development of organic alternatives to halide salts is one promising option.
Organic materials, such as agricultural residues, also can reduce salt use. Residue from sugar beet processing is mixed with some salt to make an effective deicer. Sugar beet molasses with added salt is better retained on roads thanks to the sticky molasses. It biodegrades when the weather warms, making it unlikely to build up in the environment. Biodegradability sounds great — but can also create challenges. Oxygen depletion is observed when the degradation happens in lakes and rivers. Beet residues are also implicated in harm to insects in the watershed. All deicing options require use in moderation.
Developments of alternatives that forgo use of deicing compounds are also ongoing. Heated pavement, already used in airports, offers one option. Development of options that can be economically deployed is one area of research. Many options are already available for homeowners interested in reducing their salt footprint. Snowplows don’t immediately seem like an area ripe for innovation, but developments are occurring. Use of polymeric edges and design changes are improving mechanical removal, reducing salt use.
Depressing as the data are, there are things each of us can do, things more costly but better for the environment. Homeowners, unbound by the cost constraints plaguing tax-funded road commissions, can choose more plant-friendly options, less damaging to infrastructure, and more environmentally benign. I personally sparingly use urea, costing about five times more than rock salt.
We can ask our public works to do better. We can support initiatives to better apply salt to roads and improve mechanical removal. Citizen science can play a role too. The Izaak Walton League is offering free chloride test kits to better identify problem areas. Grab a sample, do some analytical chemistry and be part of the solution. Most of all, if you are reaching for deicing salt, choose carefully and apply sparingly.