New England is about to get dumped on, and that means we all have one thing to look forward to: salt.
Every year, municipal and highway authorities across 26 states apply an estimated average of 21 million metric tons of road salt to keep roadways safe for cars and sidewalks safe for pedestrians. The amount of salt applied varies from state to state, ranging from half a metric ton per lane-km in Idaho to 28.3 in Massachusetts.
If Ben Franklin were alive today,* he would have no choice but to add salt — as cure** for the common cold water (that is, snow) — to his list of death and taxes.***
*Do not challenge my right, as a proud graduate of Penn, to channel said patron saint
**Pun intended (see definition 5)
***I concede there’s probably too much going on in this sentence
But despite the wide variety of application practices, dissent against the fact of salting is not tolerated in any form. Seattle — The Emerald freaking City! — voted out Mayor Nickels because he refused to apply salt on environmental grounds.* Absolutely true story.
*It probably also did not help that he let the Sonics go
Admittedly, I have a personal stake in this issue: I bought a new pair of shoes over winter break (which ended exactly one month ago) and I have yet to wear them once because the pair of sneakers I have been wearing is crusted over with a salty residue I have no desire to see all over a brand new pair. But I performed the research that follows a few years back, as an undergrad taking ENVS 507 Wetlands — taught by the estimable Sally Willig at the aforementioned University of Pennsylvania — so it’s not like my perspective is new.
And while I might be biased because of the whole shoe thing, I still thought it was worth at least raising the suggestion that the implicit trade-offs might not always be worth it the cure* might be worse than the disease.
*Pun still intended
The application of road salt began in the 1950s, but came into widespread use in the 1970s. Prior to that time, sand and other abrasive materials were used to increase traction on snowy and icy surfaces.
Today, applied road salt is sometimes calcium chloride, and sometimes magnesium chloride, but more often, it is sodium chloride — table salt — because NaCl is the cheapest of the three (I feel ironic saying this after hearing both R. James Woolsey and Daniel U. Markovits* mention that salt was once valued as currency over the past three days alone). As you have probably figured out, sodium chloride lowers the freezing point of water, turning dangerous ice slicks into slush that cars can drive through with little difficulty, and spray all over the sidewalks with reckless abandon.
*The U. stands for ‘Unknown’; Woolsey got a three-part name, so I had to supplement
Chloride is extremely soluble in water – and once dissolved, it is very difficult to remove – so its application directly and unambiguously affects nearby fresh water. The study of one metropolitan area detected high levels of chloride in runoff from salt depots, highways, snow piles, and urban streets, and positively correlated chloride content to paved surface area. On a slightly broader scale, another study found that 70% of the salt applied in the Twin Cities metropolitan area was retained in the watershed, while the other 30% was flushed downriver by the Mississippi.
Runoff wetlands – artificially constructed wetlands built alongside highways to help prevent flooding – were sampled in a different study, which found that their chloride content could reach levels half as salty as sea water. Remember, we’re talking about water that is ostensibly fresh.
The salt does not only affect water directly alongside roads. While most salt is deposited within five to ten meters by tires splashing through puddles, one study in the Adirondacks found that road salt could be detected up to 172 meters from the nearest highway. But salt does not only travel aboveground as runoff, or in shallow subsurface pathways through soil, but can also take deeper and slower pathways through underground aquifers. While the first two kinds of pathways affect surface water salinity, the deeper and slow alternative can pose a long-term threat to drinking water supplies.
And salt can even show up in the air. One study showed Chicago – and presumably other large, urban areas – enveloped by a cloud of salt dust throughout the winter. Leaving aside, for the moment, the negative health implications of living inside a cloud of salt dust, the study showed that small amounts of sodium can be transported by aerial means to even greater distances than previously thought.
Meanwhile, sodium chloride is not the only component of road salt. Up to 5% of the stuff is composed of elements other than sodium or chloride, including phosphorus, nitrogen and copper. To keep salt from congealing on roadways into unspreadable chunks, anti-caking agents such as sodium ferrocyanide are also added to the mixture. Although sodium ferrocyanide makes up only about .01% of the mixture’s dry weight, this can amount to two pounds of cyanide for every mile of a four-lane highway. Cyanide toxicity begins at 20 parts per billion, but cyanide has been detected in urban streams at levels as high as 270 ppb directly following runoff events. Finally, chloride has been shown to increase the bioavailability of heavy metals, including cadmium, lead, chromium, copper, and mercury.
And while increased salinity in rivers and streams due to application of road salt deposition has been studied extensively, less attention has been paid to its impact on wetlands. Because wetlands are shallow and typically slow-moving, they are in theory highly susceptible to the effects of saline runoff.
Studies have found that wetlands are typically flushed by fresh rainfall and weigh in at zero salinity around midsummer, but researchers are monitoring the year-round baseline to determine if it might begin to creep up from salt deposited in soils. And even if sodium chloride content returned to normal every summer, its presence throughout the spring growing season would still have an impact on wetland communities: the sodium component alone can destroy soil structure, reduce aeration, increase alkalinity, reduce water holding capacity, and increase susceptibility to erosion.
Increased salinity has negative effects on wetland biological diversity. First, it represents a fundamental change in the ecosystem’s chemical condition, which in turns allows invasive, saltwater-loving species to gain a foothold and crowd out native freshwater plants. One study in Massachusetts found that areas of a wetland invaded by phragmites showed much lower levels of species diversity than areas that had not seen their native communities crowded out. Increasing salinity nationwide will eventually lead to the creation of wetland monocultures, destroying a set of valuable ecosystems and a tremendous amount of natural diversity in the process.
On an organism level, sodium and chloride ions interfere with the osmotic balance of cells, disrupting plants’ abilities to regulate the absorption of water. Saltwater plants have developed various adaptations to deal with the stress of salty water, but freshwater plants have not. Measured chloride levels in affected wetlands can range from 250 to 3000 mg/L, paving the way for invasion by introduced annual saltmarsh aster, salt meadowgrass, narrow-leaved cattails, seaside goldenrod, and especially phragmites reeds. These species, common to coastal wetlands, are beginning to spread across formerly freshwater ecosystems in the interior of the continent. While this shift is most pronounced near highways, many valuable ecosystems and rare plants live in even these marginal areas.
Road salt can have tremendous secondary effects as well. For one, it can be directly harmful to animal species. One study found that spotted salamander (A. maculatum) embryos and larvae in saline wetlands showed a lower rate of survival and higher rate of malignant mutation than those raised in freshwater environments. Another study found that wood frogs (R. sylvatica) are affected similarly. If saline concentrations are too high, they can lead to local extinction of the species. Salt can affect the composition of invertebrate communities, as well. And road salt also indirectly harms animal species, by destroying shoreline vegetation critical to erosion control and as habitat for birds and other organisms.
There are a number of measures we could take to cut down on the impact of road salt. One study showed that driving at 45 miles an hour or less spread significantly less snow than higher speeds, and prompted Chicago to implement winter speed limits of 40 mph.
The construction of better storage facilities can have a large effect, as well. One uncovered storage pile leached salt for over a decade in the Indiana Dunes National Lakeshore’s Pinhook Bog (which I visited a few summers ago). Forty years later, the ecosystem has not returned to its native freshwater state.
Another effective measure is to ‘pre-wet’ the salt so it sticks better to the road, works faster, and less must be used to achieve similar effect. Further, sodium chloride is ineffective at temperatures lower than 15 degrees F, and there is no reason it should be applied at all when it gets that cold.
And at higher temperatures, research suggests that much lower quantities – no more than three cups of salt for every 1000 square feet – could be as effective as current practices. The University of Minnesota, which adopted these measures, was able to reduce its salt use by 41% and saved more than $50,000 in the first year of the program alone.
$50,000 worth of a salt is a lot of salt.
Of course, none of these piecemeal measures would be as effective as discontinuing the application of road salt entirely. And while that solution might be closer to the outcome I would prefer, at least, to current practices, such a proposal is likely a non-starter. Road salt may pose a sizable and underappreciated environmental threat, but it is understandably considered an issue of public safety.
But the truth is that road salt is not necessarily a better choice than some alternatives. Recall, for a moment, what happened in Seattle: Mayor Nickels applied sand in place of road salt and was voted out after the winter of 08-09 overwhelmed its ability to alleviate dangerous driving conditions.
Now, the punchline: the following year, his successor applied salt to no better effect.
And sure, sand comes with its own set of environmental issues, but it’s at least an option to consider instead of blindly flinging salt on every single paved surface every single time it snows.
At the very least, we might consider adopting salt-mitigation efforts outlined above, in addition to more judiciously attempting to determine which roadways must be cleared, and which can probably stay snowy for a few days until they melt or someone digs them out.
And at the end of the day, all I ask is that decision-makers consider that road salt causes corrosion, increases harmful aerosols, pollutes drinking water, is full of toxins, and is often no more effective a measure for reducing accidents than would be simply reducing the speed limit. And that list does not even broach road salt’s effect on the non-human environment.
There are better ways to ensure public safety than by turning our wetlands into microcosms of the Dead Sea. Maybe next year, we can try them out.