Too much of a good thing

31 January 2014 by Malcolm Campbell, posted in Biology, Environment, Science

The wonderful purity of nature at this season is a most pleasing fact.” from A Winter Walk by Henry David Thoreau (1817-1862)

Winter is full of wonderful things.

An overly salted sidewalk is not one of them. salt1

Instead of a blanket of pristine snow – a powdery coat of the greatest purity – salt creates a grey mess – a frigid, sooty, milky slush. Far from the “wonderful purity of nature” that Thoreau lyrically described on his winter walk, salt creates a decidedly contaminated reconfiguration of the winter landscape. How did this happen?

In recent years, around here at least, pitching fistfuls of rock salt at the ground has supplanted snow removal the good old-fashioned way – by shovel and muscle power. It’s not hard to understand why. Shovelling snow is tedious and time consuming. What’s more, shovelling snow is sometimes backbreaking work. Hefting shovelful upon shovelful of wet snow off of the walkway and driveway can be physically exhausting. “Managing” snow and ice by throwing salt is fast, easy, and requires little physical exertion.

And there are good scientific grounds to apply rock salt to well-travelled routes.

Salt serves three purposes in managing snow and ice, where people or vehicles tread. In the first instance, on clear surfaces, salt can prevent the build up of both snow and ice (anti-icing). In places where ice and/or snow have accumulated, salt can cause them to melt (de-icing). Finally, the presence of salt in slush inhibits compaction and refreezing to form ice again (anti-compaction). In all three instances, application of salt is intended to prevent the creation of icy surfaces where people or vehicles might slip or slide.  It is the reasonable hope that this will decrease the likelihood of accidents leading to injury, or worse.

Salt accomplishes this task through the elegant simplicity of chemistry.

Snow and ice are, of course, frozen water, H2O. At 0oC, water becomes solid, organised into hexagonal crystals as many, relatively weak bonds form between the hydrogen atoms of adjacent water molecules. Salt disrupts the process of water crystal formation. salt3

Salt, or sodium chloride (NaCl), is a simple ionic compound comprising one atom of sodium (Na) for each atom of chlorine (Cl). It is very soluble in water. That is, it is able to dissolve in it readily.

When added to frozen water, salt dissolves in the water molecules at the surface of the ice, which are poorly integrated into the solid. Water that contains dissolved salt is less able to form the nice neat hexagonal crystals that make up ice, effectively because the ions from the salt get in the way. In fact, the presence of the salt lowers the freezing temperature of the water – it can only form crystals at a lower temperature.

What’s more, there is a negative correlation between the concentration of salt in the water and its freezing temperature. That is, the higher the concentration of salt, the lower the freezing point of the saltwater solution.

Of course there’s a limit, but high concentrations of sodium chloride can drive the freezing temperature of water down to much lower temperatures. For example, a 10% salt solution will drive the freezing temperature down to -6oC; whereas, a 20% solution will drive the freezing temperature down to -16oC.

When salting walkways, 100% solid sodium chloride is added to the ice or snow. As it dissolves in the water on the surface of the ice, it lowers the freezing temperature. This enables more liquid water to be integrated from the surface of the ice, which dissolves more salt, which lowers the freezing temperature, enabling more water to be taken up, and so on. This continues until either the salt or ice run out, or until the temperature drops below the newly-established freezing temperature, or both.

And therein lies a problem. salt5

There is a limit to how much salt is available, as well as how well it will dissolve in cold water, and therefore, how low the freezing temperature can be reduced. Consequently, if the temperature is cold enough, the saltwater will freeze. That’s how we end up with a slushy mess.

Quite apart from being an eyesore, the slushy mess has a troubling legacy.

The salt that finds its way onto walkways and roadways is truly a chemical out of place.
The rock salt that we use to manage snow and ice is generally mined from underground. For instance, the salt that is used in this region originates from the world’s largest salt mine, which runs for over 5km, covering 13 km2, 500m under Lake Huron of the Great Lakes. That mine provides 7 million tons of rock salt for de-icing purposes annually.

The mined rock salt is halite. Halite is a sedimentary rock, meaning that it came from something that settled or precipitated at the Earth’s surface or within a body of water. In the case of halite, a mineral comprising large crystals of sodium chloride, it arose as ancient lakes and seas dried, leaving behind vast quantities of salt. The halite in the world’s largest salt mine under Lake Huron originated from a vast, tropical sea that existed some 400 million years ago.

Today halite feeds a multi-billion dollar road salting enterprise, through mines like the one that lies under Lake Huron. Over the past fifty years the application of halite on roads has increased dramatically. In 1960, around 3 million metric tons of salt were spread on North American roadways; whereas, today, more than 20 million metric tons of halite are distributed on North American roadways annually.

Over this same time period, there has been encouragement to apply salt to walkways, but the figures on the extent of application are not available. The quantity of salt applied to walkways is likely to pale by comparison to roadways. This said, in cities, where application of salt to walkways occurs at the highest density, the numbers could be significant. salt6

The cumulative effects of halite application to roadways and walkways are marked. Some freshwater river watersheds have seen salt levels rise between 100% and 250%. And these levels rise every year. Even if road salting were to stop immediately, it would still take years for river systems to purge the salt, and be restored to their original levels.

Not surprisingly, the rise in salt in ecosystems has an effect on the organisms living within those ecosystems. Most terrestrial ecosystems are based on freshwater, not salt water (or salinised water). This said, in keeping with our ancient evolutionary origins in the world’s oceans, even terrestrial beings are reliant on an appropriate balance of sodium and chloride to sustain cellular function. In our own branch of the evolutionary tree, sodium and chloride play important roles in helping us move other chemicals around our cells, enabling our cells to keep their shape, and even in firing our neurons, and in contracting our muscles. As a consequence, we have evolved means by which to taste the presence of salt, and to control its quantity within our bodies. This said, high levels of salt can throw things out of balance – it can be toxic to the point of having a lethal effect. In keeping with this, having the right amount of salt available is necessary for sustenance, while avoiding toxicity.

In the past, salt was often a precious commodity within terrestrial ecosystems. Animals would travel to “salt licks”, locations where halite was exposed, to acquire the requisite salt in their diet. Even humans prized this rare compound, to such an extent that people were paid in amounts equivalent to quantities of salt – giving us the root of the word “salary”.

Nowadays salt is relatively cheap and ubiquitous. The extension of salt’s ubiquity to terrestrial and freshwater ecosystems is problematic.

Increasing salt, or salinisation, of ecosystems can have a provide effect on the species that are found there. For example, Dudley Williams and colleagues, examined 23 freshwater springs in a large urban setting, with varying degrees of salinisation at each. Some had low levels (<2mg/L) of chloride ions originating from salt; whereas, others had relatively high levels of salt-derived chloride ions (>1200mg/L). Williams and colleagues looked at the species composition at these springs. They asked if there was a difference in occurrence of certain species. Unsurprisingly, they found that species composition did vary. salt8

Williams and colleagues also surmised that some species had tolerance to the higher chloride concentrations, and some species were much more sensitive. One species was found to be diagnostic of chloride contamination, a small crustacean, Gammarus pseudolimnaeus. The absence of G. pseudolimnaeus, especially if nymphs of the stonefly, Nemoura trispinosa, were present indicated that the water had moderate to high chloride contamination. In light of the fact that the crustaceans serve as foodstuff for other organisms in their environment, their absence creates a necessity to find alternative foodstuffs – not so problematic for generalist eaters, but problematic if your normal meal is Gammarus.

Despite the fact that salt application takes place throughout the winter months, the impact is felt well into the summer. Salt’s impact extends not merely to animals, but is observed prominently in plants as well. Most land plants, and those that are adapted to freshwater ecosystems, fare poorly when soil and groundwater becomes salinised.  Some of these impacts may be indirect – due to shifts in the populations of microbes that reside in the soil and interact with the plant. Other effects may be direct – impinging on plant growth and development. Plants are able to take up sodium and chloride ions in the soil, and transport them from the roots to the aerial tissues, where they elicit toxic effects. Minimally, plant growth and development is impaired by chronic exposure to elevated salt concentrations. This may, in turn, effect the composition of plant species present. Some species may be highly susceptible to the salt and succumb to it, while species that have greater salt tolerance thrive and dominate the ecosystem.

The long-term impact of salinised water and soil on ecosystem composition and function could be profound. The 50-year increase in the application of rock salt to manage ice and snow looks likely to proceed unabated. If neighbourhoods here are anything to go by, rock salt is likely to continue as a replacement for snow and ice removal by physical means on walkways.

Unfortunately , oftentimes, when salt is used on walkways, salt creates the very problem that its users aim to solve – it creates a walking surface that is more difficult to navigate. Flat, hard-packed snow, or, better, a shovel-cleared walkway, is far more readily traversed than one that is clogged with frigid slush. salt9

But this is only a minor, immediate irritation. Salt has a lasting legacy that we need to address.

At first glance, salt appears an easy, low-cost solution to a short-term, acute problem. In fact, the use of salt is creating a persistent, long-term problem that is costly to the health of our environment.  We need to ask if our widespread application of salt, is time and money well spent. We must ask if we are applying salt under conditions where it is actually useful, when it is actually needed, rather than simply using it as the default “solution” whenever frozen precipitation of any sort is on the way.  Perhaps an adulterated ecosystem is a price we are willing to pay for what appears to be an easy fix. It seems a rather high price for convenience. It’s for this reason that some of us are happy to keep using our shovels, except when salt is sensible.

It's worth noting that rock salt is something simple to criticise at this time of the year. Walkways that are a slog to walk through, salt-stained trouser cuffs, and the knowledge that verges will soon be showcasing salt-burned plants – they all make rock salt an easy target. On the other hand, rock salt is emblematic of a problem that plays itself out time and again with our species. Sometimes, it seems, we take too much delight in our cleverness. Elegant, innovative fixes seem too easy not to use, iteratively. In our delight with our cleverness, we overlook the long-term consequences of our clever solutions.

Our capacity to invent good things doesn't mean that we should use them, all the time, without restraint. It is possible to have too much of a good thing. Somehow, we need to hold ourselves in check, take a look at our clever solutions, and ask if their benefits outweigh the costs.

Images: All photographs by Malcolm M. Campbell.

References:

Davison AW (1971) The effects of de-icing salt on roadside verges. I. Soil and plant analysis. Journal of Applied Ecology 8:555-561

Findlay SE, & Kelly VR (2011) Emerging indirect and long‐term road salt effects on ecosystems. Annals of the New York Academy of Sciences1223:58-68

Godwin KS, Hafner SD, & Buff MF (2003) Long-term trends in sodium and chloride in the Mohawk River, New York: the effect of fifty years of road-salt application. Environmental pollution 124:273-281

Jackson RB, & Jobbágy EG (2005) From icy roads to salty streams. Proceedings of the National Academy of Sciences 102:14487-14488

Kaushal SS, Groffman PM, Likens GE, Belt KT, Stack WP, Kelly VR, & Fisher GT (2005). Increased salinization of fresh water in the northeastern United States. Proceedings of the National Academy of Sciences 102:13517-13520

Ke C, Li Z., Liang Y, Tao W, & Du M (2013) Impacts of chloride de-icing salt on bulk soils, fungi, and bacterial populations surrounding the plant rhizosphere. Applied Soil Ecology 72:69-78

Richburg JA, Patterson WA, & Lowenstein F (2001) Effects of road salt and Phragmites australis invasion on the vegetation of a western Massachusetts calcareous lake-basin fen. Wetlands 21:247-255

Williams DD, Williams NE, & Cao Y (2000) Road salt contamination of groundwater in a major metropolitan area and development of a biological index to monitor its impact. Water Research 34:127-138

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