As awareness for pollution increases, other forms of pollution are
defined. Almost everyone knows about toxic waste and carbon dioxide pollution,
but not many people have heard of salt pollution. Salt pollution has been on
the increase since the evolution of the automobile. With more pressure on
government agencies to keep the highway clear and safe, an increase in the use
of salt has developed. It is important to understand why salt is used and how
it work as well as the environmental effects to understand the salt pollution
Salt is a necessary and accepted part of the winter environment. It
provides safety and mobility for motorists, commercial vehicles and emergency
vehicles. Salt is used as the principal deicer because it is the most available
and cost-effective deicer. Rock salt is preferred because it is cheap and
effective. It costs 20 dollars a ton where as an alternative like calcium
magnesium cost around 700 dollars a ton. Some 10 million tons of deicing salt is
used each year in the U.S. and about 3 million in Canada.
Salt is used to keep snow and ice from bonding to the pavement and to
allow snowplows to remove. When salt is applied to ice and snow it creates a
brine that has a lower freezing temperature than the surrounding ice or snow.
Salt is the ideal deicing material because it is:
the least expensive deicer
easy to spread
easy to store and handle
harmless to skin and clothing
Salt pollution is broken into two main groups. Water, which includes
the effects on ground water, surface water and aquatic life and land.
Most of the salt applied to the roadways eventually ends up in the
ground water. It is estimated that 30% to 50% of the salt used travels into the
ground water. Salt effect two areas of ground water, chloride concentration and
sodium concentration. Chlorides may be present in the form of sodium chloride
crystals or as a ion in a solution. Normal concentrations in the water are
average around 10 mg/litre. Concentrations found in ground water near major
highways have been recorded as high as 250 mg/litre which is around the
threshold of taste.
The main factor with ground water pollution is the risk to human health.
The raised level in sodium in water can cause high blood pressure and
hypertension. With people who already suffer from these problem it is necessary
to keep their salt intake relatively low, they should not drink water above 20
mg/liter. Although this is recommended, a study of private well water in
Toronto showed that half the wells exceeded this limit, twenty percent exceeded
100 mg/litre and six percent exceeded 250 mg/litre. This increase in sodium and
chlorine can also cause problem with water balance in the human body.
As well as surface water, ground water is also affected by road salting.
Although the effects are not as great as ground water, they still pose problems
to the environment. The problems are based on the salt ions. The salt ions
interact with heavy metal that fall to the bottom of the body of water. An
example of this is when sodium and chlorine ions compete for mercury to bond
with. This cause the release of mercury into the water system. The risk of
mercury poisoning is far greater than that of sodium or chlorine. This increase
of sodium and chlorine as well as mercury and other heavy metal also cause
changes in the pH of water.
The increase of salt around bodies of water also effect aquatic life in
the area. Two main areas that are effected are osmotic regulation in fish and
the death of micro-biotic life in ponds and lakes. Most fish life can only
tolerate a narrow range of salt content in the water. The increase of salt in
the water produced by road de-icing cause freshwater fish to swell up with water.
The increased salt cause a lower concentration of water in the fishes cells.
To compensate, the fishes body takes in water to restore equilibrium. This can
kill fish if the salt concentration becomes to high.
Just as important as fish, microorganisms are also effected in a
detrimental way. Microrganisms are tiny organism that sustain aquatic life in
all bodies of water. They are more susceptible to the effect of salt pollution
than fish. These microorganism are at the bottom of the food chain, when they
die, it doesn’t take long for the rest of the food chain to follow. Large
increase in salt concentration can cause 75% – 100% death for these
microorganisms, The effect of salt is almost immediate. Most of the organism
are only one cell big and blow up in contact with increased amounts of salt.
Water insects are also effected by the increase in salt in the
environment. The number of insects lowers because the inability for water
insects to reproduces in the presence of high salt concentrations. With the
decreasing numbers in microorganisms, insects and fish, it is easy to see the
effect it would have on the rest of the food chain even though other animal may
be more salt tolerable.
Salt pollution also is a major factor to land. It can also be broken up
into the effects on soil, vegetation and animals.
The effect of salt on soil may seem relatively less important than the
other topics mentioned so far, but it leads up to more important things. The
effect salt has on soil is that it alters the soil structure. Sodium chloride
actually deteriorates the structure of the soil. This cause a decrease in soil
fertility. In most cases calcium in the soil is replaced by sodium in a anion
exchange. The make the soil less usable by vegetation. This also occurs with
magnesium. This depletion of calcium and magnesium also causes the soil to
increase in alkalinity with pH of nearly 10. Normal pH for the soils tested
were between 5.4 and 6.6.
High concentrations of sodium in the soil also makes the soil less
permeable. In some case soil may be encrusted in a layer of salt. As a result,
moisture content in the soil may be drastically decreased. High concentrations
of salt may also cause clay to have a decreased concentration of water. This
makes the clay harder and vegetation is less likely to grow.
Although salt already effects the soil vegetation grows in, it also can
directly effect vegetation itself. Vegetation can be dehydrate to the point of
death when in contact with high levels of salt. This occurs because the osmotic
stress put on the plant make it react like it was in a drought. A decrease in
roots production and burns to leaf tips cause the plant to go into shock.
Salt injury will also occur when plants come into contact with increased
levels of salt. Salt injury is when foliage damage is present by leaf burn,
die-back, defoliation and brooming. It can also cause fruit trees to have
reduced quantity and quality of fruit. This occurs with only a small amount of
salt comes in contact with the plant. It only take 0.5% of the plants tissue
dry weight to become salt before the plant reach toxic levels. Increased
chlorine levels can also cause salt injury to a plant in the same way. Salt
injury also effect trees as well as small plant life. Growth of plants in also
effected by the presence of sodium and chlorine.
Animal are also greatly effected by roadway de-icing. Although animals
tolerance to salt intake is quite high using salt for de-icing road presents
unusual dangers. Moose and deer become susceptible to salt pollution because of
their attraction to salt. Deer and moose are know to drink the salty water
around roads. It becomes an addiction to them and reduces the level of fear
when in contact with cars and people. They have also been found licking the
gravel and the side of the road and even the road itself in search for salt.
Small animals are effected more by the toxicity of high levels of salt.
Increased levels of salt in small wildlife caused kidney hemorrhaging,
depression, excitement, tremors, incoordination, coma and death. Rabbits seem
to be the most susceptible because their inability to stop consuming salt.
Household pets are also effected. once outside, salt collects on their feet.
Pets consume a lot salt when cleaning their feet. This causes cats and dogs to
get inflamed stomachs.
As one can see, the effect of roadway de-icing on the environment are
tremendous. The use of salt causes a great burden to both land and water. One
must weigh the pro’s and con’s of de-icing when learn about the effects of salt
on the environment.