Is Road Salt a
Problem?
Directed Study Fall 2003
Ruqaia Muradagha
Supervised by Mr. Thomas Kelley
Abstract:
My Honors Directed Study topic came
about from the work done by HFCC students and faculty in cooperation
with Ford scientists. This group has decided to examine the effects-
both good and bad- of road salt on roads and the environment in
Dearborn
As a first step, my project is to
examine some of the variables which need to be considered in order
to answer the question: Should Road Salt Be Avoided?
I read many articles and examined
many graphs during my research and came up with a base for my cost-
benefit analysis project. Salt (NaCl), Sodium Chloride (CaCl), and
Calcium Magnesium Acetate (CMA) are the three most common deicing
chemicals used in Dearborn.
To make up my decision about salt, I
started with comparing the prices of the three deicers. Then, I
looked at the positive and negative effects of each deicer on the
road and on the environment. Cost wise, salt is clearly the best
since it costs about $30/ton where CaCl costs $160/ton and CMA costs
about $700/ton. At first glance, one might think CMA is too
expensive and it is impossible to afford it. However, when I
considered some of the other variables I found that CMA reduces the
indirect cost of road and vehicle repair and it has almost no
negative effect on the environment. As shown on the graphs, CMA
actually decreases corrosion and its spalling effect on concrete is
even less than water. This makes it ideal for bridges and hard to
repair surfaces. When compared to salt, the percentage loss of CMA
is about six times less than that of salt. In addition, it is
effective in temperatures that are twice as low as the temperature
in which road salt works.
CaCl is worth looking at. It has a
reasonable price of $160/ton and it works in temperatures as low as
-67oF. Its spalling effect on concrete is less than salt
but still much higher than CMA. However, high concentrations of
CaCl have greater corrosion rates than that of 100% road salt. This
would mean a great increase in indirect costs.
The Problem
Road salt mixes with H2O
(ice) to create brine which contains sodium and chloride ions. Both
of these ions have been proved to have some negative effects on the
environment.
There
are a few de-icing chemicals used in the United States:
Ñ
Sodium chloride (salt):
Ø Salt
is the most commonly de-icing chemical used and it is very
inexpensive and very available
Ø
Comes in various sizes of solid form and works very well
Ø Works
by lowering freezing point of water; (23.2% concentration of
salt freezes at -6oF)
Ø
Cost about $30/ton
|
|
Pound Per
Two Lane Mile |
|
|
Pavement
Temperature |
Weather
Condition |
100% Salt |
Application
Frequency |
|
30+ oF |
Snow
Freezing Rain |
200- 400
200 |
As needed
As needed |
|
25-30 oF |
Wet Snow
Freezing Rain |
400- 500
300
200 |
As needed
Initial
Repeat |
|
20-25 oF |
Wet Snow
Freezing Rain |
500- 600
250
400
300 |
Initial
Repeat
Initial
Repeat |
|
15-20 oF |
Dry Snow
Wet Snow |
400
600- 800 |
Sand hazard
area
Sand as needed |
|
Below 15 oF |
Dry Snow |
|
Sand hazard
area |
Guidelines used by City of
Duluth employees to
determine how much pounds of salt to apply per two lane mile to keep
the roads safe. (Road Salt: Can we have safe roads and healthy
steams?)
Ñ
Calcium
chloride:
Ø
It comes in solid as pellets or flakes or in solution of various
concentrations of natural brine.
Ø
Second commonly used deicing chemical
Ø
Works by lowering freezing point of water; (29.8% concentration
of CaCl freezes at -67oF)
Ø
Costs about $160/ ton (Road Management Journal)
Ñ
Calcium Magnesium Acetate:
Ø
This
is the least harmful de-icing chemical present these days
Ø
Have
no corrosion
Ø
Used
for bridges
Ø
Very
expensive compared to salt
Ø
CMA
gets mixed with the snow and interferes with the ability of the snow
particles to stick to each other or to the surface. As a result,
the snow will be lighter and drier improving the traction of the
surface.
Ø
CMA
sticks more to the road since it doesn’t create running brine. This
means that less of the chemical is needed to be applied during the
storm.
Ø
Costs
about $700/ton
|
CMA Customer
Application Rates |
|
Agency |
Location |
Rate lb/lane mi |
g/m2 |
Daily Traffic
(vehicles/ day) |
|
Michigan,
DOT |
Zilwaukee
Bridge |
300 |
24 |
45000 |
|
Massachusetts,
DAW |
ROUTE 25 |
300 |
24 |
20000 |
|
Norway |
Mjosa
Bridge |
375 |
30 |
7000 |
|
Japan |
City of Sapporo |
375 |
30 |
25000 |
Liquid Chemicals have an advantage of staying on the road,
can’t be bounced off by trucks, and act faster in melting ice.
However, the loss and waste for liquid chemicals is 20% to 30% as
compared to waste of solid chemicals that are illustrated in the
graph below:
(Cryotech CMA®
Calcium Magnesium Acetate)
Some graphs that
compare salt to other chemicals:
This
graph shows how the corrosion rates vary from one deicer to
another. If we considered the corrosion rate of 100% rock salt as
the reference and it is 1.0, then 100% CMA, as a deicer, decreases
corrosion by 40 %. 30 % CaCl has a corrosion rate of about 86%, and
38 % of CaCl has a corrosion rate of about 121%
Spalling
Effect:
The chipping crumbling of concrete.
Factors
That Affect Deicing Actions:
Concentration:
-
Applying a chemical with a very high concentration, some of it may
not be needed and therefore be wasted.
-
Applying a chemical with a very low concentration may not be
sufficient to melt the ice or the ice might refreeze.
Eutectic
Point: Minimum
Melting Point
Temperature:
-
The
lower the temperature of the ice, the more deicing chemical is
needed.
-
A
little drop in temperature means a significant increase in the
amount of chemical applied
-
The
amount of salt needed to melt ice at a 20oF is five times
greater than at 30oF.
Time:
-
Time of reaction: the longer the time of reaction the greater the amount of ice is melted
-
Time of application:
o
In a
case of blowing snow and cold dry temperature, deicing chemicals
should be avoided. The chemicals will make the ice melt and the
cold dry weather will refreeze it, sticking to the road surface. If
no chemical is applied, the snow will just get blown off the road.
o
In a
case when the air is moist and snow has just started to drop, early
application might be a great idea. In the early stages, the ice is
loose and will easily turn into slush, which is 15% to 30% water.
Then, that solution can then be removed by plows. It is always
better to reapply then to over apply initially. The chemical should
be given enough time to melt ice before it is removed.
Weather Conditions:
-
Temperature of snow is very sensitive to weather of pavement.
o
On a
sunny day, the radiant heat will make the temperature of the
pavement 10oF greater than the temperature of the air.
This will assist in melting the ice. Therefore, a little amount of
deicing chemical will do the job.
o
At
night, the pavement temperature will be lower than the temperature
of the air. Therefore, the amount of deicing chemical needed is
greater.
Road Surface:
-
A
concrete surface absorbs heat rapidly and gives up heat rapidly. As
a result, the snow will melt faster on a concrete surface than on a
less heat absorbing surface.
Topography:
-
Topography ice occurs in topographic conditions such as high banks,
vegetation, or shaded areas. The longer the shadow stays on the
area, the more ice is formed on that area. Consequently, more
deicing chemical is needed to treat the area.
-
Because bridges and overpasses are in the air, they have lower
temperatures. Also, they usually have slopes. For these reasons,
they should be treated before roads. (Road Management Journal)
Environmental
Impact:
-
The
damage in the soil and vegetation occurs within 60 feet from the
road.
-
The
acceleration of deteriorating concrete and steel structures is
directly related to the use of de-icing chemicals on the roads
-
Poor
storage of these chemicals which may cause some of it to melt and go
into the soil or flow into any water course. As a result, wells can
be contaminated by the groundwater carrying de-icing chemicals.
(Road Management Journal)
A
Danish Study:
Ø
A
Danish study compared the groundwater concentration of chlorine and
found that the downstream concentrations are three times higher than
upstream concentrations even three months after the period of
heaviest road salting. The major problem is that chlorine ions,
chlorides, are almost impossible to treat once they are dissolved in
water.
Ø
Chlorides put many plants and small creatures in danger of
dehydration. The chloride and the sodium ions are too big to get
into the cell of an organism. They stay in the fluids surrounding
the cells, increasing the osmotic pressure between the fluid and the
cells. To equalize the pressure, water flows from the cell to the
fluid and then gets the plant or animal dehydrated. (The
Hamilton
Spectator, Buist, 2002)
In
New York:
Ø
A
study done by the St. Lawrence County Cooperative Extension
Community Forestry Program showed that both low and high levels of
salt in the soil have a great damaging effect.
Ø
Lower
levels interfere with nutrient availability in the soil causing the
tree growth to slow down.
Ø
Higher levels cause dehydration in young plant tissues causing them
to die. Deciduous trees are damaged by absorbing salt from the soil
or by soil structural collapse caused by the salt.
Ø
The
salt can be taken up through the needles of coniferous trees when it
is sprayed by the trucks or splashed (brine) by the moving taffic.
An excellent example of such trees is the evergreens.
(Road Salt and Trees)
|
|
Environmental Impact CMA Versus Road Salt |
|
Environmental
Impact |
CMA
|
Salt (NaCl)
|
|
Soils |
Biodegradable
in soil.
No adverse effect on soil compaction and strength.
Increases soil permeability |
Sodium may
accumulate in soil
Breaks down soil structure, increases erosion.
Causes soil compaction which decreases permeability.
|
|
Vegetation |
Little or no
adverse effect.
May stimulate roadside plant growth.
Acetate ion is the most abundant organic acid metabolite found
in nature. |
Osmotic stress
and soil compaction harm root systems.
Spray causes foilage dehydration damage.
Many plant species are salt sensitive. |
|
Groundwater |
Poor mobility
in soil, unlikely to reach groundwater.
Ca, Mg increases water hardness |
Mobile Na and
Cl ions readily reach groundwater.
Increases Na and Cl concentrations in well water along with
alkalinity and hardness.
|
|
Surface Water |
Potential for
oxygen depletion through biological oxygen demand(BOD) at
concentration greater than 100 ppm in closed systems.
Decomposes in 5 days at 20°C, 10 days at 10°C, 100 days at 2°C.
Will not stimulate algae growth. |
Causes density
stratification in ponds and lakes which can prevent
reoxygenation.
Increases runoff of heavy metals and nutrients through increased
erosion. |
|
Aquatic Life |
Less toxic to
trout than salt.
Minimal effect on trout eggs up to 5 times expected maximum
runoff concentration of 1000 ppm.
No effect on food chain up to up to 1000 ppm. |
Monovalent Na,
Cl ions stress osmotic balance.
Toxic levels: Na 500 ppm stickleback, Cl 400 ppm trout |
|
Human/Mammalian |
Mild skin and
eye irritant.
Vinegar odor.
Acute oral LD50 in rats greater than 5000 mg/kg.
Essentially nontoxic. |
Sodium linked
to heart disease, hypertension. Cl causes unpleasant taste in
drinking water. Mild skin and eye irritant.
Acute Oral LD50 in rats approximately 3000 mg/kg.
Slightly toxic. Contributes to winter road kills of wildlife. |
|
Water Treatment
Plants |
No significant
increase in BOD or impact on bacterial activity. |
No significant
impact at expected concentrations. |
|
Air Pollution |
Can reduce sand
use and resulting particulate emissions. |
Can reduce sand
use and resulting particulate emissions. |
Volumes
of Deicers Used Per Year:
These are the visual of the amount
of deicer per mile if we took all the amount applied during the
year to that mile and put it all at once.
Salt (NaCl):
Density: 70 lb/ cubic ft; 300 lb/ln/mile per year
Volume: 4.285 cubic ft
10 ft
8.117 X 10-5 ft
5280 ft
CaCl:
Density: 55lb/ cubic ft; 24 lb/acre/per year
Volume: 0.529 cubic ft
10
ft
1.002 X 10-5 ft
5280 ft
CMA:
Density: 40lb/ cubic ft; 325 lb/ln/mile per year
Volume: 8.125 cubic ft
10 ft
1.539 X 10-4 ft
5280 ft
A
Basic Breakdown of Costs and Benefits of Winter Maintenance, Using a
Deicing Chemical:
|
Costs
Direct:
-
Material cost (deicer)
-
Equipment
cost
-
Labor
cost
Indirect:
|
Benefits
Direct:
Indirect:
-
Reduction in liability claims
to road authority (associated to hazardous driving
conditions).
-
Maintain the economic activity
(production, transportation, and earnings)
-
Maintain
access to social activities.
|
(A compendium
of Costs & Benefits Indicators)
References:
A Compendium of Costs & Benefits
Indicators. “Regulatory Economic Analysis Branch of Winter Road
Maintenance.
http://www.ec.gc.ca/nopp/roadsalt/pdfs/winter_e.pdf. Online.
November 15, 2003.
Cryotech Dicing Technology.
Cryotech CMA® Calcium Magnesium Acetate.
http://www.cryotech.com/cma.htm.
Online. October 19, 2003.
Great Lakes Directory. Buist, Steve.
The Hamilton
Spectator.
http://www.greatlakesdirectory.org/zarticles/33roadsalt.htm.
Online. September 25, 2003.
Road Management Journal. Using
Salt and Sand for Winter Road Maintenance.
http://www.usroads.com/journals/p/rmj/9712/rm971202.htm. Online.
September 25, 2003.
Duluth Streams Organization. Road
Salt: Can we have safe roads and healthy streams.
http://www.duluthstreams.org/understanding/impact_salt.html.
Online. September 25, 2003.
St. Lawrence County Cooperative
Extension Community Forestry Program. Road Salt and Trees.
http://www.co.st-lawrence.ny.us/Cooperative_Extension/forestry/18roadsalttrees.html.
Online. September 25, 2003.
