Chloride toxicity to aquatic organisms.
Conductivity (EC25) values were estimated from the DuluthStreams standard curve on the
previous page and rounded to the nearest 100 uS/cm.
||synthesis of many tests; prolonged exposure
(>30 days) will eliminate 10% of the species
||Minnesota chronic standard for trout
(assumed to be for a >100 day adult exposure)
||Federal & State Secondary Drinking Water
Standard (mostly for taste)
||chronic toxicity (>30 day
||chronic toxicity (>30 day exposure)
||significant (25%) adverse effects on trout
eggs, embryos & adults in 7 days
||lowest observed effects after 7 days
||acute toxicity (50% mortality in 4 days)
||lowest observed effects after 7 days
||acute toxicity (50% mortality
in 4 days)
||lethal unless you're a sea bass
Salt damage to roadside pines.
photo by U of North Dakota Extension Service
Vegetation and soil:
The most visible impacts of road salt are
usually on roadside vegetation where a fringe of dead or dying trees and shrubs may be apparent
on major highways and streets. Also, if there is only a small
strip of land between the road side and a stream or wetland,
the shoreline vegetation may receive relatively high amounts
of road salt. The impacts may be simply aesthetic - the trees
look terrible. But remember that shoreline vegetation is extremely
important to aquatic ecosystems because it helps prevent erosion
and provides habitat to aquatic organisms as well as birds
and other animals.
Here is more information (314 KB pdf) from the MN Extension Service on how to diagnose salt damage to plants.
Photo credit: Plant Disease Clinic,
MN Extension Service
-click to enlarge-
The actual damage is mostly caused by the chloride portion
of the salt, and is toughest on young trees and evergreens.
It essentially creates an extra period of drought conditions
for the plants. However, the extremely high concentrations
on leaf and twig tips from direct salt spray from vehicles
can directly damage plant tissues. Leaves on roadside vegetation
along larger highway arteries, managed by the County and State,
may also be brown due to herbicide applications for reducing
plant material that may clog runoff ditches.
Soils can also be damaged by roadsalt but chiefly by its sodium
content. Excess sodium destroys soil structure which reduces
its ability to retain water and increases its susceptibility
How much is actually used?
Road salt is often applied in areas with narrow buffers to surface waters.
It depends on the weather. The amount
of road salt used depends on the number of snow and ice events.
Thus a milder winter with light snow, but lots of "events"
may actually need more salt than a winter with more snow overall.
In Duluth, the street maintenance department carefully calculates
application rates based on temperatures and the nature of
the snowfall. Because this is an expensive budget item decisions
are made to maximize safety while keeping costs reasonable.
Over the past four years (1999/00 through 2002/03) the amount
of salt used annually has ranged from 7659 tons to 12,224
tons. The State of Minnesota used over 225,000 tons at a cost
of almost $7 million in 1996, the year of record snowfall.
The total US annual usage is approximately 14 million tons
and Canada uses about 5 million tons per year. (see Ref 1
and 5 below).
A number of alternative products have been introduced.
The City's Public Works Department continually investigates
these products seeking viable alternatives. To date, the alternatives
are significantly more expensive and not more effective. In
a time of deep budget cuts, safety and cost effectiveness
become major considerations.
Any other problems caused by excess roadsalt?
Actually there are several other adverse effects that aren't
very obvious. Fortunately at present (Spring 2003), there
is no evidence to suggest they are a serious problem in the
Duluth area. They include:
- increased metal toxicity - higher TDS can increase the
availability and toxicity of heavy metals already present
in stream, wetland or lake sediments or in urban runoff (Ref
- corrosion of concrete and metal - besides our cars and
trucks, the estimated cost of damage to pavement, bridges,
culverts, etc is $40-90 per ton of applied salt (Ref 1);
- pond stratification - high salt content runoff into ponds
and small lakes next to intensively salted roads can form
a dense layer over the bottom that restricts oxygen transport
from overlying water in contact with the air. This can lead
to oxygen deficiency problems in addition to exposing bottom
(benthic) organisms to high salt concentrations for extended
time periods (Ref 1, 10);
- dust - Denver, CO has to worry about the excess dust from
both salt and sand that compounds their urban air pollution
problems (Ref 7);
- anti-caking agents - recent studies by the Minnesota Pollution
Control Agency indicated that another cause for concern is
from the cyanide in anti-caking agents. While the salt that
homeowners use contains no anti-caking agents, road salt includes
the additive to prevent salt crystals from congealing into
chunks impossible to spread on the roadways. And this stuff
contains cyanide that is extremely toxic to aquatic organisms.
The diluted amounts sprayed on roads isn't the problem -it's
the concentrated runoff from improperly managed salt piles
(see more by visiting Ref 9). Actually, even without the cyanide
issue, it's important to carefully manage both salt and snow
piles carefully to minimize their impact on nearby water resources.
What effect does the salt that I spread on my driveway
and sidewalk have on Duluth's streams?
Good question. The answer isn't really as simple as "Lots"
or "Little". Of course we often need to add salt
for public safety. We also know that too much salt can have
negative effects on stream critters, soil and vegetation and
that these effects must be added to all the other stresses
we contribute. Salt also costs money, not only the cost of
purchasing 10 or 20 pounds, but the added cost of your car
rusting out, and the City's pavements, roads, culverts, bridges,
etc all rusting or corroding.
So the answer is that you should use as little as you need
to and be aware that all the stuff you sweep or shovel off
your sidewalk and driveway can end up in a stream. As much
as possible, try to sweep up residue and dispose of it properly.
When the roadside snow piles melt in the spring, it's better
to spread the sand and grit on turf than to sweep it off the curb into the street.
The mouth of Miller Creek in early May, 2003
If it rains before a street sweeper gets
there, debris in the gutter will be washed down catch basins
and end up in a stream or the lake. For a visual illustration
of how much debris is generated each year, visit the mouth
of Miller Creek (22nd Ave W) and watch the delta build up
in the Spring.
Residential streets are swept once in the
and once in the fall.Heavy traffic areas are swept more frequently
and business areas are swept weekly. The sweeping keeps six
street sweepers and a crew of at least three per sweeper busy
starting from the first snow melt of spring until the first
snow fall in autumn. In 2002 sweeping continued into December
and street sweepers were out again in January 2003.
For more about the effectiveness of street sweeping in the
Minneapolis-St. Paul area and in Wisconsin cities check out:
Best Practices for Street Sweeping.
What about our lakes and ponds?
We don't know of any area-wide studies specifically addressing
road salt impacts on our lakes and ponds but it is likely
that the effects are generally small compared to the other
pollutants that wash into them.
The Wisconsin DNR has examined
some long-term records for a number of lakes and estimated
rates of increase of about 0.1 to 0.2 mg/L of chloride per
year over a 10 year period for a relatively small lake and
about 0.3 mg/L for the very large Lake Mendota in Madison
over the period from 1910 to 1980. In both cases the actual
chloride concentration for the main was still far below levels
of concern (Ref 8).
The City of Madison prepared a road salt report in
2004 (913 KB pdf file)
that summarizes 30 years of data relating to their use of road salt and
its effects on lakes and wells.
Toxicity Table Citations
- Environment Canada. 2000. Priority
Substances List Assessment Report: Road Salts [pdf]. August 2000 draft for public
comments, 156 p. A huge summary of information.
- Minnesota Rules, Chapter 7050.0220 SPECIFIC STANDARDS
OF QUALITY AND PURITY BY ASSOCIATED USE CLASSES (http://www.revisor.leg.state.mn.us/arule/7050/0220.html)
- EPA. 2003. Secondary
Drinking Water Regulations: Guidance for Nuisance Chemicals (http://www.epa.gov/safewater/consumer/2ndstandards.html). US Environmental Protection Agency, Washington, D.C. USA.
- EPA. 1988. Ambient water quality criteria for chloride.
US Environmental Protection Agency, Washington, D.C. USA.
- Shoultz, B. 1997. Road Salt: Minnesota's need to
examine its use and effects. Lakeside Magazine - the Official
Publication of the Minnesota Lales Association, January/February
- Shoultz, B. 1997. Road Salt Prt 2: Salt alternatives
and methods to minimize salt impact. Lakeside Magazine - the
Official Publication of the Minnesota Lales Association, January/February
- Keating, J. 2001. Deicing salt: Still on the table.
Stormwater 2(4) May/June p. X: xx-xx.
- Young, R. And K. Schreiber. 1999. Salt of the earth:
Does road salt affect our waters. Lake Tides 24 (1): 1-3.
U. of Wisconsin Extension, Stevens Point, WI 54481 http://www.uwex.edu
- Minnesota Environment. November 2000. Worth
his salt. http://www.pca.state.mn.us/publications/mnenvironment/fall2000/salt.html
- Novotny, V., D.W.Smith, D.A.Kuemmel, J. Mastriano
and A. Bartosova. 1999.Urban and Highway Snowmelt: Minimizing
the Impact on Receiving Water. WERFProject 94-IRM-2. Water
Environment Reserach Foundation, Alexandria, VA.
Who's trapped in the salt shaker?
Duluth Streams and WOW Principal Investigator, Rich Axler.