How much salt is a problem?What other environmental effects can salt have? Freshwater organisms: Here are some values for comparison. We used the calibration curve between chloride and EC25 to convert regulatory criteria and various toxicity study results to the conductivity (EC25) values measured routinely by DuluthStreams. Remember that it is a combination of the concentration of a pollutant and the time of exposure that determine the dose to a fish or bug, and therefore their risk. Short-term high values, if infrequent and not too high, may pose less risk than a lower level that is sustained for many months. These situations refer to acute versus chronic toxicity. Concentrations of chloride found in waste snow piles are typically very variable but in the range of 100s to 1000s of mg/L (Reference1 below). |
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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.
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 to erosion.
How much is actually used?
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:
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.
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
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.
REFERENCES Toxicity Table Citations
Other
Who's trapped in the
salt shaker? |
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For more information about salt and the environment, visit:
spring 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.