Environmental Impacts Environmental Impacts 
Trees and other vegetation along the side of a road
can be injured by salt primarily in two ways. The first is by
high levels of salt accumulating in soil and soil water,
resulting in salt absorption through roots. The second, is salt
concentrating on foliage and branches due to splash and spray
from traffic. Some physiological symptoms of vegetation suffering
from salt injury are leaf scorch, late summer coloration, early
fall defoliation, reduced shoot growth, and dying twigs and
branches in the crown of plants. Damage from high salt
concentrations usually first inhibit the growth of the plant,
then there are specific injuries to foliage and limbs. In some
cases, the end result is death. Grasses generally are more
resilient than trees and shrubs.
Factors such as temperature, light, humidity, wind, soil texture and drainage, precipitation, plant size, salt exposure, species tolerance, and especially water availability contribute to the degree of salt injury. In general, chloride is more harmful to vegetation than sodium. Once absorbed through the roots, chloride tends to accumulate in plant tissues over a long period of time, causing osmotic stress. Osmotic stress can in turn lead to dehydration.
Damage to vegetation is especially acute in areas where there is a high water table. High concentrations of salt particles in soil can damage root system and inhibit root growth. Damage increases with the amount of salt applied and with traffic volume and speed, but decreases with distance from the roadway. The Connecticut Department of Transportation found that airborne salt traveled as far as 300 feet from the roadway under heavy traffic conditions on the Interstate and other primary highways. Also salt spray can be transported downwind for distances greater than 500 feet under high wind conditions.
A considerable portion of the deicing salts that are applied to roads eventually enter the groundwater aquifers one way or another. Opinions vary as to how much deicing salts actually end up in groundwater. The true amount is hard to determine because it is a function of site specific features such as permeability, slope, vegetative cover, and infiltration. However, the fact still remains that sodium and chloride ions that infiltrate into the aquifers could lead to human health risks. The biggest concern is increased sodium levels in drinking water supplies. Sand, gravel, and coarse-textured soil allow fast infiltration, while clay and fine-textured soil slow infiltration.
Chlorides are naturally present in groundwater, usually 10 mg/L. Chloride ions move faster through the soil than sodium and penetrate aquifers because they:
Natural concentrations of sodium exist in groundwater anywhere from 6 mg/L to 130 mg/L. Sodium is positively charged and adsorbs on soil minerals with high cation exchange capacities. Such minerals are usually clay and organic particles.
The trend through time has indicated that there has been an increase in the amount of sodium present in groundwater. As to whether deicing salts is the cause of this increase is not clear. Some researches say deicing salts are the cause of increased sodium in the groundwaters while others report that this is not the case. However, the fact remains that these high concentrations are of concern for public and domestic drinking water supplies and for terrestrial and aquatic plants.
All surface waters contain a natural amount of salt. This natural salinity is from rock runoff of drainage basins, atmospheric precipitation, and balance between evaporation and precipitation. However, during and after storms and spring melt, highway runoff contains high concentrations of sodium and chloride. Fortunately, these high concentrations are diluted once they enter large water systems. Salt concentrations upon mixing vary depending on the size of stream or water body, salt intensity, precipitation, topography, and drainage patterns. Thus, small streams and creeks are most affected by deicing salts. These salt concentrations decrease with water volume and distance from roadway.
| Type of Water | Chloride Concentration (mg/L) |
| Rainwater | 0-2 |
| Upland surface water | 0-12 |
| Unpolluted river water | 0-15 |
| Springwater | 0-25 |
| Deep well water | 0-50 |
| Sewage water | 70-500 |
| Seawater | 20,000 |
Roadway runoff reaching surface waters usually contain deicing salts in solution. If runoff containing high amounts of these salts reach a lake, the density of inflow may be such that inflow falls straight to the bottom. If these conditions persist, the lake will not undergo the usual spring overturn. Without this mixing, stratification will occur. As a result, oxygen will not be dispersed throughout the layers of the lake, which soon leads to anoxic conditions. In addition, nutrient accumulation could occur, leading to eutrophication.
The extreme chloride concentrations that are harmful to fish (400 - 1200 mg/L) are rarely generated by highway deicing salts. Deicing salts do, however, impact aquatic life in two main ways. The first is by causing stratification in lakes through changes in the density gradient. This lack of seasonal mixing decreases water circulation and reaeration in lower depths, which in turn lowers the dissolved oxygen content in the lake. These conditions result in the death of many aquatic organisms. In one study, there was a large reduction in dipteran larvae (blood worms) and oligochaetes which would otherwise be abundant. Only the most pollution tolerant species remained. The second way that deicing salts affect aquatic life is related to the fact that osmotic regulation is significantly affected by variations in salinity. Sodium is one of the most important inorganic solutes that influence this process.
While organisms have come up with many mechanisms to regulate salt and water content, most freshwater bacteria and blue-green algae are relatively homoiosmotic. This means that they retain the initial internal osmotic concentration and only tolerate a narrow range of salinity. However through genetic change, homoiosmotic organisms have been able to adapt to increased salinity. Most aquatic flora and fauna are adaptable to a wide range of salinity. Invertebrates and microbes are more sensitive.
In general, sodium chloride is not found to have significant deleterious impacts on aquatic biota in large or flowing bodies of water due to quick dilution. If the salt concentration begins to exceed tolerance levels, though, aquatic life will be affected and die off rates will increase.
In general, large terrestrial animals are fairly tolerant to salt. However, poisoning of smaller animals can occur if they consume salt residue on dirt and gravel. The most serious problem that deicing salts pose to wildlife is the traffic hazard created by salt hungry animals as they seek roadside salt accumulation. Researchers believe that moose have an increased dietary need for salt in the spring to facilitate horn growth in males and lactation in females. As a result, they can often be found along salted roadsides. Another concern is that once a moose has visited a roadside salt pool, evidence shows that this same moose will return the following spring, possibly leading the way for other newcomers.
