Salt and the natural environment

Roadside vegetation

Roadsides are a stressful environment for vegetation. They are always man-made environments, created when the roadway was constructed. Often, soils are compacted. The exposure to wind and traffic – and toxic contaminants deposited by that traffic – make roadsides a dry and harsh environment for plants. Salt can add to that stress. High concentrations of chloride can interfere with a plant’s absorption of moisture from soil and cause browning or burning of leaves. High sodium concentrations may affect plant growth by altering soil structure, permeability and aeration. The additional harm to vegetation which salt may inflict depends on six characteristics: the amount of salt, type of soil, total precipitation, distance from the roadway, wind direction, and plant species. In short, the impacts are highly site-specific. Assessing the environmental impact of salt requires an understanding of the concentrations and durations of the exposures and the types of plants that are exposed. Different soils tolerate sodium differently. Different plant species tolerate chlorides differently. Different climates affect the frequency and duration of wintertime exposures. Exposures vary by season from the high chloride loadings of winter and spring to the low exposures during summer and fall. Elevated soil levels of sodium and chloride decrease over the growing season due to leaching of the ions by rainfall and run-off. Soil measurements in summer and fall indicate a decrease to background soil levels following elevated spring levels. Recent studies have indicated that some plants under stress are able to fight off diseases better when salt application is added than the same species of plants not exposed to salt.36 Again, these variables differ from one locale to another.

Different rates of precipitation affect the saline concentration of the runoff. The Federal Highway Administration has studied highway runoff and concluded: “highway runoff is generally cleaner than runoff from buildings, farms, harbors, or other non-point sources…it is important to recognize that highway runoff need not be and most often is not a serious problem.” After more than 50 years of salting, it is theoretically possible that sodium buildup in roadside soils may indirectly affect plant growth. A solution would be to chemically amend excess salinity from the soil by adding gypsum or anhydrous ammonia.

Currently, gypsum treatments appear to be the most efficient and least expensive reclamation method. Some general observations about the ten-year impact of deicing salt on roadside vegetation and soil were made in the study report: Although there was a general cumulative trend of sodium ions, it was far below sodium levels that are considered damaging. Chloride ions leached out of the soil fairly rapidly and thus had no cumulative effect. The overall effect diminished as distance from the roadway increased and became insignificant beyond 80 feet. Potassium chloride and urea are common fertilizers that are sometimes used for roadway or sidewalk deicing. They are commonly thought of as safe products to use around vegetation, but application rate determines vegetative damage and melting ice usually requires dosages far greater than recommended fertilizer application rates.

Sensible Salting can reduce the salt loadings to the roadside environment, but key contributions can be made by good engineering so that roadside environment to be salt-tolerant. Just as car manufacturers have “salt-proofed” their vehicles, highway agencies can “salt-proof” the roadside environment. The roadway right-of-way is not a natural environment; it is engineered to create a roadway. Good highway engineering practice channels runoff to facilitate drainage and prevents adverse environmental impacts. Trees adjacent to arterial roadways or major highways are generally removed as safety hazards. Replacing grass, shrubs and trees (where they can be located safely) involves a choice. Environmentally-conscious highway planners choose species which can tolerate the severe operating conditions of the roadway environment they are creating. Of course, all the adverse impacts of roadways diminish with distance from the travelway, with lesser impacts recorded up-hill and up-wind as well. There are species of plants, trees and shrubs that have a high salt tolerance and other species which have a very low salt tolerance. Oaks, locusts, Scotch elm, Russian olive, hawthorne, and silver and gray poplars all have high resistance to salt. On the other hand, sugar and red maples, Lombardy poplar, black walnut, and rose and spirea bushes would be poor choices for locations exposed to salt runoff and spray from deicing operations. The United States Department of Agriculture Research Service has done extensive testing on the salt sensitivity of 13 different pine species. Seedlings sprayed with salt solutions were compared with control groups sprayed with distilled water. Three of the 13 species did very well even under extremely salty conditions, which were saltier than the worst roadside conditions—Pinus thunbergii and P. nigra showed an 89 percent survival rate and P. ponderosa had a 95 percent survival rate. The noted survival rate is % of control - so 100% would be “normal” under lab conditions.

Drinking Water and Sodium

The amounts of sodium and chloride being consumed by humans in drinking water are rarely a significant source of either element so neither has a health standard.56 Sodium concentrations above 20 mg/L are monitored to provide consumers information useful if they are placed by their doctors on medically-supervised low-salt dietary therapy. At 20 mg/L, regulations of the Food and Drug Administration would consider beverages “sodium free,” US EPA has conducted several rulemakings designed to de-emphasize concern with sodium in drinking water,58 and state citizen notifications have been abolished.

Some people can detect an unacceptable taste in water with chloride concentrations exceeding 250 mg/L. Though drinking water chloride levels rarely ever reached the 250 mg/L concentrations triggering concern for human palatability, long term trends show steady improvements in the environmental releases of chlorides. Water quality trends in the Great Lakes show a declining trend in chloride. A recent search of the literature shows that progress in reducing chloride discharges has so diminished their environmental impact in the Great Lakes that they are no longer mentioned in “the State of the Great Lakes” reports.

Hexacyanoferrates

Humid conditions and precipitation cause salt crystals to “cake” or stick together. Salt producers add several anti-caking agents to highway salt and to table salt to keep them free-flowing. Among the most popular is sodium hexacyanoferrate(II). It is an FDA-approved food additive. Common names for the two most popular hexacyanoferrates, sodium hexacyanoferrate(II) and iron(III) hexacyanoferrate are yellow prussiate of soda or YPS and Prussian blue; other common names are sodium ferrocyanide and ferric ferrocyanide. This has led to confusion with some people anxious about the safety of these additives because free cyanide and hydrogen cyanide are highly toxic. Hexacyanoferrates (or ferrocyanides) are not toxic; they are chemically-stable metal complexes and completely non-toxic. To make the point, one study gave rats a solution of 20,000 mg/L ferric ferrocyanide in drinking water for up to a total intake of 3,200 mg/kg (bw)/day for 12 weeks and the rats showed no signs of toxicity. In highway salt, concentrations range between 20 and 150 mg/L. Despite their threaten¬ing names, these stable, complex metal cyanides (YPS, Na4Fe(CN)6•10H2O and Prussian blue, Fe4[Fe(CN)6]3) should not be confused with highly toxic free cyanide (CN , hydrogen cyanideHCN or simple metal cyanides such as sodium cyanide NaCN or calcium cyanide, Ca(CN)2).

Salted Roads and Animal-Car Collisions

There is little scientific information about vehicle-wildlife collisions related to the presence of highway salt along the roadside.41 Some have argued that animals’ need for salt attracts wildlife to salted roads in the wintertime, increasing vehicle crashes. It is difficult to gauge the motivation of wild animals; we must observe their behaviors. According to the Wisconsin Department of Transportation, the highest incidence of motor vehicle-deer crashes (38 percent) occur in October-November and the second highest period (16 percent) occur in May or June.42 Obviously, these are not months when highway salt is used on the roads. A survey of wildlife mortalities due to vehicle accidents in Canadian national parks confirmed that the majority of kills occurred in spring or fall, not when salt is applied to the road.
A study by the Michigan Department of Natural Resources (Langenau, et al. 1997) found that most wildlife-vehicle collisions occurred on paved local roads rather than Interstate Highways. A higher frequency of deer-related accidents are reported where roadsides are planted with foods deer prefer, “such as rye, alfalfa and clover.” Mowing keeps much of this roadside vegetation green and lush, which attracts deer. The authors said half of all deer-vehicle collisions in Michigan occurred in autumn during the breeding season for deer. A second peak occurs in the spring when deer move to summer range from winter concentration areas. A 1998 Wisconsin Safety Council publication45 refers to a recent study by the Wisconsin Department of Transportation stating deer-vehicle collisions peak during the October-November “deer breeding or rut” season, which is also the state’s deer hunting season.

Salt Tolerance in Fish

Healthy fish need salt too. Aquaculturists use salt as a medication to combat certain fish diseases and add salt to the water when they transport freshwater fish. But even a good thing can be overdone. Freshwater fish usually tolerate high salinity well. Again, exposure is a combination of concentration and duration. When chloride enters a stream as runoff, it creates a chloride “pulse” which will travel down and out of the stream in a relatively short time (i.e., days to weeks, depending on the width, gradient and length of the stream) because the water is constantly flowing through the stream. Different fish species exhibit a range of tolerance to different salts according to the time of exposure, salt concentration, temperature and character of the test water. The short-term effects of salt on channel catfish (Ictalurus punctatus), bluegill sunfish (Lepomis macrochirus), smallmouth bass (Micropterus dolomieu), rainbow trout (Oncorhynchus mykiss), yellow perch (Perca flavenscens), fathead minnows (Pimephales promelas), brown trout (Salmo trutta), lake trout (Salvelinus namaycush) and walleye (Stizostedon vitreum) survive well at test conditions involving a concentration of 10,000 mg/L NaCl for 24 hours (with water temperatures of 12° C and water hardness at 140 mg/L CaCO3 ). All species showed 0% mortality, with the exception of smallmouth bass, which had 3% mortality.

Oral toxicity (The Registry of Toxic Effects of Chemical Substances, 1986):

Acute aquatic toxicity (U.S. EPA, Ambient Water Quality Criteria for Chloride, 1988):

EPA says that the chlorides of calcium, magnesium and potassium are generally more toxic to fresh water species than sodium chloride. Some Antarctic species depend on salt to protect them against the cold.

Sodium and chloride occur naturally in soils and waters, and are added by residential, commercial and industrial activity. Aquatic organisms and vegetation, including crops and roadside grasses, shrubs and trees, tolerate various concentrations of sodium and chloride. The following classification is used by the U.S. Department of Agriculture to indicate the degree of hazard of saline soils to food crops. It is based on conductivity and salinity hazard. (Conductivity can be converted to approximate mg/L dissolved solids).

USDA Salinity hazard ratings:

New developments in toxicity testing are giving us a better understanding of the impact of chlorides on the environment. The current US Environmental Protection Agency chloride toxicity criteria were established using a limited number of species in a distilled water-based medium. This resulted in conditions that did not reflect the real environment under consideration. When the water quality criteria were established, the EPA was unaware of the extent and the role of water chemistry on chloride toxicity. As a result, except where challenged, current standards do not reflect the mitigating impact of water hardness on chloride toxicity. In addition, when a broader number of species are tested, particularly those that are present in the location under investigation, more meaningful toxicity data are derived. Read more... (pdf 945.23 kB)