Necessary for the efficient movement of people, goods, and services, roads are a ubiquitous feature of urbanized landscapes, but they may also have significant ecological impacts. One such impact may result from the use of deicing salts such as sodium chloride, magnesium chloride, calcium chloride, and potassium chloride, which are used for winter road maintenance on hard-surfaced roads and as dust suppressants on gravel roads. Significant ecological consequences may result from these salt applications because the chloride and metal ions contained in road salt spray and runoff contaminate adjacent vegetation and soils. The objectives of this research were to improve our understanding of the effects of deicing salt on roadside woody vegetation and soils and to improve the linkages among road engineers responsible for safety, the public that uses the roads for daily transportation, and resource managers responsible for maintaining vegetation. Four sites with symptomatic (those exhibiting yellowing or chlorotic foliage and showing at least some degree of canopy dieback) ponderosa pine (Pinus ponderosa C. Lawson) were paired with four sites containing asymptomatic trees. Soil and pine needle foliage samples were taken on both sides of the road at measured intervals. Additional site-specific information (i.e., slope, vegetational structure and composition, and variety and quantity of salts applied each season) was also recorded. The research was conducted along South Dakota state highway in the Black Hills routes known for high snowfall and salt use. Research areas were mapped out using geographic information systems (GIS) in order to chart areas exhibiting foliar damage possibly due to deicing applications. South Dakota Department of Transportation (SDDOT) records were consulted to determine the types of deicing salts the SDDOT used, the amounts used, and the dates they applied it, whenever possible.
A total of 557 km of roads were surveyed and approximately 50 percent exhibited at least some degree of canopy dieback and discoloration. This study found that only a few sources of variation were significant with regard to treatment elements at the selected paired test sites. The most statistically significant soil sources of variation were correlated with season sample times, symptoms, distance, depth, sample times and orientation, symptoms and distance, symptoms and depth, and distance and depth. Soil sodium and magnesium were both found to be statistically significant. Chloride, however, was not found to be significant across any source of variation. Foliage sodium, magnesium, and chloride were found to be significant with one or more of the following sources of variation including sample times, symptoms, distance, sample times, and distance. To supplement these findings, t-test tables, averages with standard errors, and ANOVA’s with Pr > F values were applied and accompany this research. Pearson correlation coefficient tests were run to determine if nutrients and elements in the soil had been absorbed by the vegetation. Strong relationships between extractable soil elements affecting foliar ion concentrations were difficult to find in data obtained in our research. This research did not find strong statistical differences in elemental concentrations in foliage or extractable ion concentrations in soil between symptomatic or asymptomatic sites. Additionally this research shows that elements in the deicing salts can translocate to distances of 20 meters, however, as distance increases from the road (beyond 6 meters) foliage Na+ and Cl- element concentrations quickly approach background levels. Given ideal situations and topography, deicing salt ions may translocate by air and through soil great distances from the road. But as other researchers found, though, this study demonstrates detectable amounts are minimal and should not pose a serious health risk to vegetation.