Changes in the Heavy Metal Levels in Highway Landscaping and Protective Effect of Vegetative Materials

Anthropogenic activities due to increasing population and traffic density are responsible for a great portion of highway pollution. The heavy metal accumulation in highway routes poses a risk both for agricultural areas and residential areas. The study investigated the changes in heavy metal accumulation along a 200 km long portion of the D300 highway passing through Elazığ, Bingöl, and Muş, cities located in the Eastern Anatolia region of Turkey. The heavy metal accumulation in 46 soil samples collected in 2018 and 2019 from 5 different land classes was analyzed using the ICP-MS device in an accredited laboratory. The analysis results were explained using different statistical methods depending on the standard, annual change, land class, and vegetation. Although the majority of the soil samples were within acceptable levels, the chromium (Cr) and nickel (Ni) levels of certain samples were above the standard levels. Considering the land classes, compared with other areas, residential areas (RA) contained higher levels of zinc (Zn); agricultural areas (AA) contained higher levels of chromium (Cr), cobalt (Co), nickel (Ni), zinc (Zn), cadmium (Cd), and lead (Pb); and unqualified areas (UA) contained higher levels of copper (Cu). Considering vegetation, the tree- and bush-covered soil samples contained lower amounts of Cr, Co, Ni, Cu, and Cd but higher levels of Zn and Pb compared with herbaceous or bare soil samples. A similar case also applies to the soil samples that were covered with Quercus sp., a natural plant cover on the route. The results and other similar studies have shown that there should be at least 15 m long ecological corridors (pollution-resistant tree-bush vegetation) between highway routes and both agricultural and residential areas