Environmental assessment of metals in road dust: what do geochemical indices really tell us about pollution?

Pollution of road dust has been studied for decades because of potential health and environmental risks resulting from elevated levels of metals in this medium. In many studies, environmental assessment of road dust pollution has been made with the use of indices that were originally introduced as a tool to assess the quality of air, soils and sediments. Examples of the most popular geochemical indices are the enrichment factor, the contamination factor, the index of geoaccumulation and the pollution index. We calculated these indices using the element concentrations (Ba, Ca, Cu, Fe, K, Mn, Pb, Si, Sr, Ti, Zn and Zr) in three grain fractions of road dust samples (< 2 mm, < 1 mm and < 0.063 mm) collected in the city of Kielce, Poland. The study showed that critical parameters to be considered in data interpretation are the selection of geochemical background value, the use of appropriate reference element and grain size fraction. Application of local geochemical background instead of the Earth's crust composition, grain fraction < 2 mm and appropriate reference elements are recommended for calculation of geochemical indices in road dust samples. This study shows that the use of geochemical indices for environmental assessment of road dust pollution should be carefully considered.