Concentration profiles of arsenic, cadmium, chromium, copper, lead, mercury, nickel, zinc, vanadium and polynuclear aromatic hydrocarbons (PAH) in forest soil beside an urban road

Abstract

A relatively busy road with a load of 3200 vehicles per day has since 1961 run through the Aplerbecker Forest near the town of Dortmund in West Germany. In October 1991 the soil in the forest (brown earth) was examined for the metals arsenic, cadmium, chromium, copper, lead, mercury, nickel, vanadium, zinc and for fifteen polynuclear aromatic hydrocarbons (EPA). It was established that — with the exception of arsenic, chromium, copper and mercury —the concentrations of all the heavy metals under investigation and of the individual PAH substances present in the forest soil decreased exponentially as the distance from the road increased. It was also established that the background concentration of the PAHs and the metals were reached after 3–5 m and 5–10 m, respectively. The concentration of lead and cadmium in the soil at the edge of the road is five times higher than that in the natural forest soil whereas chromium, nickel, vanadium and zinc show a twofold to fourfold increase in contamination. In the soil lying directly at the roadside edge, contaminations were found of up to 2 mg Cd/kg, 64 mg Cr/kg, 34 mg Cu/kg, 0.12 mg Ni/kg, 32 mg Ni/kg, 245 mg Pb/kg, 51 mg V/kg and 241 mg Zn/kg. The PAH concentrations in the road edge soil were higher by a factor of up to a 100 compared to those in the forest soil further away from the road (benzo[a]pyrene, factor 70) and were measured as follows: 22 ppm naphthalene, 6.3 ppm phenanthrene, 3.7 ppm anthracene, 32 ppm chrysene, 20 ppm benzo[a]pyrene and 43 ppm benzo[g,h,i]perylene. In this paper, the thick vegetation structure of the forest and its barrier effect on airborne hazardous substances will he discussed as a reason for the heavy accumulation of the metals and, above all, of that of the PAHs detected in the roadside soil.