Heavy Metal Diffuses Pollution Caused by Atmospheric Deposition in Natural Soils.

The heavy metal backgrounds in natural soils came from the geological substrata. Nevertheless, many times these soils show trace elements concentrations differing from those expected when only the bedrock lithology is considered.

In order to assess the existence of heavy metals diffuse pollution caused by atmospheric deposition in non disturbed areas the concentration of 16 trace elements (Mn, Zn, Cr, Pb, Tl, V, Co, Ni, Cu, Ba, As, Mo, Ag, Cd, Sb, Hg) were measured in 117 soil samples spread over an area of 10000 km² located in the North of Spain. With the aim to obtain non polluted soil samples pristine hillslopes were selected at the longer distance possible from the main pollution focuses.

 The sampling was designed taking pairs of hillslopes, exposed to dominant rain fronts which crossing the pollution focuses and opposite to the former. For each hillslope, two soil samples were taken (at the top and bottom) in order to identify the pattern of trace elements distribution in slopes caused by runoff mobilization.

 When considering the whole data pool coming from the samples, higher values for many elements was found at the exposed slopes revealing diffuse deposition of such elements. These differences become statistically significant for Mn, Co, Ni, Cu, Ba, Mo if only samples closer than 40 km to the main pollution focus were considered.

 The main conclusions of this study are that in despite of many areas are considered as pristine and being protected by different laws, atmospheric deposition of trace elements could be of importance where singular climatic and topographic circumstances co-occur. Also, the determination of natural geochemical backgrounds for some trace elements identify such process is necessary in order to no overestimate those levels, which are commonly used in developing environmental management strategies and legislation related to these concentrations thresholds.