Arsenic distribution in the environment: The effects of scale

摘要

The distribution of As in a variety of sample materials was studied at different scales, from continental to local, combining published data sets with the aim of delineating the impact and relative importance of geogenic vs. anthropogenic As sources. Geochemical mapping of As with a variety of sample materials demonstrates that variation is high at all scales (local to continental) - from sample densities of 400 sites per km(2) to 1 site per 5000 km(2). Different processes govern the As distribution at different scales. A high sample density is needed to reliably detect mineralisation or contamination in soil samples. In both cases the impact on the much larger geochemical background variation is limited to a local scale. Distribution patterns in geochemical maps on the sub-continental to continental scale are dominated by natural variation. Given that the geochemical background is characterised by a high variation at all scales, it appears impossible to establish a reliable single value for "good soil quality" or a "natural background concentration" for As for any sizeable area, e.g., for Europe. For such a differentiation, geochemical maps at a variety of scales are needed. Different sample materials can pick up dissimilar geochemical sources and processes, even when collected from the same survey area. Weathering (soil formation) leads to an As increase in soils when compared to rocks. Soils and stream sediments return very comparable median As concentrations. Plants are usually well protected against As uptake. There are, however, plant species that can accumulate unusually high As concentrations. The marine environment is generally enriched in As. Several geochemical mapping projects deliver indications for crustal As degassing as an important process leading to As enrichment in the surface environment.