The large-scale distribution of Cu and Zn in sub- and topsoil: Separating topsoil bioaccumulation and natural matrix effects from diffuse and regional contamination.

ABSTRACT

A realistic estimate of diffuse contamination requires to recognize and assess the dominant natural and anthropogenic element sources. For eight large-scale geochemical surveys, the relations between geogenic, anthropogenic and biogenic Cu and Zn sources are estimated by comparing the cumulative distribution functions (CDF) of the elements in top- and subsoil using cumulative probability (CP) diagrams. Strong local contamination distorts the high-concentration end of the distribution function considerably in topsoil. In contrast the impact of diffuse contamination can best be recognized at the lower end of the data distribution. Copper and Zn are important plant micronutrients, studying their concentrations in a variety of plant materials and soils along a number of transects demonstrates that both are adjusted to narrow concentration levels in many plant materials. Plants regulating the element concentrations to certain fixed levels will distort the low-concentration end of a topsoil CDF, the bio-adjustment thus limits the accuracy of diffuse contamination estimates. Combining CDF analysis with spatial mapping provides insight into the dominant contamination processes that distort the topsoil CDF relative to the subsoil CDF. For Cu a most likely diffuse contamination signal of 1-2 mg/kg with a maximum of 5 mg/kg is obtained for soils at the European scale. The higher estimate is clearly influenced by bio-adjustment. For Zn diffuse contamination appears to be higher on first glance, about 5-10 mg/kg, but again the lower end of the investigated CDFs is strongly shifted by biosphere adjustment, plants striving to avoid Zn deficiency. The true input through diffuse contamination will thus be considerably lower. Data from projects that sampled minerogenic instead of organogenic topsoil lead to lower estimates for diffuse Zn contamination in the range of <1-5 mg/kg at the continental scale.