Spatial Variability of Heavy Metal Sorption in Soils – Evaluation by Scale Factors.

One of the behaviors of soils in retardation of heavy metals is sorption. The ability of soils to sorb heavy metals is expressed by sorption isotherms, measured in the lab. A function that is often used to parameterize measured sorption isotherms is the Freundlich equation. This equation has two parameters, k and n, and relates heavy metal concentrations in soil solution to the stock sorbed at the soil matrix. The spatial variability of heavy metal sorption isotherms across fields of apparently “homogeneous” soil is often very high, and makes upscaling of point measurements to larger scales problematic. This may be overcome by scaling of sorption isotherms, a method that potentially reduces the wide spread of the isotherms into an average relation, but preserves the variation through calculated scale factors. For an agricultural field on a Luvisol developed in loess material near Hannover, Germany, we investigated the spatial variability of sorption isotherms of cadmium (Cd) and zinc (Zn). 50 samples were taken along a 250 m transect, and sorption isotherms for Cd and Zn were measured in lab. Furthermore, soil properties as pH, CEC, TOC, texture etc. were analyzed. The isotherms (Cd, Zn; topsoil, subsoil) were spatially very variable, and were successfully parameterized by the Freundlich equation. In our contribution we will present results on i) the efficiency of scaling to reduce the variability of Cd and Zn sorption isotherms and preserve the variation in scale factors, ii) and a statistical analysis of correlation between the scale factors of Cd and Zn, and scale factors and other soil properties. Briefly, some important results are: Scaling reduced the variability of the isotherms successfully by maximum 89% (Zn topsoil). Between scale factors of Cd and Zn significant correlation was found, but correlation between scale factors and soil properties was mostly weak.