Parameterization and regionalization of Cd sorption characteristics of sandy soils. II. Regionalization: Freundlich k estimates by pedotransfer functions

The aim was to describe Cd sorption in spatially variable sandy soils of the ‘Fuhrberger Feld’ catchment area as a prerequisite for prognosis of Cd transport in soil and of the threat of groundwater pollution. Thus, the possibility is evaluated to derive a generalized Freundlich equation based on multiple regressions relating the retention parameters k and M (from isotherm data of part 1 of this study) to basic soil properties (pedotransfer functions). For the parameter M (exponent), the correlation ‘measured vs estimated’ was weak (r² < 0.5) whereas k was well predictable by pedotransfer functions. The best regression was obtained if organic carbon (OC). clay content and H⁺ activity were combined as independent variables (r² up to 0.96). The obtained k values were much higher than those from comparable literature models, probably due to lower ionic strength and different composition of our background solutions used for the isotherms. As a critical evaluation, the estimates for k were used to derive solute Cd concentrations (C_est) which then were compared to measured data (range 0.1–3 μg L^?1). The best but still unsatisfactory r² was 0.77, obtained if G_est was compared to Cd in 0.01 M_c Ca(NO₃)₂ equilibria (Cd₀). Cd in fresh soil solution (Cd_z) showed no significant correlation with C_est, except for one relationship where OC had been excluded from the preceding multiple regression of k. Generally, the role of the variable OC remained unclear. Direct multiple regressions of measured solute Cd vs soil properties (bypassing k, no sorbed fraction) yielded much closer correlations, with r² = 0.9 for Cd₀ vs OC, H⁺ activity, clay (log data, OC decreases C) and r² = 0.7 for Cd₂ vs OC, H⁺ activity, clay (log data, OC enhances C).