Modeling the Concentrations of Dissolved Contaminants (Cd, Cu, Ni, Pb, Zn) in Floodplain Soils.

Central European floodplain soils are often contaminated with potentially toxic metals. The prediction of their aqueous concentrations is a prerequisite for an assessment of environmental concerns. We tested the aqueous concentrations of cadmium (Cd), copper (Cu), nickel (Ni), lead (Pb) and zinc (Zn) derived from multi-surface adsorption modeling (adsorbents: hydrous iron, aluminum and manganese oxides, clay and soil organic matter) against those analyzed in situ in the soil solution of four horizons of floodplain soil profiles at the Elbe River, Germany. The input data for the reactive metals were derived from a seven-step sequential extraction scheme or from extraction with 0.43 M nitric acid (HNO3) and evaluated in four modeling scenarios. In all scenarios, measured and modeled concentrations were positively related, except partially for Pb. Close reproduction of the measured data was obtained using measured data of accompanying cations and anions together with amounts of reactive metals from both the sequential extraction or from 0.43 M HNO3 extraction, except for Cu, which was often strongly overestimated, and partially Cd. We recommend extraction with 0.43 M HNO3 to quantify reactive metals in soil because the modeling results were metal-specific with better or equal results using the single extractant, the application of which is also less laborious. Approximations of the concentrations of accompanying ions and the water contents yielded similar results. Modeled solid-phase speciation of metals varied with pH and differed from that derived from sequential extraction. We consider multi-surface modeling an effective tool to predict both aqueous concentrations and solid-phase speciation of metals in soil.