Sensitivity analysis of physical and chemical properties affecting field-scale cadmium transport in a heterogeneous soil profile

Field-scale transport of reactive solutes depends on spatially variable physical and chemical soil properties. The quantitative importance of physical and chemical parameters required for the prediction of the field-scale solute flux is generally unknown. A sensitivity analysis is presented that ranks the importance of spatially variable water flow and solute transport parameters affecting field-scale cadmium flux in a layered sandy soil. In a Monte-Carlo simulation approach, partial rank correlation coefficients were calculated between model parameters and cadmium flux concentrations at various time steps. Data on the heterogeneity of flow and transport parameters were obtained from a 180 m-long and 1 m-deep Spodosol transect. Each soil layer was described in terms of probability density functions of five model parameters: two shape parameters of van Genuchtenʹs water retention curve, saturated hydraulic conductivity, dispersivity and soil–water distribution coefficient. The results showed that the cadmium flux concentrations at the bottom of the soil profile were most sensitive to the cadmium deposition rate and the soil–water distribution coefficient of all soil horizons. The maximum cadmium flux concentrations were also affected by variations in hydraulic conductivity of the humic topsoil horizons. Variations in shape parameters of the water retention curve did not significantly affect the field-scale cadmium flux. Variations in the dispersivity of the subsoil significantly influenced the early time cadmium concentrations. Monte-Carlo simulations involving non-linear sorption showed that cadmium flux concentrations were dominated by variations in the sorption constant and in the exponent of the Freundlich isotherm.