Modeling the Transport of Chromium (VI) in Soil Columns

Miscible displacement experiments were conducted to describe chromium [Cr(VI)] mobility and interactions in six different soils. For Calciorthid, Webster, and Norwood soils Cr breakthrough curves (BTCs) indicated that the interactions with the soil matrix were similar to those for a nonreactive solute where no or small retardation of the equilibrium type was observed. These observations are in support of earlier kinetic batch results where little retention was observed for a wide range of Cr concentrations in these (high pH) soils. The BTCs from Olivier, Cecil, and Windsor soils indicated high Cr retention capacity as indicated by increased retardation, low peak concentrations, irreversible sorption and extensive effluent tailing during desorption (leaching). A nonlinear retention/release model is proposed for the purpose of describing Cr(VI) reactions during transport in soils. The model is incorporated into the convection‐dispersion transport equation for reactive solute in uniform soils. The model was capable of providing a good description of the Cr BTCs for all soils where model parameters were obtained using a nonlinear least squares (best fit) parameter optimization scheme. The model grossly underestimated effluent concentrations for Olivier and Windsor BTCs when independently measured retention/release rate coefficients from the batch data sets were used. Model predictions overestimated the amount irreversibly retained by Cecil soil. Model predictions indicated that a unique set of independently measured rate coefficients was not capable of providing an adequate description of Cr BTCs for these soils.