Solute transport analysis of bromide, uranin and LiCl using breakthrough curves from aquifer sediment

Abstract

The transport behavior of lithium, uranin, bromide and chloride was studied in column experiments using sediment from an unconfined aquifer at the Krauthausen test site. Sorption isotherms of uranin, bromide and chloride were determined in batch experiments. In addition, a sorption/desorption isotherm of uranin was determined. Inverse parameter estimation and asymptotic analysis of solute transport processes were used to interpret measured BTCs. The log transformed BTCs of bromide and chloride showed a linear decrease of the tailing part with time. Depending on the shape of the tail in the BTCs, a CDE with retardation or a mobile/immobile model was adjusted successfully. Measured uranin BTCs were described using either the CDE with hysteretic Freundlich sorption or the CDE with linear kinetic sorption. In addition, one of the measured uranin BTCs provided evidence for the presence of nonlinear nonequilibrium sorption. The discrimination and identification of the different types of sorption processes was based to a large extent on asymptotic analysis. However, in a few experiments asymptotic analysis did not provide information on the sorption process because the tailing concentrations fell below the detection limit before convergence to one of the asymptotic solutions could be observed. For none of these cases, the models analyzed in this work were able to describe the complete BTCs (including the tail). For two BTCs showing sorption hysteresis, the observed concentrations could be predicted using data from independent measurements almost without parameter fitting. Only a slight modification of the distribution coefficient was needed in one case. Lithium transport could be described either by Freundlich sorption or linear kinetic sorption. Field BTCs of uranin showed a qualitatively similar behavior as the BTCs observed in column experiments. Evaluation of the slope of log–log transformed field BTCs suggested that uranin sorption may be described by a nonlinear nonequilibrium sorption or hysteretic Freundlich sorption model.