Stepwise Identification of Geochemical Processes and Parameters for Chromium Reactive Transport in Heterogeneous Porous Media

Sediment physical and chemical properties play major roles in controlling geochemical processes in groundwater. This study focuses on the fate and transport of Cr(VI) in natural sediments from Qiqihar, China. Column experiments were performed to evaluate the mobility of Cr(VI) in various sediments. Rather than relying on subjective trial-and-error curve fitting, we propose a stepwise inverse method to quantitatively identify hydrogeochemical processes. In each step, potential geochemical processes, such as surface complexation, reduction, cation exchange, adsorption, and mineral dissolution/precipitation, were incorporated into the multicomponent reactive transport model. Four model selection criteria (AIC, BIC, AICc, KIC) were used for model identification. Our findings indicate that Cr(VI) undergoes both surface complexation and reduction processes. The degree of the surface complexation reaction is influenced by the specific surface area of the sediment, while the extent of reduction depends on the organic matter content of the sediment. The major cations in groundwater are mainly influenced by cation exchange, with a smaller contribution from adsorption. Under the experimental conditions, the impact of mineral dissolution/precipitation on the Cr(VI) transport was negligible. Through parameter sensitivity and correlation analysis, we explained most of the uncertainty in the model. The results obtained provide valuable insights for reactive solute transport modeling and significantly enhance our understanding of Cr(VI) behavior in groundwater.