Reactive Transport Modeling of Uranium in Subsurface: Impact of Field-Scale Heterogeneity and Biogeochemical Dynamics

Understanding the fate of contaminants in heterogeneous aquifer systems is crucial to explain their transport behavior. Although it has been proven that heterogeneity has a significant control over the quantification of these processes, the extent of this impact is yet to be studied. The unique contribution of this work lies in the assessment of field-scale physical and chemical heterogeneity in modeling reactive transport processes in the subsurface. The main objective of this study is to investigate the impact of physical and chemical heterogeneity in understanding biogeochemical processes of contaminants in the subsurface environment, coupled with advective and dispersive transport in situ with mixing limitations. This study is particularly focused on an example of uranium, where especially coupled bioreduction and reoxidation processes in the presence of Fe (III) hydroxides are considered. For this purpose, 2D numerical biogeochemical reactive transport models are developed to simulate the fate and transport of uranium in a heterogeneously distributed subsurface. Results have shown that neglecting spatial heterogeneity might lead to an overestimation of uranium bioreduction, where physical heterogeneity has been observed to have a greater impact than chemical heterogeneity in the absence of adsorption reactions. On the other hand, when adsorption of uranium is included, the significance of chemical heterogeneity is more pronounced. Thus, when potential adsorption of contaminants is ignored or if chemical heterogeneity is ignored in the presence of adsorption reactions, the contaminant concentrations might be underestimated. The underestimation is more pronounced in low hydraulic conductivity zones due to the mixing limitations for soluble compounds, whereas for immobile phase interactions, high hydraulic conductivity regions became significant. The impact of U(IV) reoxidation process is also more pronounced in the presence of chemical heterogeneity and particularly enhanced in the zones with the highest mixing. The findings of this study can shed light on identifying the conditions that necessitate the accurate characterizations of physical and chemical heterogeneity in predicting contaminant transport with mixing limitations subject to competing biogeochemical reactions in the natural subsurface.