Competitive control of multiscale aquifer heterogeneity on solute transport in an alluvial aquifer

Natural aquifers can contain multiscale medium heterogeneity, whose joint impact on solute transport, however, remains obscure. To fill this knowledge gap, this study selected an alluvial aquifer as the representative medium, where the spatial distribution of multiple hydrofacies (with a typically nonuniform hydraulic conductivity (K) within each facies) could represent both regional-scale (large-scale) and subhydrofacies-scale (local-scale) heterogeneities. To obtain first-hand data, a field tracer test was conducted in an alluvial aquifer located at the Tongzhou site in Beijing, China, where the breakthrough curves (BTCs) obtained at various aquifer depths indicated a significant influence of both regional- and subhydrofacies-scale heterogeneities on solute transport. Further numerical analysis revealed that the impact of regional-scale heterogeneity on solute transport increased with increasing discrepancy in K between adjacent hydrofacies. In comparison, subhydrofacies-scale heterogeneity embedded within each hydrofacies played an important role in controlling solute transport when the mean Ks value of adjacent hydrofacies became more similar. Bimodal transport at early stages was mainly impacted by multiscale preferential flow paths, while solute retention at later times was induced by immobile zones dominating late-time transport. Anomalous transport induced by multiscale heterogeneity could be captured by the dual heterogeneous domain model (DHDM), while the dual advection–dispersion equation (DADE) or fractal mobile-immobile (fMIM) model could only provide reasonable predictions at the early or later stage, respectively. This study sheds light on the control mechanism of multiscale heterogeneity on solute transport.