Integrated multiple tracer-based tomographic inversion and heterogeneity characterization of karst limestone mining regions in southwest China

Understanding the distribution of hydraulic parameters is essential for preventing water-related hazards in limestone mines. Tracer testing is a technique to elucidate the spatial heterogeneity of hydraulic parameters and transport properties within an aquifer. In order to estimate fluid flow characteristics and the distribution of hydrogeologic properties of the limestone aquifer, this study performed an integrated tracer-based tomographic inversion and heterogeneity characterization for the Hejing limestone mine in southwest China. The temporal moment analysis was conducted on the recorded tracer breakthrough curves (BTCs), and the swept volumes and flow geometries were estimated. Furthermore, potential flow paths for water inflow were identified by inverting the travel-time dataset via tomographic inversion using the simultaneous iterative reconstruction technique (SIRT) algorithm and the staggered grid method. The estimated high-conductivity zones estimated by travel-time tomography were consistent with areas of high mass recovery, short mean residence times, and large tracer-swept volumes. Potential paths of water inflow were confirmed to be distributed in the southeast and northeast of the limestone mine. Based on the reconstructed high-conductivity distributions, three sets of grouting curtains were designed to avoid further hazards. The groundwater levels measured during grouting demonstrated that the grouting was satisfactory. The findings of this study can provide valuable references for solving the preferential flow paths and subsurface heterogeneity in aquifers and for characterizing of subsurface heterogeneity. Future joint inversion studies based on tracer, geophysical and head data are expected to refine the outcomes on stratigraphic heterogeneity.