Leaching surfaces to characterize transport in a heterogeneous aquifer: Comparison between flux concentrations, resident concentrations, and flux concentrations estimated from temporal moment analysis

For subsurface solute transport, flux concentrations are key, while usually resident concentrations are measured. Flux concentrations are frequently estimated from resident concentrations by temporal moment analysis. We tested this approach by simulating transport of an injected tracer during steady flow in an aquifer with a heterogeneous saturated hydraulic conductivity. We constructed grid cell-scale breakthrough curves (BTCs) from flux concentrations and approximated BTCs from resident concentrations and estimated flux concentrations. We assembled these BTCs into spatiotemporal leaching surfaces at various aquifer cross sections for subsequent analysis. Resident concentrations were unsuitable to assess solute movement in the aquifer. Temporal moment analysis worked well when the entire aquifer cross section was considered but performed poorer at the grid cell scale because it approximates the local velocity by the trajectory average. The leaching surfaces served as valuable tools to demonstrate and quantify the limitations of temporal moment analysis.