Multi-tracer tests to disentangle mobile-immobile regions in a highly entropic aquifer

Anomalous transport processes are frequently observed in radial flow to wells in highly heterogeneous aquifers. This is generally related to the presence of preferential flow pathways that bypass the sediment matrix, thus leading to the formation of fast flow channels, whose magnitude depends on the geological entropy of the system. Tracer tests can be effectively combined with laboratory or field-scale experimental campaigns to understand better the interlinkage between heterogeneity and preferential flow, and to distinguish hydraulically active and inactive regions. Despite considerable past research efforts, these mechanisms are only partially understood. To advance the current understanding, we study transport processes in a laboratory-built highly heterogeneous aquifer under radial flow conditions. The experimental device (200 x 200 x 100 cm) consists of 2527 randomly distributed cells (10 x 10 x 5 cm) of 12 different porous mixtures assembled in 7 layers to form a 35 cm-deep aquifer. This particularly design is intended to maximize the geological entropy of the aquifer, which is equipped with 37 piezometers placed in a radial configuration at different distances from the central (pumping) well. Multiple conservative tracer tests were conducted by injecting a mixture of deuterated water (D₂O) and Potassium Bromide (KBr) into different piezometers, and then by analysing the resulting Breakthrough Curves (BTCs) at the central pumping well. BTCs reveals features peculiar of anomalous transport, such as non-symmetry, early peaks and tailing, which depend on the injecting location. This, jointly with the incomplete mass recovery after 48 hours, suggests the simultaneous presence of fast flow in highly conductive regions, which exchange mass with quasi-immobile portions of the aquifer. By dealing with tests individually, it is seen that curves for the two tracers have a similar trend, with almost perfect overlap in the part before the peaks. Differences in the tailing of the BTCs between the two tracers, that exhibit different molecular diffusion coefficients, indicate the importance of diffusion mechanism taking place in the porous matrix.