Hydraulic conductivity assessment via tracer test data assimilation: A comparison of updating techniques

Hydraulic properties of natural aquifers, such as porosity, hydraulic conductivity, and stora- tivity, exhibit an erratic spatial variability at different scales that is difficult to recognize without expensive in situ sampling campaigns, and laboratory analyses. Nevertheless, the relevance of the heterogeneous structure of natural formations on solute transport is well recognized: the variability of the hydraulic conductivity at the local scale controls the non-Fickian evolution of contaminant plumes and the late travel time relevant dispersive phenomena. Tracer test analyses have been widely adopted to obtain a better understanding of dispersion processes and of the re- lated hydraulic properties of porous media. Recently, new interpretative opportunities are offered by geophysical approaches. In particular, the borehole Electrical Resistivity Tomography seems to be promising due to its capability to describe the spatio-temporal evolution of the injected solute. Under the assumptions that the solute spreads as a passive tracer and with high values of the Peclet number, the plume evolution is controlled by porosity and the spatial distribution of hydraulic conductivity. A coupled approach based on the Lagrangian formulation of transport and the Ensemble Kalman Filter data assimilation technique can be applied to infer the spatial distribution of hydraulic conductivity at the local scale from a sequence of time-lapse concentra- tion imaging. The capabilities of this approach are here investigated simulating a synthetic tracer test in a three-dimensional finite size domain reproducing an heterogeneous aquifer. Different as- similation scenarios are studied, varying the system state definition and the assimilated quantities. Although the method is sensitive to the procedure initialization, and some assimilation scenarios do not work as expected, the results show that the proposed approach can represent an effective tool for describing the hydraulic conductivity distribution at the locale scale.