A tracer test in a shallow heterogeneous aquifer monitored via time-lapse surface electrical resistivity tomography

We illustrate a case study of a saline tracer test in a shallow, highly heterogeneous aquifer, monitored by means of surface time-lapse ERT. The test was aimed at identifying the system's hydraulic properties. Some of the expected limitations of the method particularly caused by the strong decrease in ERT resolution with depth and the consequent problems with mass balance and moment calculation could be partly balanced by the use of direct measurements of groundwater electrical conductivity and tracer concentration at one selected location. The vast heterogeneity of the system, ranging in litholotty from clay to gravel at a scale of meters to tens of meters, reflects itself in the tracer migration and distribution over time: The tracer is trapped in the low-permeability regions and from these it is slowly released over time. High-resolution surface ERT proves effective at picturing this system behavior over time. The extreme heterogeneity is also a challenge in the attempt to translate bulk electrical conductivity into estimates of groundwater electrical conductivity and, hence, solute concentration because surface conductivity in fine sediments has an important role. The test results could be used to identify some of the key parameters for solute transport, namely, mean groundwater velocity and aquifer dispersivity at the scale of the test by means of transport model calibration.