The noninvasive characterization of the vadose zone has been the objective of intense research aiming at collecting data for the calibration of flow and transport models. This interest is motivated by the difficulty of accessing the vadose zone with direct methods without causing disturbance to the natural conditions. On the other hand, characterizing the vadose zone requires detailed knowledge of space and time variations of water distribution. Geophysical methods capable of imaging the moisture content, such as ground penetrating radar (GPR) and electrical resistance tomography (ERT), provide data with high resolution in space and time: natural infiltration and artificial injection (tracer) tests can be imaged, by repeating measurements over time (time-lapse mode). We conducted a forced-injection experiment at a test site in Gorgonzola, east of Milan (Italy). The site is characterized by Quaternary sand and gravel sediments that house an extensive unconfined aquifer. We used ERT and GPR in two-dimensional cross-hole configuration and time-lapse mode during a period of several days preceding and following a tracer experiment with the injection of about 20 m3 of fresh water in a purposely excavated trench. The calibration of a three-dimensional infiltration model on the basis of the geophysical data provides useful estimates of vertical saturated hydraulic conductivity; however, we observed a discrepancy in mass balance between calibrated simulations and measurements. This fact cannot be explained using a simple isotropic model for hydraulic properties; therefore, the method requires further investigation before being fully established.

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