1D-4D electrical and electromagnetic methods revealing fault-controlled aquifer geometry and saline water uprising

Abstract

Abstract This paper presents new results of 1D-4D electrical resistivity and electromagnetic surveys for the delineation of aquifer geometry and the detection of saline paleo-water uprising along structural discontinuities. These geophysical surveys were undertaken at the alluvial aquifer of the Oltrepo Pavese plain sector (Po Valley, Northern Italy). At the investigated test site, the alluvial aquifer is strongly conditioned by the presence of an important tectonic discontinuity whose localization and trace were better defined by this study. This fault is responsible for the sudden deepening that affects the bedrock and is also responsible for the uprising of deep, saline paleo-waters which strongly influences the chemistry of groundwater. During the campaign a variety of experimental setups were tested. This provided an ideal opportunity to cross-validate geophysical results with extensive ground truth provided by groundwater sampling and stratigraphic, electrical conductivity, temperature and redox potential logs undertaken within the wells. It also permitted an assessment of the usability of electrical and electromagnetic surveys in such a complex hydrogeological setting. Geophysical surveys revealed the presence of sharp and irregular contact between the alluvial aquifer and the underlying bedrock. This is characterized by morphological irregularities, which are likely to have been shaped either by tectonics and/or by the paleo-river’s erosion. The bedrock is affected by saline water contaminations which are likely localized along structural discontinuities which represent and facilitate the flow towards the alluvial aquifer. Detailed 3D and time-lapse imaging revealed irregular-shaped shallow saline water contaminations within the alluvial aquifer, as well as temporal variability of groundwater salinity. Spatial distribution of contaminations and salinity degree are likely to be affected by different factors like the aperture of the discontinuities within the bedrock, the hydraulic conductivity of the aquifer, as well as seasonal variations in terms of fresh water recharge.