Abstract
Since salt cannot always be used as a geophysical tracer (because it may pollute the aquifer with the mass that is necessary to induce a geophysical contrast), and since in many contaminated aquifer salts (e.g., chloride) already constitute the main contaminants, another geophysical tracer is needed to force a contrast in the subsurface that can be detected from surface geophysical measurements. In this context, we used heat as a proxy to image and monitor groundwater flow and solute transport in a shallow alluvial aquifer (< 10 m deep) with the help of electrical resistivity tomography (ERT). The goal of our study is to demonstrate the feasibility of such methodology in the context of the validation of the efficiency of a hydraulic barrier that confines a chloride contamination to its source. To do so, we combined a heat tracer push/pull test with time-lapse 3D ERT and classical hydrogeological measurements in wells and piezometers. Our results show that heat can be an excellent salt substitution tracer for geophysical monitoring studies, both qualitatively and semi-quantitatively. Our methodology, based on 3D surface ERT, allows to visually prove that a hydraulic barrier works efficiently and could be used as an assessment of such installations.
Highlights
Geophysical techniques can provide spatially and temporally distributed information on the subsurface and related processes in a non-invasive way thanks to measurements taken on the ground surface or from the sky [1]
Our results show that heat can be an excellent salt substitution tracer for geophysical monitoring studies, both qualitatively and semi-quantitatively
We demonstrated that 4D electrical resistivity tomography (ERT) can be a powerful tool to image dynamic processes in the shallow subsurface, especially using heat as a geophysical tracer
Summary
Geophysical techniques can provide spatially and temporally distributed information on the subsurface and related processes in a non-invasive way thanks to measurements taken on the ground surface or from the sky [1]. That’s one of the reasons for the emergence of hydrogeophysics for the improved understanding of subsurface processes over multiple scales [2]. To follow a physical process in the subsurface using geophysical techniques, it is necessary to have a sufficient contrast of the associated geophysical property that is measured or imaged, the importance of properly dimensioning geophysical surveys [4]. This contrast can be either natural or forced. Examples of natural processes that can be imaged with geoelectrical methods are seawater intrusion in fresh water-bearing aquifers [5], as seawater is far more electrically conductive than freshwater, and moisture dynamics in soils [6] or in the epikarst [7], since
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