Abstract
The remediation of a polluted site relies, as a first stage, on the proper delineation of the contamination sources. In classical investigations, soil and water samples are collected throughout the field. These measurements allow a quantitative characterization of the gathered materials but only provide information about the medium in the vicinity of the points where they were collected. On the other hand, geophysical techniques can provide a quasi-continuous coverage of the investigated field. This paper describes a geophysical survey that was performed on an industrial site impacted by a chlorinated DNAPL. The precise location of the contamination was needed for the treatment of the saturated zone, while the unsaturated zone was remediated by general excavation of the sediments, followed by separate treatment. As this excavation allowed to get closer to the saturated zone, geophysical measurements were conducted at the bottom of the pit. Whereas Electrical Resistivity Tomography measurements only brought little information, Ground Penetrating Radar drew the remediation operations towards an area that preliminary point measurements had not identified as a possible source location.
Highlights
Dense non aqueous phase liquids (DNAPL) are recognized as a severe source of groundwater contamination
Some of the remediation methods do not rely on a precise location of the pollutant pockets such as conventional pump and treat or containment used to control contaminant plumes emanating from DNAPL source zones, or certain source depletion technologies such as enhanced bioremediation or surfactant/cosolvent flushing coupled with groundwater recovery
This study highlighted the challenges posed by the use of geophysics for remediation purposes on old polluted sites
Summary
Dense non aqueous phase liquids (DNAPL) are recognized as a severe source of groundwater contamination Their density greater than that of water provides them with the ability to migrate down over tens of meters when spilled in the subsurface, making their detection and remediation a real challenge. Identification and delineation of the contamination sources is usually achieved through the analysis of soil and water samples These point measurements only provide information at the borehole location. Chlorinated DNAPLs are characterized by several properties allowing their direct detection, including relative permittivity, electrical resistivity, chargeability and seismic velocities Their relative permittivity commonly ranges from 2.9 to 10.9 [4], which is lower than that of water, of the order of 80. When organic matter such as chlorinated DNAPL, interacts with clay minerals, the polarization signature of the medium
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