## 1 Introduction  
Chemical industry can cause severe environmental damages, especially those that operated in the past when environmental protection measures were scarce or absent. This degradation is often not limited to the area next to the industries themselves, but instead is likely to affect a much larger area. This is particularly relevant when saturated media is affected, since its limited adsorption capability and continuous movement result in the subsurface migration and easy dispersion of contaminants. In aquifers, particularly those formed by unconsolidated materials, which are highly porous and permeable, diffusion is the most important process in contaminant migration [1]. Once in the saturated media, pollutants move according to groundwater flow paths, unless physicochemical interactions with sediments affect their mobility. Contamination effects can persist over long periods, even after their sources are isolated or removed. This can be instigated by back diffusion processes, i.e., the reverse diffusion of pollutants from low permeability layers (e.g., clayey formations, where contaminants were preferably adsorbed during high polluting water periods) to high permeability zones [2]. On the other hand, the persistence of a pollutant is also a factor of its intrinsic characteristics. Unlike several hazardous organic constituents, metal(loid)s cannot be degraded [3, 4], while there are cases of non-biodegradable organic compounds [4] and others that exhibit half-lives of hundreds to thousands of years, depending on the hydraulic behaviour [5]. The low microbial activity in groundwater is also a factor limiting biodegradation processes. Furthermore, the characteristics of many degradation products formed are still poorly understood and can present different reactivity under field conditions [5]. Although technical advances, environmental laws, and public awareness have imposed new procedures and legal restrictions to industry operations, the environmental legacy of the past is frequently difficult to overcome. That is why, in several cases, monitoring natural attenuation is the more reliable alternative to groundwater decontamination [5; 6]. A complete study of groundwater contamination requires the investigation of larger areas (i.e., not only the immediate vicinity of known pollution sources), through the evaluation of relevant physical and chemical properties and parameters, as well as their variation in time throughout the region. In this sense, geophysical exploration methods have been applied with great success in the investigation of environmental problems [78]. These methods are particularly suited to produce fast and reliable spatial maps of physical properties related with groundwater contamination issues, but they are unable to identify the contaminant. Electrical resistivity or its inverse, electrical conductivity, is an important physical property and there are many examples and case studies in the literature regarding the use of resistivity-based methods for aquifer characterization. Typically, resistivity can vary from 1 to 10 for contaminated and pristine groundwater, respectively, [9] and, therefore, resistivity can be used to map contaminated areas. Traditional resistivity methods using DC currents, or galvanic methods, have long been used for this purpose. At first, conventional resistivity soundings (VES) were used, but they provide 1D information [7]. Contamination plumes have a 3D character and the combination of traditional electrode arrays (Schlumberger, Wenner, dipole-dipole and others) along lines, electrical resistivity tomography (ERT), and grids provide 2D and 3D images of the areas under investigation [7; 10]. However, resistivity methods imply physical contact with the ground, requiring the use of electrodes to pass current into the ground, and the setup of a complete field apparatus usually takes considerable time and effort.