## 1 Introduction  
Groundwater contamination by dense non-aqueous phase liquids (DNAPLs), such as chlorinated solvents, is increasingly recognized as a serious environmental problem [1]. Chlorinated solvents, including trichloroethylene (TCE) and tetrachloroethylene (PCE), are the most important subclass of DNAPLs [2] due to their widespread use in the electronics, chemical, dry cleaning, and metal fabrication industries [3]. The physical and chemical properties of DNAPLs, aside from their relatively low solubility [4], high specific gravity [5], and tendency to remain adsorbed to organic and fine-grained materials [6], make them difficult to locate and characterize in the subsurface [7], and this can impact the effectiveness of conventional remedial technologies [8]. Over the last decade, in situ application of activated carbon-based amendments has emerged as a promising remedial technology for the cleanup of subsurface organic contaminants such as chlorinated solvents [9; 10]. The technology involves a combination of adsorption, which has been extensively characterized and well understood, and degradation [9; 10; 11]. The combination of these two processes is proposed to be more effective than conventional in situ remedial technologies that solely rely on degradation [10]. Despite the rapidly increasing number of field-scale applications, unanswered questions remain regarding the effectiveness and persistence of contaminant degradation, especially biodegradation [10]. In the present work, a colloidal activated carbon, PlumeStop(tm) (Regenesis), was considered as a versatile adsorbent for in situ groundwater remediation [11]. PlumeStop(tm) combines sorption-based contaminant removal with accelerated biodegradation [10]. The activated carbon-based amendment was co-injected with an electron donor (HRCtm, Regenesis) to provide initial biostimulation of the treatment [12]. The selection and deployment of appropriate technologies for the remediation strategy were based on initial characterization activities [13]. In environmental issues, geology-related factors control the migration of contaminant plumes and influence the effectiveness of remediation technologies [14]. The performance of activated carbon-based amendments applied in situ is heavily affected by subsurface heterogeneity and the resulting uncertainty in the delivery and distribution of reagents [10]. The high-resolution characterization of underground geological heterogeneities and the integration of information represent key elements for characterization refinement, remediation design, optimization of intervention, and performance monitoring [15]. The combination of different prospection techniques provides additional and precious information by capturing the complexity of geological heterogeneity and allowing data spatialization [16]. Conventional site investigation relies on intrusive methods, such as borehole drilling and cone penetration tests [17]. These techniques provide only point information concerning the subsurface, with possible effects of spatial aliasing [18; 19]. In addition, drilling can potentially spread contamination by opening DNAPL migration pathways through low-permeability zones [20]. Together with intrusive techniques, geophysical methods are now beginning to be applied to DNAPL contamination problems, both to assist in situ characterization and to monitor remediation processes [21]. Geophysical techniques have the advantage of reducing the need for intrusive investigations [20] and can provide spatially continuous information regarding the subsurface structure [16]. Some geophysical techniques can focus with an adequate resolution (meter) on the first meters to tens of meters of subsoil. In addition, possible links exist between relevant measured physical quantities and hydrological and environmental quantities of interest for contaminated site characterization [19]. The technique considered in this study is electrical resistivity tomography (ERT).