Application of electrical resistivity imaging (ERI) to a tailings dam project for artisanal and small-scale gold mining in Zaruma-Portovelo, Ecuador
Introduction
Socioeconomic progress in emerging countries such as Ecuador relies heavily on hydrocarbons and mining resources. This circumstance has created a pressing need for strategic infrastructure constructing, which is often a critical issue for their sustainable development. These major construction projects require, in turn, large tracts of territory to be rapidly explored to obtain rigorous geotechnical data. Mining industry is extremely sensitive to changes in financial markets and, for example, due to the recent surge in demand for metallic minerals, mining sites that were closed 50 years ago are now being reopened. This is the case of the Zaruma-Portovelo Mining District), in El Oro Province, Ecuador (ZPMD) (Fig. 1), where artisanal and small-scale gold mining has grown tremendously over the past decade (Adler-Miserendino et al., 2013). The production capacity of processing centers located along the Calera and Amarillo rivers has thus increased, and more than 90 processing centers now produce nearly 5,000 tons of tailings (Velásquez-López et al., 2011). The current scale of this artisanal mining industry, and the predictions for its future growth, caused that a tailings dam must be constructed in order to minimize the impact of mining waste on the Amarillo River waters to make the industry more sustainable (Fig. 1b) (Appleton et al., 2001, Tarras-Wahlberg, 2002, Tarras-Wahlberg et al., 2001, Veiga et al., 2014, Velásquez-López et al., 2010). Additionally, the volatility of the ore market advices that the dam must be commissioned quickly in order to ensure the repayment of its construction costs. These circumstances force the tailing dam to be projected and executed in a short term, which leaves little time for subsurface recognition of the site planned for its construction. Given the spatial scope that the planned dam will occupy and the antecedent of landslides in the area, the viability of the selected site must be assessed, requiring an extensive subsurface exploration campaign. The topographical roughness and the lack of suitable tracks to provide access for drilling rigs hindered drilling tasks. Despite these logistical constraints, high-quality data for geotechnical requests are essential for gauging a proper construction location, even more for a dam that retains mining passives whose malfunction might cause environmental severe impacts (Grangeia et al., 2011). Aside other geomechanical and dynamic–seismic considerations, the main geomorphological–hydrogeological aspects to be considered during the assessment of the viability of the site are as follows: (1) groundwater flow pattern, under natural conditions and when altered by the deposition of mining tails; and (2) susceptibility of the area to landslides.
The work reported here was aimed at demonstrating the utility of electrical resistivity imaging (ERI) in subsoil explorations supporting geo-engineering projects, such as the planned tailings dam slated for construction in the ZPMD. The main targets for the ERI surveying were identifying key geotechnical features encompassing: the thickness of the topsoil and surficial quaternary deposits, weathered and highly fractured horizons affecting the rock massif, fractures related to landslide scars, faults, high-permeability pathways for groundwater flow and the groundwater flow pattern. Resistivity DC methods had frequently been considered to offer poor resolution and to generate major uncertainties (Loke et al., 2013). Consequently, these methods were rarely used in geological engineering projects, which often require accurate quantitative models of the subsoil. However, fundamental advances in resistivity methods over the past two decades, fostered by the development of modern multichannel and multi-electrode surveying systems, and the advent of low-cost PCs and improvements in resistivity inversion routines, have led to a major increase in resolution. Large data sets encompassing thousands of apparent resistivity records, and covering distances of kilometers and depths of several hundred meters (e.g., Gélis et al., 2010, Le Roux et al., 2011), can now be collected within a few hours at reasonable cost. Regarding the purported uncertainties derived from use of ERI, these can be minimized or even eliminated by combining multiple surveying techniques and by calibrating geophysical data sets according to reliable geological information and borehole reports (e.g., Carbonel et al., 2013a, Carbonel et al., 2013b, Zarroca et al., 2014). These enhancements have prompted that actually multidimensional (2D, 3D and 4D) ERI surveys (Griffiths and Barker, 1993) are being used more widely in geotechnical, hydrological, environmental and mining applications (Loke et al., 2013). Despite its capabilities, ERI remains almost to a some extent restricted to specific geotechnical applications such as determining the depth to bedrock, exploration of buried cavities or assessment of karstic subsidence hazard (e.g., Carbonel et al., 2013a, Carbonel et al., 2014, Carrière et al., 2013, Zhu et al., 2011), landslide research (e.g., Jongmans et al., 2009, Zarroca et al., 2014) and identification and monitoring of pollution plumes (e.g., De Carlo et al., 2013, Grangeia et al., 2011). These applications involve very near-surface explorations and usually do not approach essential parts of geological engineering projects. Moreover, the utility of ERI for geo-engineering projects of singular interest has been scarcely covered in the literature (e.g., Bellmunt et al., 2012, Rucker et al., 2009, Rucker et al., 2011). It should be noted that subsoil exploration is perhaps the task that consumes greater budget in the phase of design of large-scale infrastructures. Although the use of ERI does not obviate drilling of exploration boreholes, they do allow streamlining drilling programs, which aids to significant cost reductions. Accordingly, ERI was shown as an attractive exploration tool, as it may provide information of key subsoil features at a resolution suitable for many geo-engineering applications and capable to agreeing the logistical, schedule and economic constraints.
Section snippets
Geological and geomorphological context
ZPMD is located at the Cordillera Occidental, in the forearc of the Andean active margin in SW Ecuador. The main structural element in the area is the NW-trending Piñas regional fault (PF), which has been dated to the late Jurassic to early Cretaceos (Aspden et al., 1995, Litherland et al., 1994) (Figs.1a and 2). The PF cuts through the Tertiary volcanics northward and the Precambrian metamorphic-igneous basement southward (Fig. 1a). The Tertiary volcanic sequence unconformably overlies
Geophysical survey
ERI is a DC technique aimed to imaging the resistivity pattern of the subsurface using multi-electrode systems (Griffiths and Barker, 1993). The distribution of the bulk resistivity is mainly related to intrinsic rock properties as the porosity, fluid content and fluid hydrochemistry. Since ERI is particularly sensitive to these features, it can be valuable for delineating weathered and highly fractured zones, sliding surfaces or faults that affect the rock massif. All of these elements can
Geomorphological and hydrogeological features
The landscape is dominated by dome-shaped hills that form a dense and near-orthogonally aligned drainage network. The slopes are moderate (25–35°) (Fig. 3). The local relief (measured as the height between crests and thalwegs) ranges from 50 to 100 m, whereas the absolute elevations are ca. 600–800 m asl. The main course in the area is the El Salado Stream, which is tributary to the Amarillo River at the upper Puyango River catchment (Fig. 3a).
Aerial photographs revealed the drainage network
Discussion
Given the close correlation between hydrogeological and geoelectrical parameters, ERI was expected to be, a priori, suitable for meeting the goals of the research (e.g., Chandra et al., 2008). Evaluating the quality of the inverted model entailed identifying those parts of the resistivity images that contained reliable data. The relative data misfit between pseudosections—measured vs calculated—also indicated those data points that were likely to be erroneous and, therefore, candidates to
Conclusions
ERI has been employed to assess the suitability of a site chosen for construction of a tailings dam in Zaruma-Portovelo Mining District, in El Oro province, Ecuador. Integration of geomorphological, hydrogeological and geophysical information allowed proposing a morpho-hydrogeological model and identification of critical subsoil features that might affect the planned dam. These features comprise weathered and intensely fracture horizons affecting the rock massif, sliding surfaces seated at
Acknowledgements
The authors are very grateful to the staff of the Instituto Nacional Geológico y Minero Metalúrgico del Ecuador INIGEMM for their assistance in the field work. This research was partially funded by the Spanish national project CGL2010-16775 (Ministerio de Ciencia e Innovación and FEDER) and CGL2013-40867-P (Ministerio de Economía y Competitividad). We are very grateful to anonymous reviewers for their constructive comments and suggestions.
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