Geophysical surveys for identifying saline groundwater in the semi‐arid region of the central Altiplano, Bolivia

Abstract

Abstract:In the central part of the Bolivian Altiplano, the shallow groundwater presents electrical conductivities ranging from 0Ð1 to 20 mS/cm. In order to study the origin of this salinity pattern, a good knowledge is required of the geometry of the aquifer at depth. In this study, geophysics has been used to complement the sparse data available from drill holes. One hundred time-domain electromagnetic (TDEM) soundings were carried out over an area of 1750 km 2 . About 20 geological logs were available close to some of the TDEM soundings. Three intermediate results were obtained from the combined data: (i) the relationship between the electrical conductivity of the groundwater and the formation resistivity, (ii) geoelectrical cross-sections and (iii) geoelectrical maps at various depths. The limited data set shows a relationship between resistivity and the nature of the rock. From the cross-sections, a conductive substratum with a resistivity of less than 1 Ðm was identified at most of the sites at depths ranging from 50 to 350 m. This substratum could be a clay-rich formation containing brines. Using derived relationships, maps of the nature of the formation (sandy, intermediate and clayey sediments) were established at depths of 10 and 50 m. Discrimination between sand and clays was impossible where groundwater conductivity is high (>3 mS/cm). In the central part of the area, where the groundwater conductivity is low, sandy sediments are likely to be present from the surface to a depth of more than 200 m. Clayey sediments are more likely to be present in the south-east and probably constitute a hydraulic barrier to groundwater flow. In conclusion, the study demonstrates the efficiency of the TDEM sounding method to map conductive zones.

Electrical Resistivity Tomography and Induced Polarization for Mapping the Subsurface of Alluvial Fans: A Case Study in Punata (Bolivia)

“…Characterization of the subsoil by resistivity methods in the Andean region were performed by many authors [37][38][39], where the results yield typical values of high resistivity when gravel and sand are the main content; when clay is the main compound low resistivity were obtained. In the Punata alluvial fan, the resistivity results were analyzed and the same tendency was observed.…”

Delimiting a saline water zone in Quaternary fluvial–alluvial deposits using transient electromagnetic: a case study in Punata, Bolivia

“…The TEM is a controlled source method which measures several transient electromagnetic field diffusions (Goldman and Neubauer 1994;Guérin et al 2001;Nabighian 1991). The principle of this method is that a current is alternately turned on and off.…”

“…This secondary magnetic field has an amplitude that decreases with time and will depend on the ground resistivity. Soils with high resistivity will yield magnetic fields with high initial amplitudes and quick decays, while soils with low resistivity will have an opposite behavior (Christiansen et al 2009;Fitterman and Stewart 1986;Guérin et al 2001).…”

“…The transient electromagnetic (TEM) is a ground-based geophysical method and has been used in many studies with environmental applications, e.g., Fitterman and Stewart (1986), Danielsen et al (2003), Goldman et al (1988), Guérin et al (2001), andNabighian andMacnae (1991). The TEM method provides an excellent resolution of conductive materials at large depths and highlights the contrast between high-and low-resistivity materials (Christensen and Sørensen 1998).…”

A Transient ElectroMagnetic (TEM) Method Survey in North-Central Coast of Crete, Greece: Evidence of Seawater Intrusion

“…The study area called Geropotamos, covering approximately 40 km 2 , is located on the north-central coastline of Crete (Figure 1) where the local water demand for both irrigation and tourism significantly increased in recent years. The study area lies in the northern part of Geropotamos basin (drained by Geropotamos River) and it is characterized by a combination of lowland and semi-mountainous terrain.…”