Figure 2a shows the typical trend of apparent resistivity obtained from the TEM soundings in A2. In all the soundings at later stages, the apparent resistivity decreases, which is an indicator that layers with low resistivity are reached. In Fig. 2b, it is displayed that for each sounding there are several equivalent solutions (dashed lines). This means that different resistivity values and layer thickness can yield the same response of the apparent resistivity. After performing several inversions, a final solution (solid line) is achieved. The parameter used for selecting the best solution is the residual data, which must be smaller than one. Figure 2b and c shows the layered and smooth inversion models, respectively. Both models highlight a low-resistivity layer at approximately − 70 m, and both yield approximately the same depth of investigation (DOI). The DOI is basically limited to background noise, resistivity of the ground, and the moment of the current. In this study, the bottom layers are dominated by a high clay content layer with low resistivity, which dissipates the electromagnetic signals and reduces the DOI.
All the TEM sounding curves show a trend of decreasing resistivity with depth. Generally, in A1, the resistivity in the top layers is between 100 and 200 Ωm, while in A2 the values of resistivity on the top layers are lower, e.g., 20–60 Ωm. In both areas at the bottom of the soundings, the resistivity values are similar and range from 1 to 20 Ωm. However, the TEM soundings have detected a transition zone between the top and bottom layers, where the resistivity can be as low as 0.1 Ωm. This is observed in Fig. 2b, c at a depth close to 70 m. These low-resistivity values might be associated with high concentrations of salts, e.g., Na+ and Cl−. Figure 3a displays the plot of electrical conductivity (EC) and resistivity versus the concentration of Na+ and Cl− obtained from 45 water samples within the Punata alluvial fan aquifer and surroundings. From this figure, it is observed that the increase in the concentration of these ions increases the EC (or decreases the resistivity) of the water samples. The study by UNDP-GEOBOL (1978) reported EC for borehole P305 (refer to Fig. 1a) of 97 µS/cm (~ 103 Ωm) between 0 and 100 m depth, 6000 µS/cm (~ 2 Ωm) at 150 m depth, and 30,000 µS/cm (~ 0.3 Ωm) at 250 m depth, which indicates an increase in salinization and EC with depth. The increase in salt content with depth will lead to a decrease in the bulk resistivity of the soil. The high concentrations of salts might be explained due to the fact that during the lower Pliocene the climate was mainly hot and dry, and paleolakes in the regions were evaporated. During this evaporation, period clay with high content in salts was deposited. Studies about the evolution of closed-basin brines indicate that there are commonly paths of development of brines (Eugster and Hardie 1978; Hardie and Eugster 1970), and these paths can be observed in the study area (refer to Fig. 3b). In the study area, the evolution of the brine layer might have occurred when freshwater acquired solutes by mixing water through saline deposits. In Fig. 3b, freshwater evolves from a carbonate type to chloride type; according to Hardie and Eugster (1970), the brines in the Punata alluvial fan are of the carbonate–chloride type, probably due to the partial dilution of gypsum. The occurrence of brines, which is supported by the chemical analysis, explains the very low resistivity in the deeper layers of the Punata aquifer.
Figure 4 shows the resistivity values acquired from three different methods: TEM, ERT, and borehole resistivity at two different locations. The normalized chargeability curve is also included in the same plot. There is a good agreement between all the methods. In all the cases, the resistivity values decrease with depth. However, ERT results seem to be less sensitive to high conductive layer or have less capability to resolve them, but TEM soundings highlight these layers. This is observed in Fig. 4a, b, where at depths between 120 and 140 m, TEM models have values of resistivity close to 1 Ωm, while ERT values are close to 10 Ωm. Another explanation why ERT is unable to resolve the low-resistivity layers is because of the layout of the performed surveys (i.e., spread of 800 m with 10-m electrode separation). With this layout, the maximum depth coincides with the depth of the brine layers. Since the accuracy and resolution decrease with depth in ERT (Dahlin 2001; Loke 2010; Loke et al. 2003), this might have affected the ability of ERT to resolve the low-resistivity layers. Furthermore, values of normalized chargeability increase with depth. High values of normalized chargeability might be related to sediments with high clay content, while soil with uniform particle size of sand and/or gravel might yield lower values of normalized chargeability (Alabi et al. 2010; Revil et al. 2015; Slater and Lesmes 2002). Therefore, the high values of normalized chargeability in the Punata aquifer are partially explained by the high clay content. However, as it is mentioned above, the very low values on resistivity might be explained just by the presence of brines.
The DOI often varies between ERT and TEM surveys. In the first case, it depends on the separation of electrodes, while in the second case it depends on the transmitter loop moment. Figure 4a, b shows that the effective depth of penetration in TEM is around 170 m. The depths of investigation in A1 are between 150 and 200 m, while in A2 they are around 80–100 m. This difference is because of the fact that the layer with high clay content in A2 is shallower, which quickly dissipates the electromagnetic signals. When all the final models have been estimated with the SPIA program, these are exported into Workbench. The latter was used for generating 2-D profiles. For A1, a total of 9 profiles were created, while for A2 a total of 14 profiles were made.
Figure 5 shows two example profiles obtained from the 1-D soundings, and in both cases different geological units can be interpreted. The interpretation was made with the lithological information from wells located close to the lines. The main material of each layer is shown by their initial letter (i.e., B stands for boulders, G for gravel, S for sand, and C for clay). The profile L1-2 (refer to Figs. 5a, 1a) is located in A1. This area is close to the apex of the fan, where the material is coarse in the top layers and decreases in size with depth. The same pattern occurs with resistivity which decreases from 200 Ωm in the top to less than 20 Ωm in the bottom. This profile shows a thicker layer of higher resistivity in the northern than in the southern part. As it is mentioned above, this is because close to the fan apex the rivers have more energy leading to deposition of coarser material, but further away from the apex the river energy diminishes, and finer material is deposited. In L1-2 profile approximately at a depth of 140 m (or 2620 m.a.s.l.), there is a layer with very low resistivity, which may be related to the presence of brines. The resistivity of this saline water is slightly lower than 1 Ωm.
In Fig. 5b, the profile L2-4 is shown, and the top layer seems to be dominated by sand and clay and clay solely. The high clay content in this profile is due to the fact that A2 is located in the distal part of the Punata fan, where the Quaternary sediments are of the lacustrine type. In A2, the layer interpreted as a brine has lower resistivity than in A1, and this might be explained by the less inflow of fresh water to the zone, which can dilute the salts and consequently decrease the resistivity. The brine is located approximately at depths of 70 m (or 2650 m.a.s.l.), and the resistivity of this zone can be as low as 0.1 Ωm.
Figure 6 displays the horizontal resistivity distribution for both areas. The separation between each horizontal map is not to scale. In Fig. 6a, the horizontal resistivity distribution of A1 is displayed. As expected, high-resistivity materials are found in the top layers, and the values decrease with depth. Close to 2655 m.a.s.l., a low-resistivity zone is detected, with values around 1 Ωm, and attributed to be brine deposits. When the deeper levels are reached in A1, it would be expected to locate a low-resistivity zone caused by the lacustrine deposits; however, a zone with high-resistivity values was detected (indicated in the lowest level of Fig. 6a). This high-resistivity zone is unlikely to be composed of coarse material such as sand or gravel since the brine is not sinking. Therefore, these high values of resistivity might be caused by a fault where bedrock is protruding through the lacustrine layer. The local geology shows fault lines that end in the Quaternary deposits (refer to Fig. 1a); however, if these lines are extrapolated, they cross over the area with high resistivity. Furthermore, the local geology indicates that San Benito formation is underlying the Quaternary deposits, and this formation is composed of quartzitic sandstone with resistivity values close to 200 Ωm (GEOBOL 1983). The resistivity value from the San Benito formation corresponds to the resistivity obtained from the TEM sounding. This new discovery has also been discussed in the study of Mårdh (2017) and must be further studied in order to corroborate the hypothesis.
In Fig. 6b, the horizontal resistivity distribution of A2 is displayed. Lower-resistivity values are found in A2 than in A1 most likely due to the fact that the clay content increases toward the distal part of the fan, where A2 is located. The brine zone is mainly located approximately at 2650 m.a.s.l, and the resistivity in this zone is close to 0.1 Ωm. The Punata alluvial fan is delimited by a clay fringe in the western portion (GEOBOL 1983; Gonzales Amaya et al. 2016). This fringe acts like an impermeable barrier whereby the brine cannot pass and therefore takes a southeastern dip direction.
The thickness of the low-resistivity brine layer in both areas is around 20 m. The depth of investigation in A1 is close to 200 m, while in A2 it is in average 130 m. In A1, the bottom zone with high-resistivity values might be interesting for research purposes from an hydrogeological point of view since the high values of resistivity might represent the bedrock. It might be possible to find stored water in the bedrock which later can be exploited if a secondary porosity exists. Therefore, it is advisable to perform more studies such as geophysics and drilling in order to understand this zone better.