3.2 Water flow paths by using 2D and pseudo-3D ERT data
An overall analysis of all electrical tomography data allowed the definition of a single scale of values, ​​facilitating comparisons between the sections. For the electrical resistivity data, we defined a variable scale between 200 Ω.m and 10000 Ω.m, the lower and higher values ​​measured during the field data acquisition. The definition of these values enabled the distinction of low-resistivity zones (cool colors) and high-resistivity zones (hot colors) in the 2D lines and pseudo-3D maps.
The images representing the inversion of electrical resistivity data (Figure 8) show subsurface heterogeneity; the low resistivity values may be associated with groundwater (blue colors), and in contrast, the high resistivity (dark green, brown and red colors) values ​​suggest the presence of dry soil (Keller & Frischknecht 1966; Telford et al. 2004; Loke, 2004). In the analysis of the studied wetland, low values ​​of resistivity ranged from 200 to 612 Ω.m; intermediate values from 1069 to 1870 Ω.m; and high values ​​of resistivity from 1870 to 10000 Ω.m.
All ten lines showed clear contrasts in resistivity values, with both low and high values. Lines 2, 3, 4, 5, 7 and 10 best represent the wetland contrasts (Figure 8).
Stretches of lines 5 and 10, which ran perpendicular to each other, were located exactly in the lower topographic level of compartment 3 of the wetland. In these stretches, low-resistivity zones (blue colors) are visible immediately below the soil surface, reaching the deep aquifer. These zones are where surface water interacts with the groundwater and the aquifer recharge occurs (McLachlan, Chambers, Uhlemann & Binley, 2018). Similar features having low resistivity show a lateral continuity in the distribution of the aquifer and are present in all parallel lines. Deeper in the regolith, just below the low-resistivity features (6–8 meters depth), lay sharply contrasting zones of high-resistivity features (dark green), indicating a barrier to vertical water flow. At this level, saprolite rock supports the aquifer and imposes a lateral continuity of the water body. In the soil mantle and saprolite, layers of increased saturation lead to more lateral hydraulic conductivity (Appels et al., 2015). The dense and fine-textured layers in the bedrock and soil mantle retain the water flow (Freer et al., 2002). Such stratification is typical in the Serra da Galga Formation (Fernandes & Ribeiro, 2015).
The aquifer is shallow, because the low-resistivity zones begin at two meters deep (964 m.a.s.l.). However, the aquifer has a vertical extension of at least 12 meters in some places, which is most visible in the lines 2, 3, 4 and 5. This demonstrates a connection between shallow subsurface water and aquifer-water, in a genuine interaction.
Zones with intermediate values (1069–1870 Ω.m) (light green) present features that suggest transition zones, where water may be contained in soil porosity, indicating that the aquifer is not confined. The fine soil texture occurring here can also retain water and shows lower resistivity (Greer et al., 2017).
The physical parameter of electrical conductivity is a response from the water content or the contents and composition of the soil and rock solutions. When rocks are saturated, the electrical conductivity of the solutions usually increases, decreasing the electrical resistivity values. Various studies indicate a positive relationship between electrical conductivity and both porosity and hydraulic conductivity of an aquifer; i.e. , low resistivity reflects higher porosity and higher hydraulic conductivity (Kelly, 1977; Kelly & Frohlich, 1985; Urish, 1981; Chandra, Ahmed, Ram & Dewandel, 2008; Chukwudi, 2011; Moreira, Cavalheiro, Pereira & Sardinha, 2013).
High-resistivity zones in the central region (values between 1870 and 5719 Ω.m) are a common feature of all lines and indicate the presence of low-permeability materials, with no significant presence of stored water.
Generally, the most superficial portions of the soil (0 to 2 meters deep) present high-resistivity zones, sometimes throughout their entire extent, such as in line 7. This occurs due to the presence of very clayey soils with low hydraulic conductivity, making the studied wetland an excellent natural reservoir.
In order to enhance the visualization and understanding of groundwater behavior, pseudo-3D models were produced by interpolation of data generated by the 2D inversion of electric tomography lines.
From the pseudo-3D models, resistivity maps were generated for every two meters of depth, from 966 m a.s.l. (Surface) to 950 m a.s.l. (16 meters deep). Thus, nine resistivity maps were generated (Figure 9).
The map representing the surface (966 m a.s.l.) shows that the central region has lower resistivity (values below 3270 Ω.m) than the rest of the map, indicating zones of more permeable soil and, thus, of possible aquifer recharge. At greater depths in the soil mantle, high-resistivity materials are evident in the central region, oriented in the NW-SE direction. Up to 6 meters deep (960 m a.s.l.), the water seems to flow horizontally, while from 8 meters deep (958 m a.s.l.), the resistivity in this zone intensifies, causing the water flow to move towards the southwest and, in greater quantity, towards the northeast. Yet, in the southeast region of the maps, the low-resistivity zones continue as the depth increases. This trend suggests that the low and high-resistivity zones must grow closer together at greater depths, indicating that the aquifer is connected.