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.