## 4 Results and Discussion
### Geophysical Survey
#### 4.1.2 Temporal Variation of Conductivity in the Area
Locally, groundwater roughly follows a divergent radial flow from the ICCE to the west (Figure 1; [33]). In addition, to the east of the ICCE, the thickness of the Quaternary formations decreases (Figure 2), which means that the importance of the unconfined aquifer is limited and there are no pumping wells. Therefore, a good approach to investigate temporal and spatial variations of geophysical data can be accomplished by gathering information along the lines forming a radial network from the ICCE, as shown in Figure 5. In this figure, the yellow lines depict the directions from N30\({}^{\circ}\) W to N120\({}^{\circ}\) W (with a 15\({}^{\circ}\) angular spacing) used to evaluate the variation of the measured conductivity values. The E-W line was not included because it runs into the Veiros pond. For two orientations, N75\({}^{\circ}\) W and N105\({}^{\circ}\) W, the spatiotemporal variations correspond to the areas depicting the highest conductivity values, as shown in Figure 6. This figure was constructed by plotting field conductivity values in accordance with the time of measurement and the distance from the beginning of the profiles near the ICCE. These two orientations were chosen to illustrate the procedure, but they also correspond to important directions of contamination propagation, which was obtained by crossing information from Figure 4 with the main local radial flow directions of groundwater presented in Figure 1.  
Data from orientation N75\({}^{\circ}\) W, on the left of Figure 6, showed an increase of conductivity with distance until 2006. Afterwards, VF data (Figure 6, top left) depicted a gradual decrease in conductivity values and, hence, an expected decrease in contamination as a result of the implemented remediation works. For HF (Figure 6, bottom left), or shallower information, a similar behaviour was observed, in particular for the higher conductive curves. Hence, it seemed that the contamination also decreased at shallower depths, but there were still some signs of progression at lower levels of conductivity. This could be explained by surface runoff or very shallow contamination from an origin other than  
Figure 5: Transactions used to sample the shift of the conductivity contouring lines (map coordinates: UTM; WGS84—29N zone). Figure 6: Electrical conductivity variation along profiles N75\({}^{\circ}\) W and N105\({}^{\circ}\) W over time. industrial sources. Therefore, for this orientation, the 2003-2005 remediation works appear to have been effective. On the right of Figure 6, data from orientation N105\({}^{\circ}\) W are depicted. In this case, for both horizontal and vertical fields, contamination is shown to progress as distance increases in both graphs. However, there is a distinct change in the dip of the lines. This dip is gentler after 2006, revealing a lower propagation velocity of contamination, both for shallower and deeper areas. Therefore, remediation works also showed some degree of efficiency, although lower than that observed for N75\({}^{\circ}\) W.  
Data from all the orientations in Figure 5 are summarized in Table 1. Data analysis showed similar behaviour for both shallower and deeper investigations, suggesting there is a hydraulic connection between the upper unconfined aquifer and the deeper semi-confined aquifer. This can be explained by discontinuous mud layers or their disturbing by human intervention, such as through the opening of wells or boreholes. For particular directions, there was a difference before and after 2006, although the behaviour varies with orientation. Thus, until 2001, there was a general decrease in curve displacements velocity in the direction N60\({}^{\circ}\) W, whereas for direction N75\({}^{\circ}\) W there was a general increase in contamination spread for both horizontal and vertical coils. Profile N120\({}^{\circ}\) W depicts residual displacements. Therefore, for this period, it seemed contamination receded to the northwest, but increased to the west of the mud dam.