## 4 Results and Discussion
### Hydrochemical Surveys and Water Quality
However, there were new hotspots that emerged at different sites (mainly associated with deeper aquifer measurement or vertical fields), as depicted in Figure 6 and Table 1.  
Considering that groundwater EC is the parameter that best reflects contamination, a spatial distribution map of this parameter, according to the range of values and including the data from the three campaigns, is presented in Figure 7. As can be observed, the areas showing higher EC values were located SW of ICCE, which is particularly evident for 2007 and 2016, and in agreement with the geophysical data (Figure 4). In general, the contamination plume was likely to have migrated from E to the SW, away from the ICCE, along one of the main groundwater flow paths, but it also showed some attenuation effects. This suggests that contamination spreads by diffusion because of the main geological features hosting the aquifer system, namely, highly porous and permeable media. Nevertheless, significant changes in the EC values between nearby locations also occurred for measurements carried out in the 2007 and 2016 campaigns, which reinforces the combination of the local divergent radial flow (Figure 1) with local superficial contamination hotspots (Figure 7; A, B, and C sites). In addition, high EC values were also found to the south of the sampling area, particularly in 1989 and 2016, closer to the end portion of the industrial draining ditches (Figure 7, area D). Considering the most important plume propagation path (E to SW), obtained by both geophysical (Figure 4) and hydrochemical approaches (Figure 7), a group of samples from each campaign along that path (Figure 8) were selected to illustrate the variation of hydrochemical parameters, either in terms of time or distance from the ICCE (Figure 9). Figure 7: Spatial distribution of electrical conductivity values in the studied area (map coordinates: UTM; WGS84—29N zone). Figure 8: Points defined by groups of samples in common for the three sampling campaigns, illustrating the transect of the main plume (E-SW) alignment (map coordinates: UTM; WGS84—29N zone). Figure 9: For the 1989, 2007, and 2016 campaigns, variation of electric conductivity (EC), pH, Cl, Na, SO\({}_{4}\), and Fe along the E-SW profile (displayed in Figure 8). In Figure 9, each bar represents the element concentrations in the three different campaigns. Electrical conductivity, Cl, Na, and SO4 values showed a similar spatial trend for each sampling campaign, revealing that Cl, Na (likely related to brines), and SO4 (related to acid sulphuric production) were responsible for the EC changes. This is not evident for Fe, which had highly variable values among campaigns and locations. In 2007, Fe behaviour was similar to that found for Cl, Na, and SO4 for locations 2, 3, and 4. The pH was not a differentiating parameter for the spatial and temporal contamination evolution, which can be explained by different production processes over time that generated either acid effluents (e.g., H2 SO4 production) or alkaline effluents as a result of sodium and chlorate compounds production. The most relevant aspect evidenced by these pictures is the huge increase in EC, Cl, Na, SO4, and Fe values in 2007, in particular for points 2, 3, and 4. This is likely related to the construction of a sealed landfill in the period of 2003-2005, which served as a deposit of highly contaminated waste and soils. Operations associated with these works included mechanical relocation and embolization of materials, likely promoting the oxidation and dissolution of contaminants hosted in those materials [25; 45]. In 2016, this phenomenon was not observed and the values for these parameters were closer to those in the 1989 campaign. However, the areas corresponding to points 4 and 5 displayed higher values than those for points 2 and 3. This indicates that there was a dilution/attenuation of the contamination from 2007 to 2016, and, at same time, a migration of the plume to the SW, away from the ICCE.