## 5 Discussion
### Soil Contamination Assessment  
Multivariate statistical analysis showed a clear distinction between naturally occurring trace elements and PTEs transferred from the sulphide waste piles to the adjacent marshland by both chemical and physical processes (leaching, runoff, atmospheric deposition of wind-blown dust). Several clusters are apparent on the projection of the scores on the first two principal components extracted from the PCA (Figure 8), which account for 72% of the total variance. The first principal component (PC1) is statistically dominant, showing two distinctive clusters characterized by strong and opposite scores: (1) As-Cu-Pb-Zn-Cd-Co-TI-Sb-Se, and (2) Be-Cr-V. These geochemical associations are interpreted as formed by anthropogenic and geogenic trace elements, respectively, while Ni, Ba and Sn are probably derived from mixed sources. Figure 7: General flow chart of the chemical reactions involved in the formation of the secondary sulphate minerals. By far the most abundant of the PTEs present in the sulphide waste piles are As, Pb, Cu, Zn and Sb. Consistently, the total concentrations of such elements in most soil core samples are above the median value reported for topsoil of the salt marshes of the Huelva estuary [43] and greatly exceed the regional baseline concentrations [44]. However, a site-specific assessment of soil contamination requires knowledge of pre-industrial metal concentrations to act as a local background against which measured values can be compared [20; 45]. In this study, the concentrations of PTEs measured in the least impacted level of soil (core sample 5d, transect south) served as a suitable background or baseline reference material, under the assumption that at this depth (45-60 cm) the cored soil would not be contaminated by anthropogenic inputs. The _EF_ appears to be an effective indicator to reveal the anthropogenic source of the PTEs of concern. The results from the _EF_ calculation (Table 4) suggest extremely high enrichment levels of Pb (up to 1893) and Sb (1578) as well as Tl (494), As (380), Cd (225), Zn (158), and Cu (106) in the soil of the yellow zone. It is also noteworthy that the tidal channel infill showed high _EF_ values for As (369) and Sb (133). By contrast, the _EF_ values of Be, V, Cr, Ni, Sn, and Co generally ranged around one, as they are present in most samples at near-baseline concentrations. The wetland soil affected by AMD discharges shows extensive metal accumulation. The I\({}_{\text{geo}}\) values registered in the core soil samples of the north transect were higher than those of the south transect for most PTEs (Table 5), being Sb (6.31), Pb (6.27), As (6.25) the contaminants with the largest I\({}_{\text{geo}}\) values. On average, the I\({}_{\text{geo}}\) values of the surface soil samples decreased in this order: Sb \(>\) As \(>\) Pb \(>\) Tl \(>\) Cu \(>\) Zn \(>\) Cd in the yellow zone, while the order was: As \(>\) Sb \(>\) Pb \(>\) Cu \(>\) Zn \(>\) Tl \(>\) Cd in the white zone, and As \(>\) Sb \(>\) Cu \(>\) Pb \(>\) Zn \(>\) Tl \(>\) Cd in the ochre zone. Figure 8: PCA diagram of the total concentration of trace elements. Several clusters are apparent on the projection of the scores on the first two principal components (PC1 and PC2), which explain 72% of total variance.