## 3 Results and Discussion
### Interaction between Heavy Metals and Organic Matter  
The mobility and solubility of heavy metals in the environment depend on their complexation with organic matter. We found similar patterns for Pb and Zn in sediment cores (Figure 3). Moreover, studied metals and organic matter content tended to accumulate in a similar way within the sediment layers (Table 1). Correlation among heavy metal concentrations may reveal the source, transport, and fate of those elements [44,110]. Table S4 shows the results of the Pearson correlation analysis. All elements showed a positive and significant correlation with the organic matter content (OM%) at \(p<0.05\). Pb showed a positive correlation (0.40) with organic matter. Sequential extraction experiments also showed the greater influence of organic matter in controlling Pb fate. This phenomenon may be explained by lead adsorption processes by organic colloids, as suggested by previous authors in similar environments [48]. The highest Pb concentrations occurred in lake sediments and wastewater-affected sediments/ soils. Possible explanations for the accumulation in Petrola Lake include the use of Pb in gasoline in Spain until the late 1990s or the use of gunshot pellets in hunting. Further information on sedimentation rates and Pb isotopes would be necessary to determine the origin of this element in the lake. In wastewater-affected soils and ponds (sites 2641 and 2575), concentrations may also derive from industrial wastes.  
Regarding the remaining elements, particularly noticeable were the positive correlations Hg-Zn (0.79), Cu-Hg (0.65), and Cu-Zn (0.54). Cu, Hg, and Zn seem to share a common source and/or behavior. Their concentrations were close to background values (Table 1). These findings suggest that this group of metals is derived from natural sources, such as mineral weathering [111]. Silicate minerals are abundant in the study area, potentially constituting a source. In fact, the amount of Cu, Pb, and Zn in the residual fraction (F6; Figure 2) is higher in samples from irrigation and dryland sandy siliciclastic soils. These three elements (Cu, Hg, and Zn) behave differently with respect to Hg and Cd, based on the PCA (Figure 4). PCA of the transformed variables (log) showed that the first two components explained 81% of the variance. Subsequent K-means clustering analysis revealed two main groups of explanatory variables. Cluster A includes Cu, Hg, and Zn, whereas cluster B contains Cd and Pb. Cluster A elements have the highest loading in component 1, showing that these elements accumulated mainly in flamingo tissues, particularly in the liver. In contrast, cluster B elements showed the highest loading for component 2, highlighting the accumulation in lake sediments and agricultural soils for Pb and Cd, respectively. Therefore, the influence of anthropogenic origins for the latter should not be neglected. For instance, agricultural sources involve the use of inorganic synthetic fertilizersers with considerable concentrations of trace elements (i.e., Cd) [37]. This interpretation is supported by the significant accumulation of Cd in dryland and irrigation agricultural soils. In lake sediments, heavy metals can form strongly dissolved complexes with organic matter and inorganic ligands under high ionic strength, conditions that are characteristic of Petrola Lake, with dissolved organic carbon concentrations up to 646 mg/L [75]. This may help to explain the differences in Cd concentrations between irrigation/dryland soils and lake sediments. Accordingly, transport of metals may start in the agricultural and wastewater-affected soils, where the highest Cd concentrations were found, and may end in the lake, where metals can be accumulated and immobilized in organic-rich sediments. However, Pb accumulation in subsurface lake sediments may be linked to other anthropogenic sources as discussed above (e.g., fuels, hunting). Organic-rich sediments seem to be the main sink for metals, which can probably become remobilized under changing redox conditions.