## 4 Discussion
The positive correlations of Pb with the concentrations of TOC and N\({}_{\text{t}}\) agree with the situation found for the Klingenberg Reservoir, Germany [32]. Since Pb has a relatively low element-specific mobility and no correlations with the pH value were found, it is interpreted that Pb is deposited in reservoirs in particular by the erosion of organic-rich topsoil material. Due to its ubiquitous enrichment resulting from atmospheric depositions [56], particularly in forest soils [57], it can be assumed that it is transported to the reservoirs in a similar way as Cr and V. However, due to a comparatively higher preference to be bound to soil organic matter [58,59], the deposition of Pb in reservoir sediments seems to be influenced to a higher degree by the sedimentation of organic detritus. The enrichment of Cd, Mn, Ni, and Zn is, contrarily, not explainable by soil erosion and subsequent sedimentation within the aquatic system alone. The deposition of fine sediments in reservoirs generally favors the enrichment of these elements, which agrees with the frequent presence of their highest concentrations in RS. However, the EF, by which the individual samples and sample groups can be compared largely independent of existing grain size differences [43], shows clearly increased enrichments of Cd, Mn, Ni, and Zn in SS and/or RS, which coincides with the grouping of these elements by the PCA. The preferred enrichment of these elements within aquatic sediments is explainable by additional inputs into the water systems via leaching processes from the soils of the catchment areas. This is evidenced by the comparatively high mobilizable fractions of the total element concentrations, the significant correlations to the pH value of the samples, and the joint grouping in the frame of the PCA. Further indications are given by the fact that Cd, Mn, Ni, and Zn are already increasingly dissolved under pH values of <5.5 [60,61]. Based on this process, these elements have a higher potential to be enriched in SS and RS, although the intensity of this enrichment can vary between single study areas (for details see Table S4). The CS, in turn, show the clearly lowest pH values in the comparison of the sample groups due to a stronger acid formation and leaching potential under humid climate, little acid-buffering rocks, and forest vegetation. It can be assumed that this element transfer occurring between a soil and a water body, if present, also includes anthropogenic substance accumulation in soils, as is known for the catchments of numerous reservoirs [62]. Similar to Pb, the concentrations of Cd and Zn are significantly correlated with TOC and N\({}_{\text{t}}\). It is likely that Cd and Zn are partially enriched by adsorption to organic matter or the formation of sulfides [63,64] in semi-terrestrial subsoils or organic-rich fine sediments after they enter surface water. It remains unclear how far the variable quantity and quality of organic substances between the different sampling groups--which can additionally impact metal binding [65]--might influence this process. For Mn and Ni, the possibility of sulfide formation is assumed to be less significant, since Mn can retain a comparatively high mobility even under strongly reducing conditions and its hydroxides simultaneously represent a major adsorbent of Ni [66]. The influence of point and/or anthropogenic sources, where an increased release of Cd, Mn, Ni, and Zn could occur, can be largely excluded due to the selection of the water bodies studied (headwaters of mostly drinking water reservoirs).