# Heavy Metals in Post-Exploitation Reservoirs--The Bagry Lake Case Study (Poland)
## 7 Discussion
The geochemical background for Cu, Zn, Pb and Cd in Bagry lake supernatant was exceeded in 12, 16, 17 and 12 (out of 18) measuring sites, respectively. Comparing the metals concentrations in the supernatant of Bagry Lake and water of other reservoirs (Table 9), it should be noted that the highest concentrations of Cu and Fe were found in Bagry Lake. The maximum concentration of Cu was about 5.8 times higher than the one in the Kozlowa Gora Reservoir; this reservoir is strongly influenced by anthropoppressure. The highest concentrations of Fe, Mn and other metals in the supernatant water were observed in the southeastern part of Bagry Lake where surface runoff infiltrates close to the rail track. Zinc concentrations are slightly higher than those in the Dobczyce Reservoir but definitely lower than the ones in the Czorsztyn Reservoir. Lead concentrations are much lower than in Kozlowa Gora Reservoir but definitely higher than in the other reservoirs; cadmium concentrations in Bagry Lake remain the lowest. Both the cluster analysis (Figure 18) and the analysis of the relationship between the metal concentrations in the supernatant and in the sediment confirmed that the relation is weak, e.g., a relatively low iron concentration in the sediment opposed to its high concentration in the supernatant. It may be explained by surface runoff and its infiltration into groundwater feeding Bagry Lake. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Reservoir** & **Fe** & **Mn** & **Cu** & **Pb** & **Zn** & **Cd** \\  & [g/kg] & **[mg/kg]** & **[mg/kg]** & **[mg/kg]** & **[mg/kg]** & **[mg/kg]** \\ \hline Bagry (currently analysed) & 3.16–36.03 & 61.60–566.4 & 2.11–161.0 & 1.96–54.11 & 25.35–321.0 & \(\leq\)1.831 \\ \hline [PERSON] [20] & 10.3–43.8 & 156–2355 & 9.3–44.2 & 11.5–40.7 & 16.5–264.4 & 0.33–2.4 \\ \hline [PERSON] [20] & 14.1–39.9 & 21.5–336 & 14.8–34.1 & 12–23.2 & 84.7–296.1 & 0.7–2.1 \\ \hline [PERSON] [20] & 20.6–38.1 & 619–1381 & 27.5–38.4 & 16.9–26.3 & 78.5–189 & 1.4–2.6 \\ \hline Solina [42] & nd & nd & 21.69–37.57 & 10.73–33.71 & nd & 0.12–1.4 \\ \hline Kozlowa Góra [43] & nd & 109–784 & 6.8–112.5 & 105–1373 & 290–1875 & 2.8–22.7 \\ \hline \end{tabular} nd—no data available.  
\end{table} 
Table 8: Concentrations of metals in sediments of the selected reservoirs. Since there are weak concentration equilibria between the supernatant and the sediments, it may be interesting to look into relations between metals concentrations in the supernatant and in the sediment fractions and vice versa. The interactions depend on a main direction of metal relocation between the phases, until the equilibrium is established. Due to the nature of sediments and materials submerged in Bagry Lake as well as rainwater infiltrating the area of the railway track, it is presumed that these are the key factors that determine metal concentrations in the supernatant. Metal concentrations in the sediments and in the supernatant depend not only on the metal state but also on pH and redox conditions. Soluble compounds or those which undergo hydration or dissociation move freely between phases. On the other hand, insoluble forms are transported only by mechanical movements of supernatant and sediment particles or by density currents. Many factors determine whether metals stay in soluble or insoluble forms. These factors contribute to migration of metals between phases and affect their adsorption or accumulation in sediments. The calculations of the model (6) refer to the average conditions that define the behavior of metals in the sediments and in the supernatant in Bagry Lake. Since there is a strong correlation between the volatile fraction and the fraction with particles < 0.06 mm (R = 0.9404, Table 5), metal compounds might also be present in the silly clay and clay fractions; they do not have to be adsorbed or absorbed in organic matter but may constitute a separate fraction.