# Heavy Metals in Post-Exploitation Reservoirs--The Bagry Lake Case Study (Poland)
## 7 Discussion  
The concentrations of Fe and Mn in the Bagry Lake sediment stayed below the assumed geochemical background values, which were 47,200 and 850 mg/kg d.m., respectively [40]. The concentrations of Cd, Cu, Pb and Zn exceeded the background levels at 50%, 72%, 61% and 55.5% of the measurement points, respectively; the background values for these elements were 0.3 (Cd), 45 (Cu), 20 (Pb) and 95(Zn) mg/kg d.m. [40]. Setting the geochemical background is difficult, especially for such specific ecosystems as artificial lakes. However, the higher metal concentrations in the sediment are most likely related to a large number of sunk construction materials, vehicles and boats, rails and narrow-gauge railway cars, cables, etc. [33]. Over time, the metals have been released from these objects to water and lake sediments due to dissolution, corrosion and adsorption (generally accumulation) in mineral or volatile material. Lake sediments are an integral part of the aquatic environment and provide a habitat for many aquatic organisms; high concentrations of heavy metals may pose a threat to them. It should be noted, however, that the bioavailability of metals varies depending on many factors including both metals themselves and the characteristic of the aquatic environment [41]. The LEL and SEL ecotoxicological criteria indicate that copper can be toxic for benthic organisms at two locations while the potentially harmful effects of Cd, Cu, Pb, Zn, Mn and Fe have been noticed at 3 up to 18 measuring sites, located within Bagry Lake. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Metal** & \(P_{m}\) & \(P_{v}\) & \(Q_{m}\) & \(Q_{v}\) \\  & & & [\% \(Me_{w}/\%Me_{m}\)] & [\% \(Me_{w}/\%Me_{v}\)] \\ \hline Fe & – & – & – & – \\ \hline Mn & – & – & – & – \\ \hline Cu & 0.8057 & 0.1943 & 4.149 & 104.7 \\ \hline Pb & 0 & 1 & – & – \\ \hline Zn & 0.01159 & 0.9884 & 0.3051 & 3.832 \\ \hline Cd & \(4.207\times 10^{-16}\) & \(\sim\)1 & 0.2025 & 1.777 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Average share of mineral and volatile fractions in the sediment vs. metal concentrations in the supernatant. Due to the fact that no literature was found on the concentrations of metals in the water environment of post-exploitation reservoirs similar due to the exploited raw materials (natural aggregates and clay) and uncontrolled flooding, the concentrations of heavy metals contained in its water environment were compared to those of other dam reservoirs located in southern Poland. These reservoirs, unlike Bagry Lake, are flow reservoirs. They were created for flood protection, leveling the water levels in the rivers below the reservoir, supplying the population with water, the operation of hydropower plants and recreation. Comparing Bagry Lake with other reservoirs (Table 8) the highest Cu concentration was noted in there; the concentration exceeds even the values observed in the Kozlowa Gora Reservoir, which strongly suffers from anthropoppressure. The concentrations of Pb and Zn are slightly higher than in the Dobczycki or Czorsztyn Reservoirs but are definitely lower than in the Kozlowa Gora Reservoir. The Cd concentrations remain similar to those in the Czorsztyn Reservoir. The Fe concentrations are the lowest while the Mn concentrations are slightly higher than in the Czorsztynski Reservoir but are definitely lower than in the Dobczycki or Roznow Reservoirs. It should be noted that a relatively high Cu concentration in the Bagry Lake sediment is site-specific. There was a rather weak relationship between the metal concentration in the sediment and in the supernatant in Bagry Lake. It is difficult to set the geochemical background for iron and manganese in Bagry Lake since the lake stays in contact with groundwater, located southeast (about 1 m below the surface) and not insulated with impermeable layers. The values 2 \(\times\) 10\({}^{-5}\), 0.002, 2 \(\times\) 10\({}^{-4}\) and 0.01 mg/L were assumed as the geochemical background for Cd, Cu, Pb and Zn, respectively [1].