# Heavy Metals in Post-Exploitation Reservoirs--The Bagry Lake Case Study (Poland)
## 3 Results
### Supernatant  
Table 2 presents the average concentrations of metals in the supernatant (layer of water just above the sediment) of Bagry Lake as well as the standard deviations and the minimum and maximum concentrations which have been calculated based on the results of analyzes for the entire lake. Only lead concentrations exceeded the acceptable level of 0.014 mg Pb/L (see the Ordinance, [35]); its concentration at B6 was 2.2 times higher. Figure 10: Cd in the sediment [mg Cd/kg dry matter]. Figure 9: Zn in the sediment [mg Zn/kg dry matter]. In water, iron at the +2 oxidation state is present in a dissolved form as Fe\({}^{2+}\) ion or as one ([FeOH]\({}^{+}\)) or di-hydroxy ([Fe(OH)\({}_{2}\)]\({}^{-}\)) ion or as dissolved carbonate and sulphate aqua complexes (FeCO\({}_{\rm{(aq)}}\)), (FeSO\({}_{\rm{4}}\)(aq)) and as FeHCO\({}_{\rm{3}}\)]\({}^{+}\) cation. Oxidized iron is present as Fe\({}^{3+}\), [FeOH]\({}^{2+}\), [Fe(OH)\({}_{2}\)]\({}^{+}\), [Fe(OH)\({}_{4}\)]\({}^{-}\), [FeSO\({}_{\rm{4}}\)]\({}^{+}\), [Fe(SO\({}_{\rm{4}}\))\({}_{2}\)]\({}^{-}\), [FeHCO\({}_{\rm{4}}\)]\({}^{2+}\), [FeCl]\({}^{2+}\), [FeCl]\({}^{2+}\), [FeCl\({}_{\rm{2}}\)]\({}^{+}\), [FeHPO\({}_{\rm{4}}\)]\({}^{+}\) and [FeH\({}_{\rm{2}}\)PO\({}_{\rm{4}}\)]\({}^{2+}\)[36]. These Fe\({}^{3+}\) forms are pH-dependent. In natural water, iron can be present in dissolved, suspended or colloidal forms, depending on pH and the presence of organic substances, oxygen and carbon dioxide. Iron may be assimilated by aquatic organisms and released from sediments to water, under reducing conditions. Insoluble forms of iron include mainly iron di- and tetraoxide and iron III hydroxide, carbonate and sulfide. Manganese in natural surface water is present mainly as Mn\({}^{2+}\), Mn\({}^{3+}\) and Mn\({}^{4+}\)[36]. It is easily oxidized by dissolved oxygen and by manganese bacteria. Its forms are similar to iron compounds and ions (see above). In the presence of oxygen, manganese is oxidized to insoluble forms of manganese oxides MnO\({}_{\rm{x}}\) while in anoxic zones it is reduced to soluble compounds. In its insoluble form, manganese occurs mainly as manganese di-, tri- and tetraoxide and is usually accompanied by iron compounds. Studies show that soluble manganese transfers between sediment and water more easily than a soluble iron [37]. Copper in water is present as Cu\({}^{+}\), Cu\({}^{2+}\) and Cu\({}^{3+}\) but most often at the +2 oxidation state [36]. In a dissolved form, Cu forms a copper carbonate aqua complex and various complex ions combining carbonate and hydroxy ions (similar to Fe); Cu(OH)\({}_{2}\) dissolves better at pH > 7, while Cu\({}_{2}\)(OH)\({}_{2}\)CO\({}_{3}\) at pH < 7. Copper is adsorbed in sediments; the process depends on pH, ligands, iron and manganese oxides and sediments characteristic. Copper occurs in the free state and mainly as an insoluble sulfide mixed with iron sulfide, hydroxide or carbonate and as a hydroxide mixed with its carbonate. Lead in natural water is present at +2 and +4 oxidation states [36]. The Pb\({}^{2+}\) ion forms soluble aqua complexes with carbonate, sulfate and OH group in the form of hydroxide. In addition, it can form a series of complex positive and negative ions comprising carbonate and hydroxyl group--similar to the elements listed above. Pb reacts with organic ligands containing sulfur, nitrogen and oxygen. Currently, tetramethyllead is not emitted with exhaust gases, but in anaerobic conditions, biological synthesis of tetramethyllead is possible. Lead can be adsorbed on mineral deposits and humic compounds; its insoluble form is sulfide. Zinc in water is present at the +2 oxidation state [36]. It forms complex positive and negative ions with OH groups as well as soluble aqua complexes with carbonate and sulfate. Zinc forms inorganic and organic complexes and adsorbs onto suspensions and sediments. In sediments, it may react with iron and manganese oxides, carbonates and organic substances. Zinc occurs in an insoluble form as hydrated silicate, carbonate, sulfide or oxide.