## 4 Discussion
Similar studies have also been carried out by a number of scientists considering the migration and forms of metals found in the bottom sediment [60; 85; 86; 87; 88]. Studies conducted to assess the pollution and environmental risk, analyze the spatial variability of metal concentrations, and identify potential sources and factors in six reservoirs in Poland (Je'zewo, Jutrosin, Rydzyna, 'Sroda, Wraze'snia), revealed that pollution with Cd; Cr; Cu; Ni; Pb mainly originates from geogenic sources, while Zn concentrations originate from point sources associated with agriculture [88]. According to the results of studies by [PERSON] and other authors (2003) [89], the metal concentrations are dependent on the granulometric composition of the bottom sediment (sand, silt and clay) [2; 15; 16] and, in turn, accumulate in the areas of river inflow and near the dam zone, thereby confirming our results, which indicated that from the Ii River runoff, polluted by heavy metals, moves from the upper reaches to the central part of the reservoir, depositing in the dam zone, as indicated by the sandy bottom, as well as in the silted areas of the Kaskelen and Turgen rivers inflow. According to several authors [90; 91], the decisive role in both the presence and the concentration level of metal in bottom sediment and suspended solids is played by the pH value, depending on oxidation-reduction conditions in the bottom sediment [62]. Changes in these conditions in the bottom sediment lead to changes in the valence of metals and the forms of occurrence for natural waters of any type, regardless of their chemical composition or hydrological regime [92]. The concentration of metals is dependent on the acidity of the environment, as a result of their increased solubility, with increased concentrations of metals occurring in the bottom sediment with decreasing pH, and when entering the aquatic environment, they hydrolyze and interact with other ions, while in the pH range of waters (6.5-8.5), they are able to sparingly form soluble hydroxides, phosphates and sulfides [63; 64]. Therefore, the acidity of the environment affects the intra-aquatic processes of complex formation and transformation of various forms of metals [93]. Low concentrations of zinc, copper and lead indicate their low migration activity in the sediments of the Kapshagay Reservoir, as well as the intensity of the leaching processes. During the study period, the magnitude of the active reaction of reservoir water characterized the water as being slightly alkaline, with pH values ranging from 8.1 to 8.6. pH presented low values in the reservoir zones that were subject to the influence of the small tributary rivers and the Ii River runoff. Another important factor is to determine the concentration of metals of primary and secondary origin in bottom sediment, which is associated with the processes of redistribution of pollutants in bottom sediment, worsening the conditions for the existence of bentnos in the aquatic ecosystem and, accordingly, their balance [88]. It is generally known that, along with the accumulation of heavy metals in bottom sediment, various inorganic and organic substances (petroleum products, pesticides, etc.) are deposited as a result of physical, chemical and biological processes that can easily be subjected to decomposition processes. For example, up to 30% of oil products are absorbed by bottom sediment, and up to 86% of metals [94]. When the accumulation of metals reaches a high level, they become toxic [12; 95]. Independently of the fact that the accumulation of metals in bottom sediment is higher than in water, it is necessary to consider that these interactions between water and sediments are still only poorly studied, and it is not known which part is more mobile and accessible to hydrobionts [62; 96; 97; 98].