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Redox processes affect the chemical quality of groundwater in an aquifer and can mobilize or immobilize heavy metals which occur naturally in the aquifer materials. Determining types of redox processes that occur in the aquifer and understanding how they affect concentrations of heavy metals are essential to assessing and describing the groundwater quality. In the alluvial aquifer of the Drava River which consists of Quaternary and Upper Miocene deposits (gravel, sand, silt, clay), elevated concentrations of heavy metals have been observed in the Koprivnica region. The groundwater is mainly used for water supply of the town Koprivnica and surrounding settlements. In order to determine hydrochemical processes controlling the groundwater quality, groundwater samples were collected from piezometric wells in monthly campaigns (from July 2013 till November 2014) and analysed in the Hydrochemical laboratory at Croatian Geological Survey. It was observed that in waters with higher concentrations of dissolved oxygen and nitrates, lower concentrations of iron and manganese were measured. In waters with higher concentrations of iron and manganese, lower concentrations of dissolved oxygen and nitrates were measured. The aquifer matrix contains silicate and carbonate minerals, organic materials, and sulphide minerals such as pyrite. The hydrochemical interpretation and geochemical modelling using NETPATH_WIN and PHREEQCI showed that during the oxidation of organic matter and pyrite, reduction of dissolved oxygen is observed. Since oxygen tends to be consumed along the aquifer, under the anoxic conditions, the next most energetically favourable available electron acceptor is nitrates. Because of these processes iron and manganese concentrations are higher and dissolved oxygen and nitrates are very low. Moreover, it was observed that, in the circumstances, when oxygen and nitrates were consumed next electron acceptors were manganese (IV), ferric iron and sulphates. |
