Hydrogeochemical And Isotopic Assesment of The Open Mine Pit Lakes Near Kesikköprü Dam Area, Türkiye

In cases where the excavation depth reaches the groundwater level as a result of open-pit surface mining operations, open mine pit lakes (MPL) form by the discharge of groundwater to the surface. Processes known to affect the MPL chemistry include groundwater inflow, direct precipitation, pit wall-precipitation contact, groundwater-wall rock interaction, evaporation, and other geochemical and limnological processes taking place within the MPL. While changing the hydrogeological system, mining activities might also affect the groundwater chemistry. Degradation of groundwater quality might occur as a result of the interaction between wall-rock and groundwater within the MPL. For this reason, the chemical characteristics of these mine lakes should be revealed in order to protect the groundwater systems and also before using the water inside MPL’s for different purposes like irrigation. Kesikköprü Dam is a dam located in Bala district of Ankara province, the capital of Türkiye, and has been providing drinking water to Ankara in recent years. Although mining activities have been carried out around this dam since the 1950s, where one of Turkey's richest iron deposits is located, no hydrogeological or hydrogeochemical studies have been carried out prior to this study. Therefore, the aim of this study is to evaluate the hydrogeochemical and stable isotopic characteristics of the mine lakes formed as a result of iron mining activities around Kesikköprü Dam. Within this study, three MPL’s were evaluated whose surface areas range from 0.27 ha to 0.92 ha. The pH and Electrical Conductivity values of these three lakes vary between 8.47-8.58 and 883-2500 µS/cm, respectively. According to the major ion chemistry analyses results, Na, Ca and Mg are the dominant major cations and SO₄ and HCO₃ are the dominant major anions. The major ion chemistry of these lakes have been influenced by reverse ion exchange processes. The MPL waters are oversaturated with respect to certain minerals like dolomite, aragonite, calcite and magnesite. There is dissolved arsenic in concentrations up to 16 µg/l. Dissolved arsenic is found in the form of pentavalent arsenic (As⁵⁺). The stable isotopes of hydrogen and oxygen suggest that there is evaporation from the lake surfaces and the isotopic signatures of MPL water samples prior to evaporation unveiled that there is contribution of groundwater inflow from a shallow groundwater system in the area. The abandoned MPLS’s should be rehabilitated so as to prevent exposure of the groundwater systems which become vulnerable to contaminatation.

Keywords: Open Mine Pit Lakes; Hydrogeochemistry; Stable Isotopes; Dissolved arsenic; Kesikköprü Dam, Ankara; Türkiye