Groundwater Recovery and Hydrogeological Response to Pit Lake Formation After Mine Closure: A Case Study of Nikolaevsky Open-Pit Mine, East Kazakhstan

Open-pit mines extending below the water table require continuous dewatering, which creates large hydraulic depression cones, reverses or accelerates natural gradients, and progressively mobilizes hazardous elements in groundwater through sulfide oxidation—effects that persist long after mine closure. The cessation of dewatering operations at the Nikolaevsky polymetallic open-pit mine in the Shemonaikha District, East Kazakhstan Region, Kazakhstan, in August 2012 terminated approximately 52 years of active dewatering (December 1960–August 2012), during which groundwater levels in the fractured Paleozoic aquifer were drawn down from a pre-mining static head of about +340 m a.s.l. by approximately 400 m. During the dewatering period, sulfide oxidation generated acidic metalliferous drainage in seeps, fundamentally altering the hydrochemical regime of the area. This study examines groundwater table recovery and subsequent hydrogeological changes under two scenarios. The first scenario, caused by the termination of mine dewatering and subsequent natural pit lake formation, focused on the evolution of groundwater levels, development of a pit lake, influence on regional groundwater flow, and potential impacts on the Ust-Talovka municipal reservoir intake located 3 km from the pit. The second scenario evaluates the co-disposal of local process tailings into the flooding pit as an alternative to surface tailings impoundment, assessing its effect on refill rate, water chemistry, and hydrogeological risk. Field surveys, hydrochemical sampling (62 water samples collected in July and October 2019), laboratory physicochemical modeling of tailings pulp–pit water interactions, water balance calculations, and a three-dimensional numerical groundwater flow model were employed to predict the consequences for local groundwater availability and quality. Pit lake water quality has improved substantially since dewatering stopped (pH increased from below 4 during mining to 6.85–7.29 by 2019); however, although metal concentrations have begun to decrease, Zn, Cd, Mn, and Pb remain above drinking water standards. Under median climatic conditions, the analytical water balance forecast indicates that the pit lake level will approach equilibrium at approximately +335 m a.s.l. after ~375 years; sensitivity tests show that the equilibrium level (≈320–340 m a.s.l.) and the filling time are strongly controlled by the adopted evaporation and precipitation values. In-pit tailings disposal shortens the filling period by approximately 12–15 years. Advective transport modeling over 200 years, under the stated conservative assumptions, predicts that metal plumes do not reach the Ust-Talovka municipal water intake, whereas a proposed alternative surface tailings facility would lie within the intake’s 200-year capture zone, which favors in-pit disposal of the tailings. These findings provide a scientific basis for post-mining hydrogeological risk assessment and demonstrate the utility of combined analytical and numerical modeling for evaluating pit lake evolution and tailings disposal options at comparable deep pits in arid continental climates, subject to site-specific verification.