Hydrogeochemical sources and processes controlling water chemistry of abandoned coal pit lakes in Kütahya province, SW Türki̇ye

Pit lake water quality represents a major environmental concern after mine closure. However, contrary to the common assumption that all pit lakes are acidic and of poor water quality, their hydrochemistry varies considerably depending on deposit type and local geological-hydrogeological conditions. This study presents a hydrogeochemical assessment of pit lake waters and nearby springs in the abandoned coal mines of the Kütahya Coal Basin (Türkiye) to evaluate pit lake water quality and identify the geochemical processes controlling water chemistry. The results indicate that both pit lake and spring waters exhibited similar ionic compositions dominated by SO₄ 2- , HCO₃ - , Mg 2+ , and Ca 2+ , suggesting a shared geochemical origin associated with carbonate- and sulfate-bearing lithologies. Pit-lake waters are characterized by slightly alkaline pH values (7.56–8.11) and electrical conductivity (EC) values approximately 130% higher than those of spring waters. In contrast, spring waters display lower EC values and narrower compositional ranges, reflecting relatively stable background hydrochemical conditions. The elevated solute concentrations in pit-lake waters are attributed to evaporative concentration, prolonged water–rock interaction within the pit environment and precipitation-derived seepage from surrounding waste-rock dumps. The co-enrichment of Mg and SO₄²⁻ is further explained by the preferential precipitation of Ca-bearing minerals under saturated conditions, the relatively slow precipitation kinetics of dolomite, and the high solubility of Mg-sulfate minerals such as epsomite. Although oxidative dissolution of sulfide minerals may contribute to sulfate enrichment to a minor extent, the neutral to alkaline pH values, low dissolved iron concentrations, and absence of visible acid mine drainage (AMD) indicators suggest that carbonate buffering effectively suppresses AMD generation.