Major ion chemistry and metal distribution in coal mine pit lake contaminated with industrial effluents: constraints of weathering and anthropogenic inputs

Ion chemistry of mine pit lake water reveals dominance of alkaline earths (Ca²⁺ and Mg²⁺) over total cation strength, while SO₄ ²⁻ and Cl⁻ constitute the majority of total anion load. Higher value of Ca²⁺ + Mg²⁺/Na⁺ + K⁺ (pre-monsoon 5.986, monsoon 8.866, post-monsoon 7.09) and Ca²⁺ + Mg²⁺/HCO₃ ⁻ + SO ₄ ² (pre-monsoon 7.14, monsoon 9.57, post-monsoon 8.29) is explained by weathering of Ca–Mg silicates and dissolution of Ca²⁺-bearing minerals present in parent rocks and overburden materials. Silicate weathering supposed to be the major geological contributor, in contrast to bicarbonate weathering does a little. Distribution coefficient for dissolved metals and sorbed to surface sediments is in the order of Cd > Pb > Fe > Zn > Cu > Cr > Mn. Speciation study of monitored metals in surface sediments shows that Fe and Mn are dominantly fractionated in exchangeable-acid reducible form, whereas rest of the metals (Cr, Pb, Cd, Zn, and Cu) mostly in residual form. Cd, Pb, and Zn show relatively higher recalcitrant factor that indicates their higher retention in lake sediments. Factor loading of monitored physico-chemical parameters resembles contribution/influences from geological weathering, anthropogenic inputs as well as natural temporal factors. Ionic load/strength of lake water accounted for geochemical process and natural factors, while pollutant load (viz BOD, COD and metals, etc.) is associated with anthropogenic inputs through industrial discharge.