Chemical composition of groundwater in abandoned coal mines: Evidence of hydrogeochemical evolution

Abstract

Flooding of coal mines often results in changes in the hydrochemical background and leads to the formation of new geochemical types of water in the upper part of the underground hydrosphere. There is an impact on the safety of the groundwater basin and, as a consequence, on the main sources of water supply in the regions, as well as on the surface water. In this work, hydrogeochemical studies were carried out in the areas of abandoned coal mines of the Partizansk coal basin (Primorsky Krai, Russian Federation) to identify the specific features and patterns of the composition of groundwater formed in the natural-technogenic structures. Our data indicate that water circulating in natural-technogenic structures increases salinity (>6799 mg/L), mainly by raising the content of sodium ions, bicarbonates, and sulfates. The contribution of rare earth elements to the water salinity was insignificant. We found that, as salinity increases, bicarbonate cation-mixed natural water acquires sulfate-bicarbonate and sodium sulfate composition. The proportion of complex compounds of migration forms grows leading to the formation of authigenic minerals (silicates, carbonates, oxides, and hydroxides) that control the content of components in water. The key processes forming the composition of groundwater in natural-technogenic structures are incongruent weathering of silicate rocks, ion exchange, and secondary mineral formation. This study will provide the basis for the development of hydrochemical predictions and measures to minimize the impact on the environment in coal mine areas.