Multivariate statistical analysis of water chemistry in evaluating groundwater geochemical evolution and aquifer connectivity near a large coal mine, Anhui, China

Major ion chemistry of groundwater from main aquifers near the Dingi Coal Mine (Anhui, China) was analyzed with a suite of statistical techniques, in an effort to understand aquifer connectivity and groundwater geochemical evolution. Fifty-seven groundwater samples were analyzed from four aquifers: the Cenozoic top, middle, bottom aquifers and a Permian coal strata aquifer. With increasing depth of the aquifers, groundwater became more mineralized showing greater hardness and salinity. The dissolutions of halite and sylvite, carbonate, sulfate, and silicate minerals were the primary processes controlling groundwater chemical compositions in the Cenozoic bottom and coal strata aquifers, while (K+ + Na+) and HCO3- originating from silicate mineral weathering were more enriched in the coal strata aquifer. Principal component analysis (PCA) identified two principal components, with the first component representing hardness variation and accounting for 57.5 % of total variance, and the second component (23.5 % of total variance) controlled primarily by salinity and sulfate reduction process. The Cenozoic bottom aquifer had greater hardness than other aquifers whereas the coal strata aquifer was characterized by higher salinity and larger influences of sulfate reduction. Cluster analysis revealed that the coal strata aquifer was largely isolated from the Cenozoic aquifers with possible infiltration at discrete locations, indicating that the high-yield, thick Cenozoic aquifers may serve as a potential additional source for groundwater inrush into coal mines.