Evaluation of changes in groundwater quality caused by a water inrush event in Taoyuan coal mine, China

The effects of a water inrush event on the chemical composition of groundwater in a coal measures sandstone aquifer that is important for water supply was investigated at the Taoyuan coal mine in the Huaibei region in China. Changes in groundwater quality caused by the inrush event were investigated using statistical techniques, a Piper trilinear diagram, saturation index, ion combination ratios and a fuzzy comprehensive evaluation method. These investigations indicated that Na⁺ + K⁺ and SO₄²⁻ were the main cation and anion, and the average contents were 272 mg/L and 732 mg/L before water inrush, and 394 mg/L and 779 mg/L after water inrush, respectively. The concentrations of anions in groundwater before the inrush event were in the order SO₄²⁻ > HCO₃⁻ > Cl⁻ and the cation concentrations were in the order Na⁺ + K⁺ > Ca²⁺ > Mg²⁺. The chemical composition of groundwater was mainly SO₄·Cl–Ca·Mg before water inrush event, and SO₄·Cl–Na and Na–HCO₃–Cl after this event. According to the saturation index method, gypsum, calcite, magnesite, and aragonite controlling the chemical composition of groundwater were reactive minerals in groundwater system. The oxidation of pyrite, dissolution of sulfate and carbonate minerals were main geochemical process before and after the event; while the oxidation of pyrite and cation exchange were enhanced after the water inrush event. The fuzzy comprehensive evaluation results showed that the type V water accounted for a large proportion of poor-quality groundwater. Groundwater quality in the aquifer near the mine generally deteriorated after the water inrush event.