Analysis on the hydrochemical and sulfur isotope characteristics of the groundwater in cross-strata pollution control area of Hongshan and Zhaili coal mines in Zibo

Since the closure of Hongshan and Zhaili coal mines in Zibo City, Shandong Province, the pumping and drainage of groundwater has stopped, leading to the rise of water level in coal mines. Consequently, Ordovician limestone water was polluted to different degrees through the hydraulic connection channels such as broken well pipes. The main pollution factors, like ${\rm{SO}}_4^{2-}$ , TDS, total hardness, etc., resulted in the deterioration of karst water below the drinking standard in the study area, and hence seriously affected local residents’ living and economic activities. The main water-bearing strata in the study area include loose rock pore aquifers, clastic rock fissure aquifers or interlayer karst fissure aquifers and carbonate rock karst aquifers. As the main coal measure aquifer in the study area, the fissure aquifer is the direct water source of coal mining. In the natural state, there lacks hydraulic connection between the fissure aquifer and the underlying Ordovician karst aquifer. But because the impermeable layer was damaged by coal mining, the water channel has been formed. During the mining process, the fissure water in the coal measure strata was basically drained, and the Ordovician limestone water entered the pit by the way of jacking recharge. After the coal mine was closed, the level of the coal mine water rose, and the Ordovician limestone water was replenished through the connecting place.

In order to provide a scientific basis for future remediation of groundwater pollution in Hongshan and Zhaili coal mine areas and other similar mining areas, this study was carried out on the hydrochemical and isotope characteristics of the water after treatment in the study area, and a qualitative and quantitative analysis was conducted on the quality characteristics and treatment effects of the groundwater. To conduct the hydrochemical and isotope analyses based on hydrogeological conditions, characteristics of groundwater flowing field and sampling points before treatment, a series of monitoring points were set up along the groundwater flowing direction from the upstream of the pollution source area. Sampling types include coal mine water, Ordovician limestone water, rain water, surface water, etc. Coal mine water includes mine water, goaf water and mine drainage.

Through the sampling test, the results of hydrochemical and isotope analyses show that there is a hydraulic relation and mutual influence between Ordovician limestone water and coal mine water in the study area. The sulfate in groundwater in this area mainly comes from the oxidation of sulfide minerals in coal-bearing strata, and the Ordovician limestone water is polluted by the coal mine water in cross-strata, which leads to the increase of sulfate concentration in Ordovician limestone water.

Results show that the hydrochemical type of Ordovician limestone water is complex, and some Ordovician limestone water is characterized by high ${\rm{SO}}_4^{2-}$ concentration, high hardness and high TDS. The concentration range of high ${\rm{SO}}_4^{2-}$ is basically consistent with the coal mine area and its downstream. The normal Ordovician limestone water environment in the area has been disturbed, because of the mixture of extraneous water. The hydrogen and oxygen isotopic compositions of some Ordovician limestone water and coal mine water are similar, and δS values present positive and negative deviations. According to the characteristics of sulfates and sulfur isotopes, the sulfates in Ordovician limestone water mainly come from the cross-strata pollution of coal mine water, and the pollution is more serious than it is before treatment. It is speculated that there is still a hydraulic connection between coal mine water and Ordovician limestone water. Therefore, further treatment is suggested to identify and control the water channel, strengthen the pumping and drainage of coal mine water and encourage the comprehensive utilization. Besides, the water level of mine pit should be controlled to avoid the pollution of Ordovician limestone water.