Groundwater influencing mechanism in water-ponding process of Huainan coal mining subsidence area

Ground subsidence caused by coal mining activity commonly exists in the coal belt of China eastern plain area. Subsidence area is on average 0.18-0.33 hm2 per ten thousand tons of coal mining. The subsidence coefficient there is about 0.8-0.9, which means coal mining of one cubic meters result in ground subsidence of 0.8-0.9 m3. In the environment with high groundwater level, the situation is worse, for once the ground sinks due to coal mining, large area of water-ponding occurs. Due to limited source of backfill material, high cost, the long time needed to wait for the stable state of subsidence and other factors, traditional land remediation pattern for coal mining subsidence in China faces difficulties, and land reclamation rate by now is only 12%. From other point of view, coal mining subsidence area is potentially useful water storage structure, and recently some researchers start to study a new remediation pattern of using them for water resource development. However, a prerequisite for the new pattern is to learn where the water comes from. Related to the environment background with high groundwater level, it seems that water stored in coal mining subsidence areas may mainly come from groundwater, but by now this viewpoint has not been proved in detail yet. The article researched the law of water-ponding process in coal mining subsidence area by means of hydrological reasoning and water cycle simulation. In this paper, Huainan coal mining area is taken as study area, which is the largest coal production base of East China area with its coal reserves accounting for 74% of Anhui Province, and 50% of East China area. According to statistical data in 2010, after long-term coal mining activity of dozens of years, the maximum subsidence depth of Huainan coal mining area has reached 7.6m, and the ground subsidence area has reached 108.3 km2, along with the total subsidence volume of 0.314?109 m3. Some meaningful conclusions are drawn through our study. Firstly, the reason that the ground subsidence area is easily to be ponded with a large area of water is due to the close volumes of annual precipitation and evaporation in Huainan district. Water balance analysis reveals that even though there is no groundwater replenishment, an isolated subsidence area there still can reach a large water-ponding area ratio of about 71% in normal year due to the supply of precipitation falling on water-ponding area and runoff yielded from non-ponding area of the isolated subsidence area. And hydrological reasoning indicates that the higher the ratio of precipitation to evaporation, the higher the ratio of water-ponding area to subsidence area. Secondly, proved by both hydrological reasoning and water cycle simulation, due to the flat terrain in Huaibei plain area where the Huainan coal mining subsidence areas are located, the lateral flow of groundwater is relatively weak for lack of gradient driving force, and the ratio of groundwater supply in ponding water of coal mining subsidence area can't be too high. As an example of a named Gubeiguqiao subsidence area in our study, the ratio of ground water supply in its ponded water may only account for 4.0%, however, precipitation falling on its water-ponding area accounts for 86.6% and runoff yielded from non-ponding area within its subsidence area accounts for 9.4%. Thirdly, transformation between groundwater and ponded water in coal mining subsidence area depends on the water level difference of groundwater and ponded water, rather than the one-sided groundwater level. Hydrological simulation reveals that there are obvious stage characteristics between the transformation of groundwater and ponded water in a hydrological year. The law is that water quantity leaking from ponded water to groundwater is more than that recharging from the groundwater to ponded water in the flood season, and in non-flood season the situation is in the opposite direction