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Figure 1 shows the number of coal mines in China over the past 20 years from 1998 to 2018. The number of coal mines decreased from 80,000 in 1998 to 5,800 in 2018, a reduction of approximately 92%. Closure of the mines resulted from exhaustion of coal resources in old mining districts and integrated resources management by strategically abandoning smaller coal mines. The trend of coal mine closure is anticipated to continue in China in the near future. Such massive mine closure is not limited to China but has also been experienced in many other countries including the Unites States, United Kingdom, and South Africa (Aljoe 1994; Younger 2000; Younger et al. 2006; Wolkersdorfer 2006; Li et al. 2014). Where mining of coal extends below the groundwater table or under potentiometric pressure, sustained dewatering is necessary throughout the life of the mine to prevent flooding of active parts of the workings (Younger & Thorn 2006). As a consequence, the natural hydro geological conditions are disrupted, and the local and regional hydro geological regime is controlled to a large extent by the dewatering. In many coal mines or coalfields with multiple mines, the scale of hydro geological disruption can be many hundreds or thousands of square kilometers. Coal mine closure usually comes with discontinuation of mine dewatering, which results in an increase in the groundwater level. The groundwater level rise floods the underground voids generated due to mining activities or flowing back into the formations surrounding the mine. This flooding process is often referred to as groundwater rebound. Predicting the groundwater rebound in response to mine closure is important because the rebound leads to the following hydro geological consequences pertinent to environmental impacts, resource conservation, and hazard risk management. Abstract: Groundwater rebound occurs after closure of coal mines that are below groundwater levels and cessation of dewatering. While a number of modeling approaches have been developed in recent years to predict such rebounds and facilitate post-closure proactive management, the lack of detailed input data and complex mine void structures make these modeling approaches less effective. Traditional Darcian-flow based groundwater flow models do not provide the correct simulation of multi-porosity flow with preferential flow along mine voids including mined-out areas, roadways, and shafts. Models that account for the non-Darcian flow in mine voids include black-box model, pipe-network model, and pipe-pond models. Depending on volume of mine voids and their connections, these models can predict the general trends in groundwater rebound and water quality. They can help determine the general timeframes to approach the full recovery. In particular, the black-box approach, which does not require detailed geometric and hydraulic data of mine voids, takes advantage of the routine mining records on mining extent and dewatering activity. Examples are presented to show that the predictions can provide useful information for more sophisticated modeling of groundwater flow dynamics after mine closure. The hydrogeological insights into the mine void system that are gained from such exercises are also helpful in future re-use of flooded voids as underground reservoirs or resources for low-carbon heating systems.

Approaches of Applying Non-Darcian Flow Equations to Predicting Groundwater Rebound in Response to Coal Mine Closure