The effects of longwall coal mining on overlying aquifers

Abstract

The hydrogeological effects of longwall mines are vertically zoned. The heavily fractured strata immediately above the mine dewater, but they are typically overlain by a zone of low permeability that prevents shallower aquifers from draining to the mine. However, shallow bedrock aquifers experience head changes caused by fracturing during subsidence. New fracture void space takes up water, causing large head drops especially in confined aquifers. Increased fracture permeability affects heads because upper aquifers in high relief areas lose water through fractured aquitards to lower aquifers, and because the higher permeabilities lower hydraulic gradients and up-gradient heads, and increase downgradient discharge. In addition, a secondary drawdown spreads out laterally through transmissive aquifers from the potentiometric low in the subsiding zone. After undermining, water levels may recover due to closure of fractures and to recharge flowing back into the affected area. Studies at two active longwall mines in Pennsylvanian coal measures in Illinois support the conceptual model, with variations. Unconsolidated, unconfined aquifers were not significantly affected by mining. At one site, heads in a moderately transmissive sandstone declined due to mining but recovered fully afterwards. Increased permeability led to enhanced well yields, but water quality deteriorated, probably because of oxidation and mobilization of in situ sulphides during the unconfined and recovery phases. At the other site, heads in a poorly transmissive sandstone fell rapidly during subsidence and did not recover; hydrogeological responses varied at the site scale due to variations in bedrock-drift continuity. Predictions and monitoring schemes can be guided by the general conceptual model, but must consider local hydrogeological variations. Effects in shallow aquifers not in direct contact with the mine can be simulated using readily available flow models.

Grain Size Distribution Effect on the Hydraulic Properties of Disintegrated Coal Mixtures

“…The stress variation within the ground occurs from the seam level all the way up to the ground surface, this can lead to fractures and changes of the permeability of the adjacent ground zones by several orders of magnitude [42,49]. As a result the hydraulic properties of the overburden strata are changed, particularly near the excavated zones, and therefore high permeability pathways are frequently formed [49,50]; this can cause water influx within the ground. The same experimental setup was formerly applied to study the flow characteristics of disintegrated sedimentary rocks [4,41]; however, the former work did not take into account the effect of grain size distribution on the compaction and water flow properties of sedimentary coal samples, which is the main focus of this study.…”

Exploitation Contradictions Concerning Multi-Energy Resources among Coal, Gas, Oil, and Uranium: A Case Study in the Ordos Basin (Western North China Craton and Southern Side of Yinshan Mountains)

“…The coal mining face is not a settled working place but a moving place. To compensate for the space in which the coal is found, the overburden roof strata are disturbed in the order of severity from the immediate roof toward the surface [16,17]. In general, if the ratio between the coal occurrence depth and mining height is larger than 40 [18], then four zones of different fracture degrees can be formed along the vertical direction, namely, the caved zone, fractured zone, continuous bending and deformation zone, and soil zone [19][20][21], as illustrated in Figure 5.…”

Use of Modflow Drain Package for Simulating Inter-Basin Transfer of Groundwater in Abandoned Coal Mines

“…Use of MODFLOW to model groundwater flow has been considered inappropriate in some mined settings due to high fluid velocities, turbulent flow, and variably saturated media (Booth, 2002). Adams and Younger (2001) note problems with spatial and temporal discretization that make convergence of MODFLOW models in mined areas difficult.…”

“…al., 2011;Zaidel et. al., 2010) although MODFLOW limitations need to be considered prior to modeling in partially saturated or geologically complex mined environments (Booth, 2002;Adams and Younger, 2001). Extensive field data from the Elkhorn study are used to support assumptions in the development and calibration of the groundwater flow models.…”