No 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.

A review of mining‐engineering concepts and studies in mine hydrology suggests a conceptual model linking the strata deformation, hydraulic property changes, and ground‐water impacts due to underground coal mining. The pressure‐arch deformation pattern about a small opening creates a local zone of increased permeabilities and dewatering in the seam and immediate roof, but should not hydraulically affect shallower aquifers. Networks of supported headings, rooms, and pillars intensely drain lower aquifers but only slightly affect higher strata except in areas of naturally high permeability. Longwall mining causes extensive, high‐reaching, well‐defined zones of stress, fracturing, and hydraulic impact, the maximum permeability increases being in the tensile zones immediately above the panel and at the sides of the subsidence trough. In shallow aquifers, permeabilities and ground‐water velocities increase, and hydraulic gradients decline independently of mine drainage. A study of a deep coal mine in the Appalachian Plateau, Pennsylvania indicated: probable hydraulic connections between the mine and shallow aquifers in a principal valley area; no obvious response of water levels in shallow aquifers to undermining by supported headings; and rapid, considerable declines in such water levels in response to nearby longwall mining. These results are consistent with the conceptual model.

Longwall underground coal mining produces major changes in the hydraulic properties and groundwater levels of overlying shallow aquifers because of the fracturing associated with mine subsidence. Many aspects of these impacts remain unclear and unpredictable, particularly the variations in response between different sites. Our detailed, long-term studies of subsidence strains and hydraulic responses at sites in Illinois address this and other aspects of the conceptual model. At a study site in Saline County, inconsistent permeability changes and large rapid head drops, without significant recovery, were observed in a low transmissivity sandstone over an active longwall panel. Corresponding head changes in overlying glacial drift units reflected localized drainage from the drift to the bedrock. In comparison to our earlier study at a site in Jefferson County, Illinois, in which full recovery occurred in a moderately transmissive sandstone aquifer affected by longwall mining, the Saline site illustrates the importance of local hydrogeologic characteristics, particularly variations in transmisivity and continuity with recharge sources, in controlling site-specific responses to longwall mining.

This study examined the response of potentiometric levels and hydraulic properties to subsidence caused by a 725‐ft‐deep active longwall mine in southern Illinois. The overburden is mainly shale but includes a shallow sandstone aquifer, overlain by a shale aquitard, capped by thin drift. Pumping and packer tests indicate that subsidence fracturing increased the hydraulic conductivities of the sandstone aquifer by about an order of magnitude, and of certain horizons of the lower bedrock by several orders. The water table in the drift aquifer was unaffected by mining; however, heads in the bedrock units dropped sharply in response to subsidence, probably because tensional dilation of fractures increased storativities. The sandstone aquifer also displayed gradual potentiometric declines ahead of mining, and a rapid partial recovery afterwards. Depression of water levels and potentially higher well yields represent conflicting negative and positive aspects of the hydrologic impact of longwall mining.