CHANGES BETWEEN THE CONFINED AND UNCONFINED AQUIFER STATES AS A MECHANISM OF THE HYDROGEOLOGICAL EFFECTS OF MINE SUBSIDENCE

It is well established that the potentiometric levels in aquifers above longwall coal mines decline considerably due to the effects of underground mine subsidence, and also often recover within a period of months to a few years after mining has ceased. Commonly, the potentiometric level drops rapidly from a confined to an unconfined condition. This change has been under-appreciated and little explained, but is significant both hydraulically and geochemically. The rapidity of the initial drop to a threshold confined/unconfined condition is due to the low value of storativity for confined aquifers operating during the loss of water into the newly opened fracture space. In the unconfined state, further decline of the heads during fracture dilation becomes constrained both by the higher unconfined storage coefficient, and by limitation on air entering the aquifer pore spaces. Observation of the true hydraulic state is difficult because piezometers and wells are themselves open conduits from the atmosphere to the aquifer. The unconfined hydraulic response is further complicated by delayed drainage of water from the intergranular matrix pore spaces into the fractures. The post-mining recovery similarly occurs in a two-stage unconfined to confined sequence. Geochemically, aquifers in the newly unconfined state are now exposed to anomalous aerobic conditions. This can cause oxidation of in situ minerals, particularly of iron sulfides to soluble sulfates, which are then mobilized during water-level recovery, causing a deterioration of groundwater quality. These processes and results of the confined-unconfined state changes are illustrated with case studies, particularly from a seven-year study of longwall mining in Illinois, USA.