Confined-unconfined changes above longwall coal mining due to increases in fracture porosity

Journal Article · Thu Nov 15 04:00:00 EST 2007 · Environmental and Engineering Geoscience

DOI:https://doi.org/10.2113/gseegeosci.13.4.355· OSTI ID:21004704

Booth, C J ^[1]

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1. North Illinois University, De Kalb, IL (USA). Dept. of Geology & Environmental Geoscience

Subsidence and strata movement above longwall (total extraction) coal mines produce complex hydrologic responses that can occur independently of drainage to the mine. One response is dewatering from confined to unconfined conditions in bedrock aquifers as a result of loss of water into new void space created by fracture and bedding separations. This dewatering process has been little studied but accounts for several hydraulic and geochemical effects of longwall mining. This article presents a conceptual model of the process and reviews evidence from case studies. Confined bedrock aquifers in subsiding zones exhibit dramatically steep head drops because of the low value of confined storage coefficients relative to the volume of water drained into the new fracture void space. The aquifer changes rapidly to an unconfined condition. Tight units to which air entry is restricted may even develop negative water pressures. In the unconfined state, sulfide minerals present in the strata readily oxidize to soluble hydrated sulfates. When the aquifer re-saturates, these salts are rapidly mobilized and produce a flush of increased sulfate and total dissolved solids (TDS) levels. Observations made in our previous studies in Illinois are consistent with the confined-unconfined model and include rapid head drops, changes to unconfined conditions, and increases in sulfate and TDS during re-saturation of a sandstone aquifer. Studies reported from the Appalachian coalfield show aspects consistent with the model, but in this high-relief fractured setting it is often difficult to distinguish aquifers from aquitards, confined from unconfined states, and the fracture-porosity cause of head drops from several others that occur during mine subsidence.

• OSTI ID: 21004704
• Journal Information: Environmental and Engineering Geoscience, Journal Name: Environmental and Engineering Geoscience Journal Issue: 4 Vol. 13; ISSN EEGEFJ; ISSN 1078-7275
• Country of Publication: United States
• Language: English

Related Subjects

01 COAL, LIGNITE, AND PEAT
AQUIFERS
COAL MINING
DRAINAGE
GEOLOGIC FRACTURES
GROUND SUBSIDENCE
HYDROLOGY
LONGWALL MINING
STRATA MOVEMENT
SULFATES