Groundwater flow in the South Wales coalfield: historical data informing 3D modelling

Abstract

The South Wales Coalfield is structurally and lithologically complex. Attempts to understand the regional groundwater flow system have been largely piecemeal, focusing on single collieries or groups of collieries. Inspection of both historical data, gathered during the active mine dewatering phase of the coalfield, and of contemporary post-mining groundwater rebound data, allows a picture of the whole groundwater flow system to be derived, along with the regional water balance. The assessment is aided by application of a 3D visualization model of the geological framework, which includes the piezometry during and after the mining phase and pumping and yield data. Other valuable historical information, which can no longer be replicated, are the analyses of samples of groundwater that entered working mines from specified horizons, rather than present-day analyses of acid mine drainage. It is found that there is little deep regional groundwater flow across the basin, for example towards the sea, and most groundwater flow is of river catchment scale draining to the main rivers that traverse the coalfield. This analysis of the coalfield and the 3D model provide a foundation for future investigation into issues such as dewatering of new opencast pits, in situ gasification or anthracite mining.

The application of the Intermittent SBAS (ISBAS) InSAR method to the South Wales Coalfield, UK

“…In fact east of a line drawn between Rhymney and Caerphilly there was a heightened risk of water inrush to the mine workings than in other areas within the coalfield. In this zone mine dewatering was able to reach high enough pressures that a burst could occur [27].…”

“…The eastern crop contains two subsidiary flexures, separated by the E-W Pontypridd Anticline, the narrow Llantwit-Caerphilly syncline in the south and the broad Gelligaer syncline to the north. Many NW-SE trending Variscan fault structures cross the basin and E-W trending low-angle thrust faults occur at the northern tip of the Variscan Trust Front in the south east Controlling high groundwater flows proved a constant engineering challenge, particularly in the east where fractured bedrock and deep valleys allow rapid recharge of the coal measures aquifer [27]. To counter this problem ingenious pumping schemes were developed and disused deep shafts and infrastructure were commonly converted to provide ventilation for connecting collieries resulting in interconnection of some deep mines, such as Bwllfa ( Figure 7).…”

“…Although the coalfield to the east of the Neath Disturbance is shallower than to the west, it is wetter; there was an increased need to pump ground water to prevent it affecting the mines [27]. The coalfield thins towards the east and the deep faulted valleys found in this area allow greater recharge of the shallower coal measures.…”

Low enthalpy heat recovery potential from coal mine discharges in the South Wales Coalfield

“…Major disturbances including the Tawe and Neath Valleys trend NE-SW, but the main fault pattern is NW-SE and WNW-ESE (Thomas, 1974). Coal mining in the South Wales valleys declined steadily after the general strike of 1926 with significant closures of coal mines in the 1980's and 1990's (Robins et al, 2008).…”

“…As mine water levels rebounded, new gravity fed discharges occurred (Robins et al, 2008) and to mitigate against impacts on the surface water environment the Coal Authority installed mine water treatment systems (Younger et al, 2004).…”