Baseline geochemical study of the Aberpergwm mining site in the South Wales Coalfield

Abstract

This paper presents the baseline geochemistry and hydrogeology of a site in the South Wales Coalfield. Three deep exploration boreholes were drilled up to 650 m for that purpose and investigated together with the surrounding surface environment. The present study reports (i) General geological description, mineralogy and chemical composition (ii) Geochemistry of the coal seam water, surface waters and water from a shallow water table (iii) Periodic monitoring of temperature, water level and (iv) Adsorbed methane on rock samples and free gas concentrations acquired from three exploration boreholes. Rock specimens from the deep (425 m to 656 m) contained a minor amount of clay minerals such as clinochlore and kaolinite. Along with the major sandstone constitutions such as quartz and feldspar, rutile was also found in minor quantities. The trace elements such as arsenic, barium and thorium were found in various depths. The water temperature was stable around 13.5 °C (at 240 m) in Borehole 1, 15.6 °C (at 255 m) in Borehole 2 and 18.1 °C (at 453 m) in Borehole 3 throughout the monitoring period. The thermal profiling indicated the temperature gradient of 1.5 °C/100 m with maximum water temperatures at 22 to 24 °C which shows the thermal energy potential of the site. The low water temperature gradient in deep strata below 580 m, where the coal seams were encountered, indicated fluid movement in the deep coal seam aquifers. The gas analysis results showed the availability of the methane in the coal seams, and the water chemistry from the boreholes also exhibited the similar characteristics of the methane-producing wells around the world. The water chemistry of the shallow aquifers at the site was different from the water chemistry of the aquifers near the coal seam. The water quality measurements demonstrated the surface water quality was not affected by the drilling operations. Overall, this paper provides insights into the geochemistry and hydrogeology of the South Wales Coalfield that represents the environmental baseline data for any future industrial exploration or development activities.