Abstract

MOKHAHLANE, L.S.; MATHOHO, G.; GOMO, M.  and  VERMEULEN, D.. Qualitative hydrogeological assessment of vertical connectivity in aquifers surrounding an underground coal gasification site. J. S. Afr. Inst. Min. Metall. [online]. 2018, vol.118, n.10, pp.1047-1052. ISSN 2411-9717.  http://dx.doi.org/10.17159/2411-9717/2018/v118n10a5.

Underground coal gasification (UCG) is the conversion of coal in situ into a usable synthetic gas. One of the major environmental concerns with UCG is the possibility of groundwater from the coal seam aquifer contaminating the shallow aquifers via hydraulic connections. The coal seam aquifers are usually confined aquifers but can have hydraulic connections to the shallow aquifers due to faults/fractures or any man-made connections, including boreholes. The aim of this paper is to study groundwater hydraulic connections across various aquifers at the UCG site at Majuba, using hydrochemistry and stable isotope (6¹⁸O and 6²H) tools. Physical and chemical processes such as diffusion and condensation generate isotopic differentiation in natural waters that can be used to deduce the origins of different waters, and in groundwater the spatial isotopic distribution can be used to deduce hydraulic connections between different aquifers. The Majuba UCG site consists of shallow, intermediate, and saline deep aquifer systems at a depth of 70 m, 180 m, and 300 m respectively. Samples were taken from each aquifer system together with supplementary samples from an on-site water storage dam. The analyses of isotopic compositions led to the determination of the possible sources of each sample. The deep aquifer is represented by an isotopic signature that is depleted in heavy isotopes with average values of -41.7% and -7.02% for 8¹⁸O and 6²H respectively, while the shallow aquifer is enriched with corresponding average values of -19.9% and -3.3%. Hydrochemical data also showed different water types: a sodium chloride type in the deep aquifer and a sodium bicarbonate water in the shallow aquifer. The results indicate that the shallow aquifer and the deep aquifer are not hydraulically connected, and therefore it is unlikely that groundwater from the gasification zone would contaminate the shallow aquifer.

Keywords : underground coal gasification; hydraulic connectivity; stable isotopes; hydrochemistry; Majuba.

        · text in English     · English ( pdf )