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technology that seeks to exploit coal reserves through gasification of in situ coal and extract a synthetic gas that can be used for electricity generation (Burton, Friedmann, and Upadhye, 2006). This is achieved by injecting oxidants through boreholes into the coal seams to induce gasification. The resultant synthetic gas (methane, hydrogen, and carbon monoxide) is piped to the surface via production wells, as seen in Figure 1. The UCG process offers some environmentally friendly outcomes such as no process tailings, reduced sulphur emissions, and low discharge of ash, mercury, and tar. The UCG cavity is, however, a source of gaseous and liquid pollutants (Liu et al., 2007). Since the UCG process occurs in a natural environment, this raises concern about the impacts on the regional groundwater system. The by-products of gasification can react with the surrounding strata or be dissolved in groundwater (Krzysztof and Krzysztof, 2014). However, this is unlikely to occur during gasification as the pressure in the cavity and the connected gas-filled voids must be kept below the hydrostatic pressure of the aquifer. This ensures that contaminants are always contained in the gasifier, as groundwater flows towards the cavity. A groundwater sink hence develops in the cavity as the gasifier consumes groundwater through evaporation, chemical reactions, and as part of the syngas in production wells. Stratification is the vertical distribution of salinity, pH, and temperature of groundwater in a stepwise or layered manner (Ryuh et al., 2017). Stratification within an underground cavity associated with coal mining is common in the Karoo coal-bearing formations (Johnstone, Dennis, and McGeorge, 2013). UGC creates an underground cavity as a result of coal being gasified in situ. Groundwater is an important input in the gasification process as water in the gaseous phase takes part in various chemical reactions to produce hydrogen gas, which forms part of the synthetic gas product. Upon completion of the gasification process, groundwater levels are expected to rebound in the gasification zone and the groundwater flow to resume. The geochemical evolution of the UCG cavity will proceed as a result of interactions between groundwater and the various residue products in the cavity, including ash, unburned coal, heat-affected surrounding strata, and hydrocarbons. Assessment of stratification in the UCG cavity is important as it may point to chemical processes such as diffusion, which may influence the evolution of contaminants. Johnstone, Dennis, and McGeorge (2013) reported stratification in cavities in coal mines at Ermelo, Mpumalanga Province, which Temperature and electrical conductivity stratification in the underground coal gasification zone and surrounding aquifers at the Majuba pilot plant