Characterization of migration of organic contaminants within the groundwater polluted by underground coal gasification

Abstract Underground coal gasification (UCG) is a promising technology, but the groundwater pollution caused by UCG is a potential risk to the environment. The measured results of the stratum in the combustion cavity resulting from UCG had proved that the combustion cavity would be filled with some UCG residues and caving rocks when UCG was finished. The pollutants in underground water around the combustion cavity include organic pollutants, inorganic pollutants, and ammonia nitrogen, and one of primary organic pollutants is phenol. The migration and diffusion characteristics of organic pollutant (taking phenol as representative) in the groundwater of the combustion cavity was investigated by breakthrough experiments and numerical simulations. The results show that the hydraulic conductivity of the coarse UCG residues is much than that of fine residues, and the hydraulic conductivity of the UCG residues with the size of -0.15 mm and 0.15–0.3 mm are 4.68×10− 6 m/s and 1.91×10− 4 m/s respectively. The dispersivity λ for the migration of organic pollutants will be influenced significantly by the size of UCG residues in fractures of the combustion cavity, while the distribution coefficient Kd will not. The dispersivity of organic pollutants in the fine UCG residues is more significant than that in the coarse residues, and the λ for the two kinds of residues are 3.868 and 1.765 cm, respectively. The shape of the migration path slightly affects the pollutant concentration distribution along the path, but the width of a path has a more pronounced influence on the concentration distribution. In this research, the influence was formulated by a new technical term, MPWIT related to transverse dispersion. Specifically, while the transverse dispersion values account for 20 and 10% of the longitudinal dispersion respectively the corresponding MPWIT values are 39.48 and 33.96 mm.