Migration of Co, Cd, Cu, Pb to the groundwater in the area of underground coal gasification experiment in a shallow coal seam in the experimental mine 'Barbara' in Poland

A numerical model of inorganic pollutants migration in the area of underground coal gasification (UCG) reactor in the post-operational phase was constructed in this study. Also the deceleration of migration and the shape of the contaminant plume in individual geological formations of the reactor surroundings, resulting from different values of sorption affinity, were compared. The simulations were carried out for four selected metals (Co, Cd, Cu, Pb), which in relation to the raw coal and post-process char showed different sorption properties. The sorption parameters of the tested systems were determined experimentally on the basis of the Freundlich isotherm equation. The retardation coefficients were calculated and the degree of sorption intensity in hydrogeological media was classified from II (in the cobalt-char and metals-coal systems) to the IV class of sorption intensity (in the copper-char system). The modeling was carried out using the finite element method (FEM) in the COMSOL Multiphysics computing environment in a two-dimensional plane. The simulation of pollutant migration was carried out for the transient state, with the assumed time horizon up to 20 years after the termination of the UCG experiment. The bottom of the cavity formed due to the coal gasification in situ (10 m x 20 m ellipse) was assumed as the source of contamination and the spread of metals in a horizontal plane 200 m long and 200 m wide was analyzed. To compare the extent to which sorption phenomena affect the spread of pollutants, an additional model was constructed, taking into account only the advection-dispersion movement of the contaminant plume. The consequence of sorption interactions for the tested systems was the delay of the contaminant plume migration depending on time since the termination of the gasification process and depending on the distance from the source of pollution. Due to high sorption affinities, the post-process char acts as an effective buffer for inorganic pollutants and slows down the migration of analyzed metals even 20 years after UCG experiment. The greatest slowdown in migration in relation to the velocity of groundwater flow as a result of sorption occurred for copper transport in the char layer.

Automated summary

In this study, a numerical model of inorganic pollutant migration in the area of an underground coal gasification (UCG) reactor was constructed. The study compared the effect of different sorption affinities on the deceleration and shape of the contaminant plume in geological formations. The results showed that sorption parameters, time, and distance play important roles in pollutant migration.