The technology of permeable reactive barriers (PRB) is one of the most frequently developed methods for protecting soil and water from pollution. These barriers are zones filled with reactive material in which contaminants are immobilized and/or their concentration is reduced to the limit values during the flow of contaminated groundwater. This article presents a study on the efficiency of the removal of contaminants from the post-processing water from the underground coal gasification (UCG) process. The tests were carried out in a laboratory using a flow-through reactor design. The post-processing water came from a UCG experiment carried out in the Barbara mine, Mikołów, Poland. Activated coal, zeolite, and nano-iron were used as the reactive materials in the experiment. The obtained results were compared to tests carried out with reference water (artificial) with strictly defined characteristics. Research has shown that activated carbon is the most effective material used in the reaction zone for removing organic contaminants from groundwater generated during the coal conversion process. A new feature is the use of PRB in a georeactor zone during the UCG process to limit the potential risk of contamination spreading in the case of uncontrolled and unpredictable operation, in emergency situations related to gas leaks into the environment, during underground fires, and for water polluted by high-toxicity substances.