The Lawrence Livermore Laboratory has conducted 2 underground coal gasification experiments at its Hoe Creek Site in NE Wyoming. Environmental studies in conjunction with these experiments, including an investigation of changes in local ground-water quality were conducted. Such changes are a consequence of the residual reaction products - coal ash, char, some of the coal tars, and roughly 10% of the product gases - which remain underground after gasification. When ground water returns to the gasification zone, leaching and dissolution processes lead to the formation of a plume of contaminated ground water, which begins to move through the coal seam. In addition, fissuring and roof collapse may destroy the integrity of the underground reaction vessel permitting contaminants to escape to the surface or into overlying aquifers. The field program is augmented by laboratory measurements and modeling studies. 10 references.