Rio Blanco Oil Shale Company has considered the modified in-situ process as one of the alternatives for extracting oil form oil shale at Tract C-a. one of the problems to be solved for this approach is the environmental consequence of underground retort abandonment. This includes possible contamination of groundwater as the natural groundwater regime is re-established, aerial subsidence, aquifer mixing and underground disposal of surface-retorted materials. This paper outlines the four-phase program under contract to the Department of the Interior, Bureau of Mines, which is being used to address the problem and details the laboratory and pilot scale testing.