Control strategies for abandoned in situ oil shale retorts

In situ oil shale retorting may result in a number of environmental impacts including degradation of local surface and groundwaters, low resource recovery and subsidence. The target of present oil shale commercialization activities is the Mahogany zone in Colorado's Piceance Creek Basin. The principal oil shale resource in this area is surrounded by two confined aquifers. During mining and retorting, these aquifers are dewatered. When the site is abandoned, groundwater will reinvade the area and flow through the abandoned retorts, leaching potentially toxic or carcinogenic materials from the spent oil shale. This material may then be transported in local aquifers, withdrawn in wells or discharged into the Colorado River system as base flow. Certain control technologies appear potentially able to protect groundwater quality at reasonable cost. These include designing retort blocks to include a hydraulic bypass around abandoned retorts (about $0.50/bbl), placing adsorbent clays in abandoned retorts to catch and hold leachable matter (about $0.50/bbl), collecting leachate and treating It on the surface (about $1.20/bbl), protecting abandoned retorts from leaching by placing a grout curtain around a block of abandoned retorts (about $2.00 to $3.00/bbl), or grouting abandoned retorts with spent, shale (about $3 to $4/bbl).