A research program was initiated to study the potential ground-water chemical changes resulting from in situ retorting of oil shale. This report covers a study of the ground-water conditions before and after retorting at the Laramie Energy Technology Center Site 12 near Rock Springs, Wyoming. Wells were drilled, cored, and sampled around the burn site. X-ray diffraction, x-ray fluorescence, acid-soluble fraction analysis, and optical microscopy were used to establish the pre-burn mineralogy and chemistry of the core. Standard field and laboratory analytical methods were used to determine the pre- and post-burn water compositions. In addition, a sixfold fractionation of dissolved organic compounds was made using differential absorption onto macro reticular resins. Factor analysis was used as an aid in the interpretation of the data. Two aquifers were recognized at Site 12 -- the upper aquifer at 70-95 feet and the lower aquifer at 218-246+ feet. The upper aquifer is unconfined and has a gradient of 5.4% to the southeast. The lower aquifer is confined and has a gradient of 17.9% to the northwest. Water movement in both aquifers is largely fracture controlled, and the regional rate of movement is estimated to be on the order of a few feet per hundred years. A typical lithologic sequence consists of calcareous siltstones/claystones interbedded with dolomitic limestones to calc-dolostones and tuffaceous rocks. The major mineral phases include dolomite, quartz, plagioclase, analcime, calcite, potash feldspar, mica-illite, and montmorillonite with minor pyrite and traces of gypsum and anhydrite. The tuffaceous rocks exhibited moderate porosity. The two aquifers were distinguished by water chemistry. The upper aquifer was characterized by major concentrations of HC03 and S04 with minor CI and C03 , and the lower aquifer was characterized by major CI and HC03 with minor S04 and C03 . As the pre-burn fracture was approached, the Na-CI content of the lower aquifer increased, probably as a result of contamination from a deeper aquifer exposed by the explosive fracturing. Post-burn waters from the two aquifers were indistinguishable from the respective pre-burn waters. Since movement of oil shale water is vertically restricted by the nature of the upper and lower aquifers and since the horizontal regional rate of ground-water movement is very slow, mixing of the oil shale water with the upper and lower aquifer waters is unlikely. This report was submitted in fulfillment of Grant No. R-804162 by the Colorado School of Mines Research Institute and Engineering Enterprises, Inc.,under the sponsorship of the U.S. Environmental Protection Agency.