Underground coal gasification (UCG) experiments conducted during the 1970s at the Department of Energy (DOE) site near Hanna, Wyoming, formed six underground cavities in the Hanna No. 1 coal seam, an aquifer of low permeability. When the first Hanna UCG experiment began in March 1973, researchers had little information about what effects the geologic or hydrologic characteristics of the area might have on the UCG process; likewise, the effects of UCG on the environment were unknown. Since the UCG experiments were completed, dilute concentrations of pyrolysis products and leachates have been detected in groundwater monitoring wells in and near some of the six cavities. Three primary UCG indicator constituents have been measured at elevated concentrations: phenols, TDS, and sulfate. The Hanna III cavity water exceeded the DOE target level for TDS and sulfate, and the Hanna I cavity water exceeded the DOE target level for phenols. The indicated phenols contamination, however, was in groundwater sampled from a well which was previously used as a production well during the experiment. Water pumped during the restoration project and a new well located approximately 10 ft from the old production well was sampled and no elevated phenols concentration was detected. Therefore, the restoration performed on the Hanna I cavity water was not necessary. The restoration was performed, however, because these indications were not available until during the restoration. Locally, various other constituents exceed DOE target levels, but concentrations are very near target levels and are well within livestock use limits. 2 refs., 7 figs., 5 tabs.