Seasonal thermal energy storage (STES) involves storing thermal energy, such as winter chill, summer heat, and industrial waste heat, for future use in heating and cooling buildings or for industrial processes. Widespread development and implementation of STES would significantly reduce the need to generate primary energy in the United States. Data indicate that STES is technically suitable for providing 5 to 104 of the nation's energy, with major contributions in the commercial and industrial sectors and in district heating and cooling applications. Aquifer thermal energy storage (ATES) is predicted to be the most costeffective technology for seasonal storage of low-grade thermal energy. Approximately 609 of the United States is underlain by aquifers that are potentially suitable for underground energy storage. ATES has the potential to substantially reduce energy consumption and electrical demand. However, the geohydrologic environment that the system will use is a major element in system design and operation, and this environment must be characterized for development of efficient energy recovery. Under sponsorship of the U.S. Department of Energy (DOE), the Pacific Northwest Laboratory (PNL) manages DOE's STES Program and directs numerical modeling, laboratory studies, and field testing of ATES at several sites. PNL is operated by Battelle Memorial Institute for the Department of Energy under contract DE-ACO6-76RLO-1830. This report describes the results of the first long-term heat injection/ recovery cycle at the St. Paul (Minnesota) field test facility (FTF). The St. Paul FTF°operated by the University of Minnesota, is the principal U.S. facility for research on relatively high-temperature ATES. The primary objectives of investigations at the St. Paul FTF are to: I) evaluate the technical issues associated with design and oepration of a high-temperature (>IO0°C) ATES system and 2) obtain data on fundamental geotechnical processes to validate laboratory and bench-scale geochemical testing and geohydrothermal modeling.