Development of the New Castle, Utah geothermal resource has proceeded slowly since its discovery in 1975. In 1993, a third commercial greenhouse operator completed construction of a 37,161 square meter (400,000 square ft) greenhouse and drilled production and injection wells. The company plans to expand their facility, possibly enclosing more than 130,000 square meters (1,400,000 square ft), in the near future. Future demands on the geothermal aquifer could stress the long term productivity of the resource. Monthly temperature monitoring, in available observation holes during the 1993-94 heating season, shows small but measurable temperature changes in the thermal aquifer. Detailed geohydrologic information, including monitored flow tests, is necessary to complete a model of coupled fluid-flow and heat transfer. Such a model is needed to guide development of the thermal aquifer and to insure the long-term, sustainable use of the New Castle geothermal system