AbstractShallow low‐to‐moderate temperature geothermal waters are found in the upper part of the Napa Valley near the town of Calistoga, California. The geology of the upper Napa Valley is dominated by Jurassic to Cretaceous eugosynclinal Franciscan assemblage and the late Tertiary (Pliocene) Sonoma Volcanic rocks. Minor post‐Pliocene normal faulting is evident in the volcanic rocks north and southeast of the study area. The thermal waters which have a sodium chloride chemistry with elevated concentrations of chloride, boron, and fluoride, are attributed to fault‐controlled hydrothermal circulation. The cold waters are characterized as mixed‐cation bicarbonate type with high concentrations of calcium, magnesium, and iron. The geothermal aquifer is believed to be recharged by the upwelling of thermal water along a linear fault zone aligned with the topographic axis of the Napa Valley. Data from 140 domestic water wells were used to evaluate the chemical character of the geothermal resource and to construct maps of chloride and boron concentrations. These maps were subsequently used to locate the source of the deep thermal water and zones of cold and hot water mixing, and to develop a conceptual model of the geothermal system.