The DF 66-21 well is located in the valley about 2 km southeast of an unnamed fumarole group near the south margin of the Dixie Valley Power Partners geothermal lease. “The wellhead is about 150 m southeast of the main road in Dixie Valley. The well was drilled in 1979 to a depth of 2988 m (9,800 ft), penetrating roughly 1250 m of basin-fill deposits, 335 m of Tertiary volcanic rocks, 870 m of primarily granodiorite, and 534 m of primarily metasediments. The lower portion of the metasedimentary section contains gabbro/diorite intrusive bodies. According to S. Johnson (oral communication, 1997), the hole bottoms in the ophiolitic rocks. An obvious water entry occurs at 1463 m depth but this entry was cased off. A temperature log obtained in September 1979 indicates a rather linear conductive gradient, with a temperature of about 150°C at 2470 m (Mackay Minerals Research Institute, 1980). When sampled in 1997 and 1998, the well was producing a small amount of water (4 to 7 l/min) at 55 to 57°C (Goff et al., 2002). No free gas was observed at the wellhead, although the present configuration does not allow for clear observation of gas emissions.

Water from DF 66-21 has relatively moderate to high contents of SiO2, As, B, Br, and Li, and the Cl concentration is about 1460 ppm. This Cl level is nearly three times higher than Cl in the Dixie Valley Production Field production waters and is the most saline groundwater (other than Humboldt Salt Marsh) that Goff and others encountered in Dixie Valley. The water has moderate Ca but low Mg concentrations. Although not part of the Dixie reservoir (Figures 2.3.9-2.3.13), DF 66-21 water has the general characteristics of high-temperature geothermal fluids. Chemical geothermometers suggest that DF 66-21 water has equilibrated at temperatures of about 210°C (Goff et al., 2002), indicating that the water comes from an entry near the bottom of the hole. Bottom hole temperature in the Dixie Valley Power Partners wells just to the northeast of DF 66-21 is over 265°C.” (Blackwell et al., 2006 (in review))