The Soda Lake area is in the west-central part of the Carson Desert, 10 km northwest of Fallon (figure). It is a Holocene phreatic explosion feature related to shallow basaltic magmatism (Garside and Schilling, 1979). The thermal groundwater is mostly present in the central part of T20N, R28E over an area of 18 to 20.7 km2. The area is along the Carson River Route of the Old California Trail; soda was mined from Soda Lakes in the middle to late 19th century.
Geothermal activity was apparently unknown or very poorly known in the area until a well drilled in 1903 to supply water for a topographic survey camp for the Truckee-Carson Irrigation Project hit boiling water at about 18 m. A cinder-block bathhouse was built later to utilize the steam and hot water (Peggy Wheat, oral commun., 1975). The well was still emitting hot steam in 1974, although the bathhouse had been torn down. Alteration in Quaternary sediments exposed at the surface probably indicates shallow subsurface boiling (Olmstead and others, 1975); a hot spring may have discharged at this site through the end of the 19th century (Hill and others, 1979).
At Soda Lake, several hundred to more than 1,000 m of Quaternary and Tertiary sedimentary and volcanic rocks overlie a Mesozoic metamorphic basement. Geothermal fluids in the Soda Lake area are believed to originate deep within the Carson basin to the east and northeast, and migrate up dip along permeable beds in a late Tertiary sedimentary unit. A northeast-striking(?) fault is thought to allow vertical fluid migration between offset portions of a permeable pumice tuff unit that makes up the reservoir (McNitt, 1990).
The extent of the thermal anomaly in the shallow subsurface has been outlined by the drilling of temperature-gradient holes (figure) by the U.S. Geological Survey and the U.S. Bureau of Reclamation (Olmsted and others, 1975) and continued drilling of production, injection and monitoring wells. Also, warm springs apparently enter the bottom of Big Soda Lake near its center, which is about 60 m deep. Breese (1968) conducted a temperature survey of the lake bottom and reported temperatures up to 30°C. Nearby temperature-gradient drill holes of the U.S. Geological Survey indicate that cool groundwater is present between Big Soda Lake and the thermal anomaly about 1 km to the northeast. A geothermal well drilled in C SE¼ SE¼ Sec. 29, T20N, R28E (The Chevron-Phillips Soda Lake 1-29) to 1,313 m reportedly encountered gabbro(?) near the bottom. A 172.2ºC stabilized bottom-hole temperature was reported from that well. The thermal anomaly >115ºC at about 600 m below the surface covers 3-4 km2 centered over SW¼ Sec. 28, T20N, R28E (GeothermEx, 2004).
The Soda Lake area is in the western part of the Carson Desert or Carson Sink, a large depression filled with unconsolidated basin fill at least 1,829 m thick. Basaltic rocks of Quaternary age are exposed at several places within the basin, including Lone Rock in the northeast Carson Sink, Rattlesnake Hill near Fallon, and Soda Lake (figure). Lone Rock may be a remnant of a volcanic plug or neck, and Rattlesnake Mountain consists of basalt flows and the eruptive vent, which is now filled with agglomerate (Morrison, 1964). Upsal Hogback is a cluster of several basaltic cones, and Soda and Little Soda Lakes are craters or maars that are rimmed by a mixture of basaltic and nonvolcanic debris blown out by repeated gaseous eruptions (Olmsted and others, 1975). The eruptions that formed the craters may be phreatic in part.
The eruptions that formed the cones at Upsal Hogback occurred chiefly during an interpluvial time in the late Pleistocene when Lake Lahontan was dry (Morrison, 1964). Upsal basaltic tephra is found in the lower Sehoo Formation, which is probably about 25,000 to 30,000 years old (Jonathan Davis, oral commun., 1977). The earliest eruptions at Soda Lakes may have been as early as or earlier than those at Upsal Hogback (Morrison, 1964), but the rim of Soda Lake (elevation 1,220 m) has not been cut by any Pleistocene lakes. Since the last lake above 1,220 m elevation was in the lowermost upper Sehoo, this would indicate that the present maar at Soda Lake was formed less than about 6,900 years ago (Jonathan Davis, oral commun., 1977). On the basis of preliminary helium isotope studies (Thure Cerling, personal commun., 1997), the eruption at Soda Lake may be younger than 1,500 years before present.
The exposed materials in the thermal area are predominantly unconsolidated Lake Lahontan sediments and some sediments reworked from the Lahontan deposits. In the vicinity of the old bathhouse steam well the sands have been altered in part to kaolinite and various iron oxides or hydroxides by hydrothermal activity, probably chiefly vapor (Olmsted and others, 1975).
Exposed faults in the area are rare, although Morrison (1964) has mapped several northeast-trending faults (see figure). The general alignment of Soda Lakes, the thermal anomaly, and Upsal Hogback along a north-northeast trend suggests faults at depth, possibly along a zone of rupture in the Tertiary or pre-Tertiary consolidated rocks (Olmsted and others, 1975).
A less intense thermal anomaly than the one northeast of Soda Lake is located over an area of about 45 km2 near Upsal Hogback (about 16 km northeast of Soda Lake). The anomaly, defined by shallow temperature-gradient wells, is centered on an area near Highway U.S. 95 about 2 km northeast of the northeast end of Upsal Hogback (Olmstead and others, 1981, Fig. 5). This anomaly results from thermal waters rising to a depth of about 245 m below the land surface before flowing laterally in an aquifer in Tertiary basaltic and sedimentary rocks (Olmsted and others, 1984). A 36ºC water temperature in a Kennametals well and ~38ºC temperatures from U.S. Geological Survey wells (see geochemistry database), both located southeast of Upsal Hogback, suggest an even larger area of thermal groundwater.
Since the early 20th century irrigation has raised the water table throughout the area. The thermal water rises from depth, mixes with the shallow nonthermal water, and moves laterally to the north-northeast (figure). The extent of the zone of mixing is not known (Olmsted and others, 1975). Further data on the geohydrology of the area are available in Olmstead and others (1984).
Currently (2008) there are two binary power plants at the Soda Lake geothermal area producing from the 182°C reservoir. The power plants are located in NW¼ Section 33, T20N, R28E, about 4 km northeast of Soda Lake. Both plants are operated by AMOR IX of Fallon, NV. Soda Lake 1 produces 3.6 MW of electricity, and the plant came on-line in 1987. Soda Lake 2 produces 13 MW of electricity, and the plant came on-line in 1991. Drilling of a new, 1,500-m production well began in April 2002.
Photos
Soda Lake, Churchill County (from lithograph, Hague and Emmons, 1877, plate 22). Currently (2008), the Soda Lake geothermal plant is operated by AMOR IX, 5500 Soda Lake Road, Fallon, NV 89406.
Cooling units at the Soda Lake power plant.
Soda Lake cooling unit. The Soda Lake geothermal plant is operated by AMOR IX, 5500 Soda Lake Road, Fallon, NV 89406.
Soda Lake condensers. The Soda Lake geothermal plant is operated by AMOR IX, 5500 Soda Lake Road, Fallon, NV 89406.
Cross section of the Soda Lake geothermal field. From McNitt (1980). The Soda Lake geothermal plant is operated by AMOR IX, 5500 Soda Lake Road, Fallon, NV 89406.