Chemical and Isotopic Characteristics of Thermal Fluids in the Long Valley Caldera Lateral Flow System, California

Abstract

Chemical and isotopic data of thermal waters in Long Valley caldera have been used to identify both the origins and characteristics of the fluids and to evaluate mixing and boiling processes occurring within the lateral flow system of the caldera. Recharge to the Long Valley geothermal system occurs in the western part of the caldera with the water being heated at depth and flowing laterally eastward in the subsurface. The lateral flow system was recently intersected by the Shady Rest Continental Scientific Drilling Program (CSDP) corehole at 335 m (1100 ft) with fluids in this 202°C zone being more concentrated than nonboiled fluids to the west. As the Na-K-HCO3-C1 thermal fluids flow eastward, they are increasingly mixed with isotopically depleted, dilute groundwaters similar to cold waters east of Lake Crowley. Near surface boiling of Casa Diablo well fluids at 100°C forms waters with the compositions of Colton and Casa Diablo hot springs. Waters to the west of the Casa Diablo area are mixtures of meteoric water and oiled thermal fluids with a composition close to that of Colton Hot Springs. There is no correlation between 3H and 36C1 in thermal fluids or between these components and conservative species, and it appears that cold fluids involved in mixing must be relatively old waters, low in both meteoric 3H and 36C1.