Hydrochemistry and isotope composition of springs in the Tecopa basin, southeastern California, USA

Springs and seeps in the Tecopa basin were sampled and analyzed for their major ion chemistry and oxygen, hydrogen, and boron isotope compositions in order to assess the possible origins of the waters. The Tecopa basin is a groundwater discharge area along the Amargosa River drainage basin south of the Nevada Test Site and proposed high-level nuclear waste repository at Yucca Mountain, NV. The spring waters are categorized into three end-member hydrochemical facies: (1) pH-neutral, Na + Ca + Mg + SO4 + HCO3 water with moderate to high total-dissolved solids; (2) mildly alkaline, Na + CI + SO4 + HCO3 water with high total dissolved solids; and (3) alkaline, Na + HCO3 + CO3 water with high total dissolved solids. The temperatures of most of the springs range from 17°C to 32°C; however, waters in the Tecopa Hot Spring area range from 9°C to 47°C. The oxygen and hydrogen isotope compositions (normalized to SMOW) of the waters are as follows. The facies 1 springs have a mean δ18O of - 12.77 ± 0.14‰ (1σ) and δ of - 94.9 ± 0.5‰ (1σ). The facies 2 springs have a mean δ18O of - 12.73 ± 0.19‰ (1σ) and δ D of - 97.7 ± 0.9‰ (1σ). The facies 3 spring has a mean δ18O of - 13.09 ± 0.17‰ (1σ) and δ D of - 103.5 ± 0.7‰ (1σ). The mean boron concentrations and isotope compositions range from 2.4 mg/I and δ11B of + 2.5‰ for facies I springs to 72.8 mg/1 and δ11B of - 3.8‰ for the facies 3 spring. The facies 1 waters have chemical and isotope compositions consistent with meteoric recharge in the Spring Mountains, NV, groundwater flow through the regional carbonate aquifer, and minor mixing with more concentrated water. The thermal, facies 2 waters are isotopically and chemically distinct from waters in the regional basin-fill and carbonate aquifer systems. These waters are interpreted to originate from fractured water-bearing strata beneath the regional carbonate aquifer that discharge only in the southernmost part of the Great Basin. The facies 3 spring is isotopically and chemically similar to basin-fill aquifer waters in the Amargosa Desert, but has higher sodium, alkalinity, and boron contents due to interaction with saline, alkaline-lake deposits of the Plio-Pleistocene Lake Tecopa beds. Geologic, geochemical, and isotope data suggest that these three distinct water types flow through hydrogeologic compartments in the Tecopa basin. The compartments are interpreted to have been created by the impermeable Lake Tecopa beds and geologic structures within the basin. © 2001 Elsevier Science B.V. All rights reserved