Deuterium and Oxygen-18 in Water From a Deep Unsaturated Zone in the Amargosa Desert, Nye County, Nevada

David A. Stonestrom, David E. Prudic, and Robert G. Striegl

Abstract

Isotopic composition of water in deep unsaturated zones is of interest because it may provide information on hydrologic processes and climate. An improved method of cryogenic distillation was used to remove water from 12 cores collected from depths of 3 to 85 m in the unsaturated zone at a 16 ha site 17 km south of Beatty, Nevada. Tests showed that method removed water from the dry alluvial sediments without substantially altering its deuterium (D) or oxygen-18 (¹⁸O) content. The isotopic composition of water from cores was compared to that of water in air sampled from 10 depths between 5 and 109 m. The

D and

¹⁸O values in water-vapor samples were close to theoretical predictions from core-water samples when temperature-dependent fractionation was taken into account. This suggests that the vapor-sampling technique produced reliable results for materials and conditions at the site. Core water at a depth of 3 m had a

D of -64 permil and an

¹⁸O of +0.4 permil.

D and

¹⁸O values in core water decreased with depth in the upper unsaturated zone, becoming uniform below about 30 m and approximately equal to values in ground water sampled at the 110-m deep water table (

D = -107 permil and

¹⁸O = -14 permil, respectively). On a

D-

¹⁸O plot the unsaturated zone data are approximated by a line with a slope of +3.3, consistent with net evaporative discharge of water at the land surface. On the basis of data from nearby areas, winter precipitation at the site has an estimated volume-weighted mean

D of -95 permil and

¹⁸O of -12.8 permil. Comparison of D and ¹⁸O in precipitation, unsaturated-zone water, and ground water indicates that water is moving from the deep unsaturated zone to the land surface in response to upward thermal and matric-potential gradients. The isotopically light water at depth is consistent with previous studies suggesting that some ground water in the region may represent recharge from an earlier climatic episode.

This abstract was published in Eos, American Geophysical Union Transactions, v. 78, no. 46, p. 302-303.