STRONTIUM ISOTOPE STUDIES OF GROUNDWATER IN PARTS OF SOUTHERN AND EASTERN NEVADA

Measurement of strontium (Sr) isotope concentrations can be useful in the characterization of groundwater in complex regional flow systems. Strontium isotope concentrations and ⁸⁷Sr/⁸⁶Sr ratios of spring and well waters have been used to characterize groundwater in parts of southern and eastern Nevada, as part of an on-going water resources evaluation.  The area of interest lies within the carbonate-rock province of the Great Basin, and is dissected by active faults with normal and right lateral slip components that form local barriers, as well as conduits in some cases, to groundwater flow.

Strontium concentrations in the groundwater vary from 0.01 to 8.2 mg/L with 5 of the 48 samples collected having concentrations in excess of 1 mg/L.  The ⁸⁷Sr/⁸⁶Sr activity ratios range from 0.70869 to 0.71587 with a mean value of 0.71170.   The computed δ⁸⁷Sr values vary from –0.73 to 9.40 per mil.  These values primarily reflect the lithology of the aquifers as a result of weathering processes and rock-water interactions. Two groups of groundwater were distinguished by the ⁸⁷Sr/⁸⁶Sr ratios. The first group is the relatively non-radiogenic samples with ⁸⁷Sr/⁸⁶Sr ratios varying from 0.70869 to 0.71022 and δ⁸⁷Sr values of less than 2. These groundwater samples are interpreted to have undergone minimal water-rock interaction, and are strongly influenced by local recharge from the surrounding mountains.  The temperatures of the springs and wells in this group vary from 19 to 28^oC.  The second group of samples is highly radiogenic with ⁸⁷Sr/⁸⁶Sr ratios varying from 0.71050 to 0.71587 and δ⁸⁷Sr values ranging from 2.2 to 9.8 per mil. The observed large variation of δ⁸⁷Sr values can be explained by water-rock interactions and the dominant recharge source. Groundwater within the regional carbonate flow system in the area is characterized by δ⁸⁷Sr values of between 4.4 and 6.8 per mil. These observed characteristics would be used in addition to data on other isotopic tracers, major ions, and temperature, to develop a better understanding of the patterns of groundwater flow in southern and eastern Nevada.