Use of U-Series Isotopic Disequilibrium to Investigate the Nature and Distribution of Actively Flowing Fractures

Groundwater transport of radioisotopes from underground nuclear tests at the Nevada National Security Site (NNSS) is of concern to the U.S. Department of Energy. Extensive testing was conducted near or below the regional water table (saturated zone; SZ) at Pahute Mesa and within the unsaturated (or partially saturated) zone at Rainier Mesa. Groundwater flow in these rocks is believed to occur mainly through a connected network of fractures. To better understand flow in these fractured rocks, we analyzed U-series isotopes (238U-234U-230Th) from drill core samples. In rock isolated from flow over the last million years, isotopes in the 238U decay chain reach a state of radioactive secular equilibrium, where 234U/238U and 230Th/238U activity ratios (AR) = 1.0. More recent water-rock interaction results in mobilization of 234U relative to 238U, and U relative to Th in migrating waters. Rock surfaces that incorporate this U or are leached of 234U and U will show U-series disequilibrium. Isotope data can thus provide time-sensitive information on hydrologic conditions in host rocks without directly observing or measuring flowing water. To investigate NNSS fracture networks, core was selected from confining units (bedded and zeolitized felsic tuffs) and aquifers (felsic welded tuffs and lavas) in five boreholes on Pahute Mesa and two boreholes on Rainier Mesa. Samples include interiors of intact core as well as natural fracture surfaces and brecciated core. Intact core and brecciated samples were crushed and powdered. Fracture surfaces were sampled using dental burs to remove the outer 0.1 to 0.5 mm of fracture surfaces, which may have thin mineral coatings of zeolites, clays, and Mn oxides. Samples were totally digested, spiked with a 236U-229Th tracer, and analyzed by a solid-source TRITON° mass spectrometer equipped with an energy filter and single ion counter. Results show that 8 of 9 intact core samples have 234U/238U AR within ±5% of 1.0, suggesting little or no water-rock interaction over the last several hundred thousand years. In contrast, discrete fracture surfaces (N=37) have 234U/238U AR ranging from 2.09 to 0.34, although the median value is 1.04. About one third of the 28 SZ fracture surfaces have 234U/238U AR within 5% of 1.0. Remaining SZ fractures tend to have 234U/238U > 1.0,indicating that U incorporation from migrating groundwater (234U/238U AR ≈ 2-4) is an important process. Furthermore, samples with isotopic disequilibrium commonly plot along the equiline (equal 234U/238U and 230Th/238U AR) regardless of sample type or location. This pattern cannot be explained by deposition and closed-system isotope evolution of secondary minerals. Instead, it suggests a quasi-steady-state balance of processes including sorption or leaching of U associated with migrating solutions and in-situ production, decay, and α-recoil of 230Th and 234U. These data will be used to help constrain numerical models of fracture-matrix interaction and spatial distribution of flowing versus non-flowing fractures.