Measuring the Stable Isotope Composition of Water in Brine from Halite Fluid Inclusions and Borehole Brine Seeps Using Cavity Ring-Down Spectroscopy.

Abstract

Naturally occurring bedded salt deposits are considered robust for the permanent disposal of heat-generating nuclear waste due to their unique physical and geological properties. The Brine Availability Test in Salt (BATS) is a US-DOE Office of Nuclear Energy funded project that uses heated borehole experiments underground (∼655 meters depth) at the Waste Isolation Pilot Plant (WIPP) in the bedded salt deposits of the Salado Formation to investigate the capacity for safe disposal of high-level, heat generating nuclear waste in salt. Uncertainties associated with brine mobility near heat-generating waste motivates the need to characterize the processes and sources of brine in salt deposits. Intragranular halite fluid inclusions are a potential source of brine that can migrate under temperature gradients toward heat sources. We developed a methodology to measure the stable isotopic compositions of water (δD _VSMOW, δ¹⁸ O _VSMOW) in brine from halite fluid inclusions using Cavity Ring-Down Spectroscopy that accounts for memory effects using a unique reference-sample-reference bracketing approach and that minimizes sample size requirements. We applied this approach to halite samples obtained from WIPP and compare these data to seeped brines collected from horizontal boreholes at WIPP after drilling at ambient conditions. The stable isotope compositions that we obtain for halite fluid inclusions (δ¹⁸ O _VSMOW = +3.24 ± 0.53‰, δD _VSMOW = -25.3 ± 5.1‰, ±1σ, n = 5) generally agree with previous measurements and likely reflect a combination of syn-depositional and/or postdepositional processes. The seep brines are isotopically distinct (δ¹⁸ O _VSMOW = +3.46 ± 0.84‰, δD _VSMOW = +7.3 ± 3.5‰, ±1σ, n = 35) and instead resemble evaporated seawater. We discuss our results in the context of prior WIPP-proximal waters and lay the groundwork for using stable isotopes of water in brine as a tool to assess the heat-induced mobilization of halite fluid inclusions in ongoing heating experiments that comprise the Brine Availability Test in Salt.