Stable Isotopes (δ¹⁸O, δ²H) of Pore Waters in Clay‐Rich Aquitards: A Comparison and Evaluation of Measurement Techniques

Abstract

Pore water collected from piezometers installed in a thick clay‐rich till were used to compare and evaluate four techniques for obtaining δD and δ18O values in these media. The techniques included mechanical squeezing, centrifugation, azeotropic distillation, and a direct soil‐water equilibration technique. Direct CO2‐core equilibration yielded sufficiently accurate and reproducible δ18O results of pore water in clay‐rich tills. In addition, this method eliminated the need for labor‐intensive complete extraction of water from the geologic media. Mechanical squeezing and centrifugation produced results similar to direct equilibration. However, both of these methods exhibited a greater degree of variability and were laborious and more time consuming. Small differences in δ18O values between piezometer water and equilibrated, squeezed, and centhfuged samples suggested that each method collected different fractions of the clay‐water reservoir. Although these subtle differences were not conclusive, they did suggest the presence of weakly bound water and highlighted the difference between these three techniques for determining the stable isotopic composition of pore water in clay‐rich aquitards. Azeotropic distillation produced a high level of discrepancy in δD andδ18O results compared to the other methods. Incomplete extraction was considered the most probable cause of this error. The results of this study suggested that direct equilibration is the best method for determining detailed δD and δ18O values of pore water in clay‐rich aquitards.

Determining the stable isotope composition of pore water from saturated and unsaturated zone core: improvements to the direct vapour equilibration laser spectrometry method

“…The analytical precision of the DVE-LS method for δ 2 H and δ 18 O (± 0.4 ‰ for δ 18 O and ± 2.1 ‰ for δ 2 H) is comparable to or better than the aforementioned extractions and isotope ratio mass spectrometry (IRMS)-based direct equilibration methods (Kelln et al, 2001;Morrison et al, 2001), and significantly better than those reported for physical and chemical water extractions (Allison and Hughes, 1983;Revesz and Woods, 1990). Accuracy is assured through the application of identical treatment (IT) approaches, using controls and standards and, where available, by field comparison with isotopic data from piezometers or by other extraction methods.…”

“…Although piezometers, wells, and lysimeters are useful for characterizing physical and chemical pore water transients in the subsurface, they generally lack detailed vertical depth resolution (< 1 m scale to detect transients) or may be too expensive to install and monitor over large spatial scales or over detailed vertical profiles. Other water isotope techniques use physical extraction of pore water from sub-samples of saturated or unsaturated cores, such as high-speed centrifugation (Allison and Hughes, 1983;Gimmi et al, 2007;Ingraham and Shadel, 1992;Kelln et al, 2001), mechanical squeezing (Kelln et al, 2001), cryogenic micro-distillation (Araguas-Araguas et al, 1995;West et al, 2006;Koeniger et al, 2011;Orolowski et al, 2013), azeotropic distillation (Allison and Hughes, 1983;Revesz and Woods, 1990), and microwave distillation (Munksgaard et al, 2014). In general, physical extraction methods are laborious and have the potential for evaporative isotopic fractionation caused by storage, multistep procedures, or by incomplete recovery of the water or evaporative loss during handling.…”

In situ unsaturated zone water stable isotope (&lt;sup&gt;2&lt;/sup&gt;H and &lt;sup&gt;18&lt;/sup&gt;O) measurements in semi-arid environments: a soil water balance

“…In most unsaturated zone studies, manual removal of soil samples and a subsequent extraction of soil water in the laboratory was necessary using vacuum extraction (e.g., West et al, 2006;Koeniger et al, 2011;Orlowski et al, 2013), equilibration (e.g., Wassenaar et al, 2008), mechanical squeezing, azeotropic distillation, or centrifugation methods (e.g., Walker et al, 1994;Kelln et al, 2001). These methods cause both disturbance to the integrity of the natural soil system and possible evaporation during the sampling procedure.…”

Intercomparison of soil pore water extraction methods for stable isotope analysis

“…Equilibrium techniques include: direct liquid-water-vapor equilibrium (e.g. Wassenaar et al, 2008;Hendry et al, 2015), in situ equilibration (Gaj et al, 2016;Garvelmann et al, 2012;Rothfuss et al, 2013Rothfuss et al, , 2015Volkmann and Weiler, 2014), Hepurging (Ignatev et al, 2013), and CO 2 -and H 2 -equilibration (Hsieh et al, 1998;Jusserand, 1980;Kelln et al, 2001;Koehler et al, 2000;McConville et al, 1999;Scrimgeour, 1995).…”