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

Abstract

Abstract.A method to measure the δ 2 H and δ 18 O composition of pore waters in saturated and unsaturated geologic core samples using direct vapour equilibration and laser spectrometry (DVE-LS) was first described in 2008, and has since been rapidly adopted. Here, we describe a number of important methodological improvements and limitations encountered in routine application of DVE-LS over several years. Generally, good comparative agreement, as well as accuracy, is obtained between core pore water isotopic data obtained using DVE-LS and that measured on water squeezed from the same core. In complex hydrogeologic settings, high-resolution DVE-LS depth profiles provide greater spatial resolution of isotopic profiles compared to long-screened or nested piezometers. When fluid is used during drilling and coring (e.g. water rotary or wet sonic drill methods), spiking the drill fluid with 2 H can be conducted to identify core contamination. DVE-LS analyses yield accurate formational isotopic data for fine-textured core (e.g. clay, shale) samples, but are less effective for cores obtained from saturated permeable (e.g. sand, gravels) geologic media or on chip samples that are easily contaminated by wet rotary drilling fluid. Data obtained from DVE-LS analyses of core samples collected using wet (contamination by drill water) and dry sonic (water loss by heating) methods were also problematic. Accurate DVE-LS results can be obtained on core samples with gravimetric water contents > 5 % by increasing the sample size tested. Inexpensive Ziploc ™ gassampling bags were determined to be as good as, if not better than, other, more expensive specialty bags. Sample storage in sample bags provides acceptable results for up to 10 days of storage; however, measurable water loss, as well as evaporitic isotopic enrichment, occurs for samples stored for up to 6 months. With appropriate care taken during sample collection and storage, the DVE-LS approach for obtaining high-resolution pore water isotopic data is a promising alternative to study the hydrogeology of saturated and unsaturated sediments. Eliminating analytical interferences from volatile organics remains a challenge.

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

“…3, center). Since the in situ approach is based on the same principle as the equilibration bag method, its applicability on samples with low water content is supported by a recent study of Hendry et al (2015), who found that the measurement accuracy of their equilibration procedure could be improved by increasing the sample amount of low water content soils to increase the amount of water available for equilibration in the bag.…”

Intercomparison of soil pore water extraction methods for stable isotope analysis

“…Ziploc® bags were evacuated, sealed and massaged to homogenize, placed inside a second Ziploc® bag, and stored to equilibrate prior to analysis. We followed protocols of Hendry et al (2015) and Wassenaar et al (2008), whereby the Ziploc® bags containing the soil samples were inflated with dry air, sealed, and allowed to equilibrate at room temperature for additional three days prior to water vapor 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).…”

Soil water stable isotopes reveal evaporation dynamics at the soil–plant–atmosphere interface of the critical zone

“…Potentially fractionating effects of interactions between soil water and surfaces of clay minerals (Oerter et al, 2014;Gaj et al, 2017a, b;Newberry et al, 2017) are of minor relevance for our study, since clay contents were low in the sampled soils (Table 1). We can further ensure that the sampled soil volumes always contained much more than 3 g of water as suggested by Hendry et al (2015).…”