Abstract
Abstract. A method to measure the δ2H and δ18O 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 2H 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™ gas-sampling 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.
Highlights
The stable isotope composition of pore water (δ2H, δ18O) in unsaturated and saturated zone geologic media is an important hydrologic tracer used to determine the origin, rate of water movement, and mixing of different waters; this method has been applied to studies evaluating resources, the water cycle, and groundwater contamination, as well as those determining soil vapour fluxes (e.g. Clark and Fritz, 1997; Gimmi et al, 2007; Mueller et al, 2014)
We compared the high-resolution isotopic depth profiles of pore water determined by direct vapour equilibration and laser spectrometry (DVE–LS) to liquid water samples obtained by both squeezing duplicate core samples and collection from four site piezometers
The effect of wet versus dry sonic core sampling for DVE–LS analyses was tested at a natural saturated site and an unsaturated waste rock pile near Sparwood, British Columbia
Summary
The stable isotope composition of pore water (δ2H, δ18O) in unsaturated and saturated zone geologic media is an important hydrologic tracer used to determine the origin, rate of water movement, and mixing of different waters; this method has been applied to studies evaluating resources, the water cycle, and groundwater contamination, as well as those determining soil vapour fluxes (e.g. Clark and Fritz, 1997; Gimmi et al, 2007; Mueller et al, 2014). We assess and evaluate several aspects, including (1) the accuracy of high-resolution isotopic depth profiles by DVE– LS; (2) contamination impacts from a range of drilling and sample collection methods; (3) the effects of long-term sample storage in plastic gas-sampling bags and approaches to correct for it; (4) alternate gas-sampling bag materials, with the aim of increasing the storage time with no loss of sample integrity; (5) water content limits for obtaining accurate isotopic data; and (6) spectral contamination by petroleum organics These topics are addressed in separate sections, including a discussion of relevant materials, results, and outcomes. We compared the high-resolution (cm scale) isotopic depth profiles of pore water determined by DVE–LS to liquid water samples obtained by both squeezing duplicate core samples and collection from four site piezometers. This difference was attributed to a recent rapid change in source water in this sand layer, as evidenced by the sharp isotopic interface across the lower boundary of the sand
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