Cryogenic vacuum distillation (CVD) methods have been widely used to extract water from unsaturated soil materials. However, recent studies have reported that extraction conditions (e.g. extraction time) and soil type can influence the stable isotope ratios of soil water extracted by CVD. We examined (i) the effects of extraction time on the stable isotope ratios of soil water extracted by CVD, and (ii) how the stable isotope ratios differed from those of soil water extracted by other methods. We first examined extraction times of 1–6 h, and observed no significant differences in the δ2H or δ18O values of extracted soil water for extraction times >2 h. However, extraction for 1 h collected only 48% of soil water, and the δ2H and δ18O values were significantly lower than those for extraction times >2 h. We then compared the stable isotope ratios of soil water extracted by CVD and centrifugation. Although the stable isotope ratios for both extraction methods were within the range of that of rainwater, the stable isotope ratios for water extracted by CVD were lower than those for water extracted by centrifugation. Our results highlight questions surrounding the use of CVD for stable isotope analysis of soil water.
This study demonstrated groundwater flow simulations to investigate a vertical trend of equivalent hydraulic conductivity of alluvial fan gravel deposits in Sapporo, Japan, considering open void connectivity. Equivalent hydraulic conductivity was defined according to Darcy’s Law for a cube of 10 m in size, consisting of one million cells assigned among fully packed (without open voids), loosely packed (with less-connected voids) or very loosely packed (with well-connected voids) deposits. The stochastic generation was performed under each configuration in terms of target depth sections for vertical trend analysis, and horizontal variogram ranges (random, high, and low connectivity, and no open voids) for open void connectivity. The logarithmic average of 100 equivalent hydraulic conductivities was calculated in each configuration, and the vertical trends were determined. The simulation results showed that the equivalent hydraulic conductivity increased when the open void frequency was large in the shallow zone and the connectivity of the open voids was assumed. In particular, the high connectivity assumption was needed to match the in situ trend with a decay exponent of 0.05 m–1. Modeling the vertical trend with such a large decay exponent was essential to obtain realistic solutions of the groundwater flow and transport system in the alluvial fan.