Influence of equilibration time, soil texture, and saturation on the accuracy of porewater water isotope assays using the direct H2O(liquid)-H2O(vapor) equilibration method

The hydrogen and oxygen stable isotopes of water ((SH)-H-2 and (SO)-O-18) are powerful tracers for studying subsurface water flow processes, but the extraction of porewater from unsaturated soil is complex and laborious. The direct liquid-vapor equilibration method (DLVE) for isotope analysis overcomes this challenge by analyzing headspace vapor in isotopic equilibrium with the porewater under closed system conditions. However, the effect of the equilibration time, soil texture, and porewater saturation on isotopic results is not fully understood yet. We tested three differently textured, disaggregated soils (sandy, silty loam and clay) to assess how the equilibration time is impacted by (i) porewater saturation (100%, 80%, 60%, and 40%), and (ii) soil surface area. For all tests, the water loss through diffusion was negligible (< 0.3%). The experiments showed that for disaggregated sandy soil, 24 h was a sufficient isotopic equilibration time regardless of water saturation level. Similarly, 24 h was sufficient for kaolinite samples with 100% saturation exhibiting little isotopic variance even after 168 h of equilibration. For saturated silty loam with 2% organic carbon content, 96 h was the optimal equilibration time, whereas saturated silty loam with 4% organic carbon content did not reach isotopic equilibrium by 168 h. The optimal equilibration time increased depending on whether soil samples were disaggregated or kept in a core cutter. Inconclusive results for silty soils with organic carbon revealed the need to further investigate the possible in-fluence of organic carbon on the DLVE method.

Automated summary

The experiments showed that the equilibration time for isotopic analysis of water in different textured soils and porewater saturation levels varies, with different optimal times for different soil types and saturation levels. The presence of organic carbon in soil also impacts the equilibration time needed for isotopic equilibrium to be reached, requiring longer times for soils with higher organic carbon content.