Determining the stable isotope composition of porewater using low temperature multi-step extraction for low water content soils

Abstract

The δ2H and δ18O values (isotope values) of porewater in soils have been used as natural tracers in many research disciplines. Conventional methods to determine the isotope values of porewater require extraction of water from the soil. To extract soil porewater cryogenic vacuum distillation is a commonly used technique. Based on extraction techniques porewater can be considered to be composed of ‘extractable’ and ‘non-extractable’ two components. Based on the goal of most studies, porewater consists of ‘active’ and ‘inactive’ two components, of which only the active part is of interest. Because different porewater components are interrelated through isotope exchange, in principle the conventional distillation technique cannot extract the active porewater without changing its original isotopic composition, regardless of whether recovery of the active porewater is complete or not. Thus, using the isotope results determined on the extracted water to represent the active porewater in soil may lead to variable results. Here, we describe a new distillation strategy for extraction of porewater for determining active porewater isotope values. The extraction uses a lower extraction temperature that is comparable to natural conditions. The isotope values are determined on the extracted water corresponding to different recoveries resulted from multi-step extraction. The final results of the active porewater can be derived indirectly from the multi-step extraction. This study demonstrates that in some cases the conventional one-step extraction technique may not be capable for accurate determination of isotope values of the active porewater. The multi-step extraction technique described in this paper can be used to give more reliable results. Multi-step extraction is similar to direct equilibration methods in that it determines the porewater component which plays an active part in the water cycle, regardless of whether extraction of the porewater is complete or not. Multi-step extraction method could provide an alternative approach to direct equilibration methods in some special cases, or provide a way to test the reliability of other methods for the determination of δ2H and δ18O values of soil porewater.