Date of this Version
Weaver, C.B. 2014. Pore Water Extraction for Unsaturated Isotope Research: An Investigation using an Immiscible Displacement Fluid and a Centrifuge (MS Thesis). University of Nebraska, Lincoln, NE
Accurate and reliable pore water extraction techniques are important to an array of scientific fields including, but not limited to, hydrogeology, soil science, and paleoenvironmental research. The aim of the current project is to test the applicability of an immiscible displacement extraction technique for stable isotopes of water under a range of textural, hydrologic, and chemical conditions. In this study, laboratory experiments were conducted to establish the extent to which the proposed method 1) achieves sufficient yield for laboratory isotopic analyses, 2) results in isotopic exchange between water and the displacement fluid, 3) conserves initial isotopic compositions of spike test standards under a range of sediment conditions, and 4) is appropriate for natural stable isotope profile interpretation.
Overall, pore water extraction from the unsaturated zone using the immiscible displacement fluid, HFE-7100, and a centrifuge has proven to be a satisfactory method for stable isotopes of water under the test conditions considered in this project. HFE-7100 causes no isotopic discrimination with water exposure times up to 24 hours (maximum exposure time tested). Results suggest that yields are sufficient under a wide array of natural conditions including in water stressed regions (consistently with moisture contents less than 6%). With few exceptions, the greatest percent yield resulted from high moisture contents, large grain sizes, and low clay contents. Persistent isotopic discriminations between spike test standards and extracts suggest that non-vaporization fractionation may be more common than is currently assumed under typical unsaturated zone conditions. The fact that pore water isotope fractionation was found in non-saline, unsaturated, highly permeable conditions suggests that the fractionation is most likely associated with water adsorption onto mineral surfaces. While fractionation-related uncertainty raises concerns about the validity of unsaturated zone isotopic profile interpretations, analysis of paired field cores suggests that fractionation-related uncertainty may be smaller than the temporal variability of meteoric water in many cases. In such instances, profiles may be interpreted reliably using standard conservative tracer assumptions.
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