Direct equilibration of soil water for delta O-18 analysis and its application to tracer studies

Current methods for stable oxygen isotopic (delta(18)O) analysis of soil wa ter rely on separation of water from the soil matrix before analysis. These separation procedures are not only time consuming and require relatively l arge samples of soil, but also have been shown to introduce a large potenti al source of error. Current research at Queen's University Belfast is focus ed on using direct equilibration of CO2 with the pore water to eliminate th is extraction step using the automated Multiprep system and a Micromass Pri sm III isotope ratio mass spectrometer (IRMS). The findings of this researc h indicate the method is less time consuming, more reliable, and reproducib le to within accepted limits (+/-0.1%parts per thousand delta(18)O). In thi s study the direct equilibration method is used to analyse delta(18)O trace r profiles in the unsaturated zone of field soils, concurrently with chlori de tracer profiles, which can be used to assess infiltration rates and mech anisms through the unsaturated zone. Copyright (C) 1999 John Wiley & Sons, Ltd.