As the absolute isotopic composition of a material is difficult to measure directly, isotope ratios are generally compared to that of a standard. In the case of 180/160, the standard is commonly Vienna-Standard Mean Oceanic Water (VSMOW), with an isotope ratio of 2.0052 x 10~3 (Gon-fiantini, 1984). Isotopic compositions are expressed as relative deviations from VSMOW, and denoted álsO = Rp/Rst - 1, where Rp and Rst are the isotope ratios of the substance of interest and the standard, respectively. Variation in the <5lsO of source (soil) water may be removed from ¿18Op by presenting the composition as an enrichment above source water (A18Op) given by: A18Op = Rp/Rs - 1, where Rs is the 180/160 ratio of source water. To a close approximation, A18On may be estimated by A18Op = á18Op — <518Os, where á18Os is the isotopic composition of source water (relative to VSMOW).

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