Does the Rayleigh Equation Apply to Evaluate Field Isotope Data in Contaminant Hydrogeology?

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• 作者：Yumiko Abe and Daniel Hunkeler
• 刊名：Environmental Science & Technology
• 出版年：2006
• 出版时间：March 1, 2006
• 年：2006
• 卷：40
• 期：5
• 页码：1588 - 1596
• 全文大小：175K
• 年卷期：v.40,no.5(March 1, 2006)
• ISSN：1520-5851

文摘

Stable isotope data have been increasingly used toassess in situ biodegradation of organic contaminants ingroundwater. The data are usually evaluated using theRayleigh equation to evaluate whether isotope data followa Rayleigh trend, to calculate the extent of contaminantbiodegradation, or to estimate first-order rate constants.However, the Rayleigh equation was developed forhomogeneous systems while in the subsurface, contaminantscan migrate at different velocities due to physicalheterogeneity. This paper presents a method to quantifythe systematic effect that is introduced by applyingthe Rayleigh equation to field isotope data. For this purpose,the travel time distribution between source and samplingpoint is characterized by an analytical solution to theadvection-dispersion equation. The systematic effect wasevaluated as a function of the magnitude of physicalheterogeneity, geometry of the contaminant plume, anddegree of biodegradation. Results revealed that the systematiceffect always leads to an underestimation of the actualvalues of isotope enrichment factors, the extent ofbiodegradation, or first-order rate constants, especially inthe dispersion-dominant region representing a higherdegree of physical heterogeneity. A substantial systematiceffect occurs especially for the quantification of first-order rate constants (up to 50% underestimation of actualrate) while it is relatively small for quantification of theextent of biodegradation (<5% underestimation of actualdegree of biodegradation). The magnitude of the systematiceffect is in the same range as the uncertainty due touncertainty of the analytical data, of the isotope enrichmentfactor, and the average travel time.