Characterizing the Transport of the Stable Isotopes of Water in Unsaturated Soils
Date
2017-11-06
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Degree Level
Masters
Abstract
infiltration. The peak shift method is used to calculate a net percolation rate by finding the location
of isotope peaks. The literature applying the peak shift methods is plentiful. However, when
extensive spreading occurs by diffusion and dispersion and a well-defined peak is no longer
present the peak shift method is not applicable and another method of analysis is needed.
The goal of this work was to develop a better understanding of the diffusive and dispersive
movement of isotopes within an unsaturated soil. Isotopes can be partitioned to the vapour phase
where they can be stored and transported in addition to the aqueous phase. A dual phase
relationship was derived to analyze isotope profiles where significant spreading has occurred.
A dual phase (i.e. vapour and liquid) diffusion – water content relationship was developed using
water isotope, carbon dioxide, and oxygen gas diffusion literature including gaseous and aqueous
phase tortuosities. This relationship was evaluated using the results from a set of double half-cell
diffusion cells. Each cell allowed for a diffusion coefficient to be measured at specific volumetric
water content. The experimental procedures proved to be challenging and required several
iterations to collect quality data. Interpretation of the diffusion cell data resulted in a best fit dual
phase model, using the gaseous phase tortuosity model of Penman (1940) and the aqueous phase
tortuosity model recommended by Padilla et al. (1999) with the saturated tortuosity from Maxwell
(1881).
The proposed diffusion model was also evaluated using the observations from a column test used
to simulate infiltration under simple field conditions. Simulated rainfall was allowed to infiltrate
and diffuse. Isotope values were measured over the column elevation at different times. These
observations were used to verify the dual-phase diffusion model as well as several different
dispersivity-water content relationships. The diffusion behavior observed in the column
experiment was consistent with the combined dual-phase model selected from the diffusion cells.
Description
Keywords
Isotopes, Stable Isotopes of Water, Hydrology, Hydrogeology, Transport, Water
Citation
Degree
Master of Science (M.Sc.)
Department
Civil and Geological Engineering
Program
Civil Engineering