Quantification of advection and dispersion in lateral subsurface flowpaths at the hillslope-scale Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/tb09j814h

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  • It is becoming increasingly important to understand fundamental hillslope-scale hydrological processes. Most hillslope-sale transport experiments have generally focused on conceptual findings or other aspect of flow behavior, rather than the quantification of the mass transport mechanisms of advection and dispersion. When the velocities have been quantified, dispersion has been mentioned as present, but has not yet quantified. This study uses a natural gradient well-injection tracer test to characterize solute transport in lateral subsurface flowpaths. The breakthrough curves obtained from the tracer tests were analyzed using traditional hillslope hydrology methods for calculating velocities, time to peak and the Mosley [1982] method, as well as CXTFIT, a program that computes average velocity and dispersion coefficients for breakthrough curves by fitting experimental data to the 1-Dimensional convective-dispersion equation using a non-linear least-squares regression technique. Well injection tracer tests at the WS10 hillslope showed advection and dispersion rates larger than reported from laboratory studies and comparable rates to those reported from field studies. Lateral preferential flowpaths appeared to significantly reduce travel time through the study hillslope. However, once tracer was stored in the subsurface, the travel times and average velocities depended largely on the applied driving force (i.e. intensity and duration of precipitation and/or injection). The tracer tests also illustrated that as less tracer remained in the flowpath the amount of water required to remove a quantity of mass increased. Through the quantification of advection and dispersion in Experiment 1, it was shown not only that one flowpath had a larger advective velocity but also mixed with the tracer-free water more due to it’s higher velocity. In addition, the dispersion coefficients showed that while Inj1 and Inj2 have very similar BTCs, Inj2's dispersion coefficient was about twice that of Inj1's, indicating that Inj2 spread longitudinally more than Inj1. Through the quantification of advection and dispersion in Experiment 2, the large effect of stored water on apparent dispersion was illustrated. As stored water plays a leading role in the hillslope hydrology, accounting for the dispersion that results from storage is imperative to accurately describing internal hillslope hydrological processes.
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