Browsing by Author "Rahman, Md. Arifur"

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Experimental investigations on transverse dispersive mixing in heterogeneous porous media
(2005) Rahman, Md. Arifur; Helmig, Rainer (Prof. Dr.-Ing.)
Transverse mixing has been identified to be a controlling factor in natural attenuation of extended biodegradable plumes originating from continuously emitting sources in the subsurface. The heterogeneity of natural formations causes both spreading and mixing such plumes with oxidants from the ambient groundwater. Out of these two processes, it is only the transverse mixing with oxidants from the ambient groundwater that facilitates degradation. This thesis offers a promising approach to the characterization and quantification of transverse dispersion coefficients in heterogeneous porous media. The aim is to deepen the understanding of transverse dispersion and mixing in natural heterogeneous porous media. Particularly, I develop an experimental method to determine transverse dispersion coefficients. Then, I investigate to what extent the heterogeneity of natural porous media enhances transverse mixing. To this end, I conduct and evaluate conservative and reactive tracer tests in one-dimensional as well as quasi two-dimensional laboratory and technical scale sandboxes. The heterogeneous filling used in these experiments mimics natural sediments including a distribution of different hydro-facies and micro-structures within sand lenses. The effective dispersion coefficient which describes mixing and dilution excludes the spreading of plumes. Without knowledge of spreading and meandering, it is not possible to determine the transverse effective dispersion coefficient from concentration profiles in vertical cross-sections. In order to determine this coefficient from such profiles, I corrected for the effects of plume meandering. The vertical transverse dispersion coefficients I determined from the experiments are fairly small and depend on travel time. This is as expected from analytical expressions for the time-dependent effective dispersion coefficient based on stochastic linear theory. The values obtained are less than an order of magnitude larger than the effective molecular diffusion coefficient. For typical groundwater flow velocities, therefore, the velocity-independent contribution to transverse dispersion cannot be ignored. Analytical expressions for the transverse vertical macrodispersion coefficient, also based on stochastic linear theory, predict only a small increase towards the large-time limit. In my experiments, I found that there is no significant increase in the transverse dispersion coefficient with increasing travel distance. This indicates that the heterogeneity has hardly any impact on vertical transverse mixing. In general, my findings are in very good agreement with the results of stochastic linear theory.