Experimental Investigation of Solute Transport in Stratified Porous Media: 1. The Nonreactive Case

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Abstract
The migration of a nonreactive solute in layered porous media was studied under controlled laboratory conditions by injecting a tracer into a thin sand layer bounded by silt layers. The experimental results show a delay in the breakthrough of the tracer in the effluent of the permeable sand layer along with a substantial decrease in concentrations after a 1‐m travel distance. The strongly dispersed and skewed shape of the breakthrough curves for the layered system is in contrast to the symmetric and weakly dispersed concentration patterns typically associated with homogeneous media. Because of the tailing of the breakthrough measurements, a departure from classical Fickian transport is apparent. Simulations of the experiments demonstrate that these effects are the result of a transient redistribution of the tracer across the strata by transverse molecular diffusion and that local longitudinal dispersion is only of secondary importance as a spreading process in such systems. These findings are consistent with recent theoretical descriptions of dispersion in stratified aquifers.