Laboratory evidence of the scale effect in solute transport through saturated porous media

Abstract

Field observations of the spread of tracers in saturated ground-water systems indicate that the dispersivity is dependent on the scale of measurement. Laboratory tests were conducted in a sandbox model (96"x42") to investigate changes in the dispersivity and deviations from expected breakthrough curves caused by heterogeneities. Measurements at several depths were averaged to give one-dimensional breakthrough curves at several points. Results from these experiments were: 1) Breakthrough curves for homogeneous sand showed little deviation from classical theory. 2) Fine sand between two layers of coarse sand gave breakthrough which deviated from homogeneous results only in the late time data. 3) Replacing one-third of the flow region by a layer of uniformly distributed blocks of fine sand, two distinct portions of the breakthrough curve were identified; one for the homogeneous coarse sand (giving a constant dispersivity), and one for the heterogeneous layer (which showed an increase in dispersivity with distance). 4) When fine sand blocks were uniformly distributed over the entire system, results indicated a continuous increase in dispersivity and significant change in the shape of the breakthrough curve with distance. To the author's knowledge, these experiments provide the first quantification of a scale effect under controlled laboratory conditions.