Laboratory evidence of the scale effect in dispersion of solutes in porous media

Field observations by other authors indicate that dispersivity is dependent on distance from source. In this study, tests were conducted under controlled laboratory conditions to investigate the changes in dispersivity caused by the presence of heterogeneities. The experiments were conducted in a 2.4 × 1.07 × 0.10 m “sandbox” with measurements of tracer concentration taken at various distances from the input. Results for various sand‐packing arrangements were as follows: (1) Breakthrough curves for a uniform coarse sand showed a constant dispersivity of approximately 0.02 m. (2) Breakthrough curves for a three‐layer arrangement of coarse‐fine‐coarse deviated from the homogeneous results only in the late‐time tails. (3) The middepth fine‐sand layer was then replaced by small blocks of fine sand surrounded by coarse sand. Two distinct portions of the breakthrough curves could be identified: one for the coarse‐sand matrix and one for the heterogeneous layer. (4) Small blocks of fine sand were uniformly distributed through the entire coarse‐sand matrix. Results indicated a continuous increase in dispersivity with distance and a change in the shape of the breakthrough curve at each of five measurement sections. These results are in general agreement with several recent theoretical developments regarding transport.