The relation between the mass transport rate and the spatial distribution of retardation coefficient, R d , has been examined by using two-dimensional (2-D), advection-dispersion model. In the calculated breakthrough curves, this study focused on the peak height and its arrival time. To compare them easily, the dimensionless mean residence time was always set at unity. In the results, as the R d values were distributed perpendicular to the flow direction, the peak height and its arrival time

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... ongly depended on the R d distribution. This study, for simplicity, considered two kinds of R d layers and assumed that the small R d and the large were arranged parallel to one another. The smaller the alternation frequency of the layers became, the higher peak and the shorter arrival-time the breakthrough curve showed. In contrast, when the frequency was large enough, the peak-arrival time almost agreed with the homogeneous case. Further, this study confirmed that the variation of the skewness of R d had no appreciable influence on the whole mass transport rate. When the 2-D distribution of R d was described by, e.g., log-normal distribution, the average mass transport rates showed agreement with those on the other probability density functions defined by the same set of the arithmetic mean and the standard deviation of R d . These tendencies mentioned above were confirmed in the range of Peclet number from 10 to 10 2 for the dimensionless standard deviation at least up to around 1.