The accurate estimation of soil dispersivity (a) is required for characterizing the transport of contaminants in soil. The in situ measurement of a is costly and timeconsuming. Hence, in this study, three soft computing methods, namely adaptive neuro-fuzzy inference system (ANFIS), artificial neural network (ANN), and gene expression programming (GEP), are used to estimate a from more readily measurable physical soil variables, including travel distance from source of pollutant (L), mean grain size (D50), soil bulk density (qb), and contaminant velocity (Vc). Based on three statistical metrics [i.e., mean absolute error, root-mean-square error (RMSE), and coefficient of determination (R2)], it is found that all approaches (ANN, ANFIS, and GEP) can accurately estimate a. Results also show that the ANN model (with RMSE = 0.00050 m and R2 = 0.977) performs better than the ANFIS model (with RMSE = 0.00062 m and R2 = 0.956), and the estimates from GEP are almost as accurate as those from ANFIS. The performance of ANN, ANFIS, and GEP models is also compared with the traditional multiple linear regression (MLR) method. The comparison indicates that all of the soft computing methods outperform the MLR model. Finally, the sensitivity analysis shows that the travel distance from source of pollution (L) and bulk density (qb) have, respectively, the most and the least effect on the soil dispersivity.
