Effect of water flux on solute velocity and dispersion

A systematic analysis of the effect of displacement length, soil texture, and steady state water flux (q) on solute transport was performed on saturated repacked loam and sandy loam soil columns to identify the simplest possible description that was consistent with the observations across all of the breakthrough curves (BTCs) for each data set. All 39 BTCs obtained from loam and 21 from sandy loam soils, with column lengths of 10, 20, and 30 cm, and a two-order magnitude variation in measured pore water velocity (ν_m), were fitted simultaneously and global estimates of parameters [dispersivity (λ) and molecular diffusion coefficient (D₀)] were obtained. We showed that a two-parameter global dispersion relationship (D = λν_m + D₀, where D is the apparent diffusion coefficient) accurately represents the spreading process for all 39 loam soil BTCs (r² > 0.99), whereas a single global parameter (D = λν_m) was all that was required for the sandy soil (r² > 0.98). The globally fitted λ was independent of displacement length and ν_m. D remained independent of ν_m in the lower velocity ranges; however, a linear relationship between ν_m and fitted D was obtained for ν_m > 0.1 cm h^-1. The results of this study illustrated the importance of molecular diffusion. In addition, we identified a nonlinear relationship between ν_m and average solute velocity (ν_s), which suggested that the anion exclusion volume decreased with increasing ν_m.