CONVECTIVE DISPERSIVE STREAM TUBE MODEL FOR FIELD-SCALE SOLUTE TRANSPORT .1. MOMENT ANALYSIS

Field-scale solute transport is typically difficult to model due to th e complexity and heterogeneity of flow and transport in natural soils. The stream tube model attempts to stochastically describe transport a cross the held for relatively short travel distances by viewing the fi eld as a series of independent vertical soil columns. This study inves tigates the stream tube model with the chemical equilibrium and nonequ ilibrium convection-dispersion equation (CDE) for local-scale transpor t. A bivariate (joint) lognormal probability density function was used for three pairs of random transport parameters: (i) the dispersion co efficient, D, and the pore-water velocity, v; (ii) the distribution co efficient for linear adsorption, K-d, and v; and (iii) the first-order rate coefficient for nonequilibrium adsorption, alpha, and v. Express ions for travel time moments as a rsult of a Dirac input were derived to characterize field-scale transport according to the stream tube mod el. The mean breakthrough time for the held-scale flux-averaged concen tration, (c) over cap(f), was found to be identical to that for the de terministic CDE. Variability in D has generally a minor effect on solu te spreading compared with variability in v. Spreading of reactive sol utes increased for negatively correlated v and K-d, even if the variab ility in K-d was relatively small, while nonequilibrium adsorption fur ther increased spreading. If alpha was variable, a negative correlatio n between v and alpha enhanced the skewness of the breakthrough curve for (c) over cap(f) while spreading was independent of the correlation between alpha and v.