THE EFFECT OF VERTICAL SOIL HETEROGENEITY ON FIELD SCALE SOLUTE FLUX

Vertical heterogeneity is incorporated in a stochastic transport model, in which the soil is viewed as consisting of an ensemble of essentially vertical, independent stream tubes. The saturated hydraulic conductivity in each stream tube is defined as the vertical average of the corresponding point random process. The statistics of the vertically averaged hydraulic conductivity and the resulting statistics of solute travel time are depth dependent. The evolution of the field scale breakthrough curve with depth is analyzed in terms of the first two moments of solute travel time to an arbitrary depth below the soil surface. The arrival time of the solute center of mass, quantified by the expected travel time, grows linearly with depth also for vertically heterogeneous soil. The temporal solute spreading around its center, quantified by the travel time variance, may exhibit a compression-expansion that is qualitatively analogous to the solute plume behavior in the studies of Butters and Jury (1989), Russo (1991), and Ellsworth and Jury (1991). The relative solute spreading, quantified by the travel time coefficient of variation, decreases with increasing depth.