STOCHASTIC MODELING OF SOLUTE FLUX IN THE UNSATURATED ZONE AT THE FIELD SCALE

Spatial variability in both the saturated hydraulic conductivity and the pore-size distribution is incorporated in a stochastic model of field-scale solute transport through the unsaturated zone. The transport model considered is based on a Lagrangian framework, in which the expected field-scale solute flux and the associated uncertainty in predictions are defined from distributions of solute travel time. Expressions for such travel time distributions are derived from statistical information on measurable soil hydraulic properties and are used to analyze the statistics of the field-scale solute flux in heterogeneous soil. The spatial variability in the saturated hydraulic conductivity and in the pore-size distribution has a similar effect on the prediction uncertainty, quantified by the variance of the solute flux, as on the expected breakthrough curve. The effect on the relative prediction uncertainty, quantified by the coefficient of variation for the solute flux, is therefore small. Both the solute flux variance and the coefficient of variation decrease rapidly with increasing size of the input domain and are considerably smaller for a planar source than for a linear one.