The Lagrangian framework presented by Dagan et al. (1992a) is used to analyze the uncertainty in predictions of the field scale mass flux of solute through the unsaturated zone. Transport of both nonreactive and degradable solutes is investigated for input sources that are located at the soil surface of fields with spatially variable hydraulic conductivity at saturation. The variances of the solute flux and accumulated mass, which quantify the corresponding prediction uncertainties, are illustrated at an arbitrary depth below the soil surface for different sizes and shapes of the input domain, and for different flow and degradation conditions. The greatest solute flux variances arise when the expected breakthrough curve has a steep slope. The coefficient of variation for the solute flux is minimum at the peak arrival time of the expected breakthrough curve; this minimum value is relatively insensitive to the assumed distribution for solute travel time and to the loss rate coefficient for degradante solute. The prediction uncertainty decreases with increasing size of the input domain and is smaller for a planar source than for a linear one. The relative uncertainty in the total leached mass of degradable solute increases with increasing loss rate coefficient.