TEMPORAL VARIABILITY OF SOLUTE TRANSPORT UNDER VADOSE ZONE CONDITIONS

The heterogeneity of the solute flux field in the horizontal plane at the field scale has been documented in several field studies. On the o ther hand, little information is available on the persistence of certa in solute transport scenarios over consecutive infiltration cycles. Th is study was initiated to analyse the recurrence of solute leaching be haviour as estimated in two soil column tests emphasizing the preferen tial flow phenomenon. Twenty-four small-sized soil samples were subjec ted to two consecutive unsaturated steady-state flow leaching experime nts with bromide as tracer. Observed breakthrough curves (BTCs) were a nalysed by the method of moments and by the advection-dispersion equat ion (ADE) to classify solute behaviour. Frequency distributions of the parameters indicating the solute velocity were heavily skewed or bimo dal, reflecting the broad variability of the leaching scenarios, inclu ding some with pronounced preferential solute breakthrough. Exclusion of the preferential flow columns from our calculations revealed an ave rage amount of 37% of immobile water. The large-scale BTCs derived fro m assembling the individual concentration courses of each run showed s imilar features, such as an early bromide breakthrough. However, two d istinct apices, viz. one preferential and one matrix, were observed on ly in the first run, whereas the concentration decrease between the pe aks was missing from the second run. A change in soil structure with c ontinuous leaching was presumed to modify the interplay of the various flow domains, thereby altering the spreading of the BTCs. Correlation analysis between parameters of both tests suggests that preferential transport conditions are likely to occur at the same locations in the field over several infiltration cycles, whereas the 'classical' or exp ected matrix flow is time variant and therefore seems to be hardly pre dictable. (C) 1998 John Wiley & Sons, Ltd.