Modelling tritium and phosphorus transport by preferential flow in structured soil

Subsurface solute transport through structured soil is studied by model int erpretation of experimental breakthrough curves from tritium and phosphorus tracer tests in three intact soil monoliths. Similar geochemical condition s, with nearly neutral pH, were maintained in all the experiments. Observed transport differences for the same tracer are thus mainly due to differenc es in the physical transport process between the different monoliths. The m odelling is based on a probabilistic Lagrangian approach that decouples phy sical and chemical mass transfer and transformation processes from pure and stochastic advection. Thereby, it enables explicit quantification of the p hysical transport process through preferential flow paths, honouring all in dependently available experimental information. Modelling of the tritium br eakthrough curves yields a probability density function of non-reactive sol ute travel time that is coupled with a reaction model for linear, non-equil ibrium sorption-desorption to describe the phosphorus transport. The tritiu m model results indicate that significant preferential flow occurs in all t he experimental soil monoliths, ranging from 60-100% of the total water flo w moving through only 25-40% of the total water content. In agreement with the fact that geochemical conditions were similar in all experiments, phosp horus model results yield consistent first-order kinetic parameter values f or the sorption-desorption process in two of the three soil monoliths; phos phorus transport through the third monolith cannot be modelled because the apparent mean transport rate of phosphorus is anomalously rapid relative to the non-adsorptive tritium transport. The occurrence of preferential flow alters the whole shape of the phosphorus breakthrough curve, not least the peak mass flux and concentration values, and increases the transported phos phorus mass by 2-3 times relative to the estimated mass transport without p referential now in the two modelled monoliths. (C) 1999 Elsevier Science B. V. All rights reserved.