A formula for predicting advection-dispersion in the vadose zone at uneven drainage intervals

Abstract

A computational method is presented for predicting one-dimensional advective-dispersive solute transport with linear equilibrium sorption, for complex solute input histories associated with uneven intervals of transporting water flux. The suggested application is monitoring of leached solutes from land use and near-surface transformations, which are assumed to be transported through the vadose zone to the underlying groundwater without further degradation. Real-time forecasts of solute concentration at the groundwater surface provide a smooth feedback signal to operational management of land use for protection of groundwater, as a surrogate for groundwater monitoring which has unacceptable transport lag. The conceptual basis is the series of mixing cells as an analogue for advection-dispersion, in the form of a linear system for which cumulative pore-water drainage is the continuous index. This mixing-cell formula is unconditionally stable for any interval of drainage and assures conservation of solute mass. A demonstration example is presented.