Numerical analysis of flow and transport in a combined heterogeneous vadose zone-groundwater system

Numerical simulations of water flow and solute transport were used to inves tigate the effect of the recharge (rain/irrigation) at the soil surface on solute spreading and breakthrough in a realistic, three-dimensional, hetero geneous, vadose zone-groundwater flow system. Results of the analyses sugge st that smaller recharge at the soil surface increases the relative variabi lity in the response of the unsaturated how system, decreases the groundwat er recharge from the unsaturated zone, and, concurrently, decreases the deg ree of nonuniformity of the groundwater flow. Consequently, smaller recharg e at the soil surface enhances the longitudinal and, especially the transve rse spreading of the solute plume and increases the spreading of the expect ed solute ETC at the lower boundary of the unsaturated zone. Furthermore, s maller recharge at the soil surface decreases the extent of the vertical pe netration of the solute plume into the groundwater, slows the peak arrival of the expected ETC, and considerably increases the spreading of the expect ed solute ETC at a given vertical control plane in the saturated region. Tt was shown that for the combined flow system considered here in which the s olute plume is displaced to a sufficiently large vertical distance from the inlet zone at the soil surface, the variable that controls the transport i s the cumulative recharge at the soil surface, and not the recharge rate. ( C) 2000 Elsevier Science Ltd. All rights reserved.