Preferential solute transport under variably saturated conditions in a silty loam soil: Is the shallow water table a driving factor?

Abstract

A shallow water table might enhance preferential solute movement by modifying both the water flow and solute dynamics. In this study, we estimated soil hydraulic and solute transport parameters through a tracer experiment in lysimeters comparing different water table levels. In a set-up of 12 lysimeters, the bottom boundary condition was set as a water table depth of 120 cm, or 60 cm, or as free drainage. A tracer solution of bromide (250 mg l−1, 40 mm) was added to each lysimeter and soil water was sampled with suction cups at different depths for the following 174 days. Soil water content and matric potential were monitored using TDR probes and electronic tensiometers at the same depths. Soil hydraulic and solute transport parameters in different soil layers were estimated by inverse modeling using HYDRUS 1D. Soil hydraulic parameters were estimated from the Mualem-van Genuchten equations, while both the advection–dispersion (ADE) and physical non-equilibrium mobile-immobile water (MIM) models were used to describe the solute transport. Moreover, the soil pore network was analyzed by means of 3D X-ray microtomography. Results showed different solute dynamics between contrasting water table managements. With free drainage, solute in the immobile domain was negligible, and its transport was fully associated with the mobile water flow. In contrast, a shallow water table affected the tracer transport, by modifying a) the soil pore network, with an increase of the macropores and a reduction of the pore connectivity; b) the flow field, with an increase of immobile water and a reduction of αMIM, indicating slow exchange between mobile and immobile regions, in turn promoting preferential pathways. Hence, groundwater pollution might be worsened by preferential solute transport of agrochemicals occurring with shallow water table conditions.