Consequences of Salinity-induced-time-dependent Soil Hydraulic Properties on Flow and Transport in Salt-affected Soils

A shortage of rain and water resources exits in semi-arid and arid zones, necessitating the use of marginal water such as treated sewage water (TSW) for irrigation. These waters may contain substantial amounts of mixed-sodium- calcium salts, which, in turn, interact with the soil matrix. The soil solution-soil matrix (SS-SM) interactions, particularly in the presence of smectite minerals (e.g., montmorillonite), may change the soil pore-size distribution, and, consequently, could affect the soil hydraulic properties. Since the magnitude of the SS-SM interactions depends on time-dependent flow-controlled attributes, i.e., water content, pressure head, and solute concentrations, the resultant hydraulic properties are also time-dependent. Numerical simulations of flow and transport in a three- dimensional (3-D), spatially heterogeneous, variably saturated soil, for the case in which the flow is coupled to the transport through the dependence of the soil hydraulic properties on solute concentrations, were employed in order to analyze flow and transport in salt-affected soil. The case under consideration was a citrus orchard planted on a structured clay soil in a region characterized by a distinct rainy period during the winter and a relatively long dry season, requiring irrigations. Results of the analyses suggest that enhanced SS-SM interactions, induced by increasing soil exchangeable sodium during the irrigation season and dilution of the soil solution during the rain season, may considerably reduce soil hydraulic conductivity during the rain season, particularly, close to the soil surface. Consequently, enhanced SS-SM interactions, may slow down the solute movement, may substantially increase the tailing of the mean solute breakthrough at a given horizontal control plane (CP), and may lead to mean and standard deviation solute profiles which exhibit bimodal distributions, characterized by a substantial, secondary peaks close to the soil surface. Practical implications of this study regarding the long-term use of TSW for irrigation are briefly discussed.