Integrated modelling of impact of re-use of waste water in agriculture 

In arid and semi-arid regions, there is often a scarcity of good quality irrigation water. Therefore, marginal water is used, like treated wastewater effluent. Wastewater can contain large amounts of contaminants. The type of contaminants and the concentration levels depend on the origin of the wastewater, e.g. domestic or industrial. To determine if wastewater irrigation is a sustainable practice, the transport of contaminants must be predicted, which is done by modelling. An integrated approach must be used, considering the effect on the quality of the root zone, soil structure and the groundwater simultaneously. Due to the large number of contaminants, not every contaminant in wastewater can be considered and thus the focus will be on transport characteristics like sorbing/non-sorbing and degrading/non-degrading. Focus will be given to the effect of spatial and temporal variability on the transport of contaminants, as they can have a significant effect on solute transport.

Major focus of this project will be on the effect of wastewater irrigation on the soil structure. Wastewater often contains large amounts of sodium. This leads to increased sodium concentrations the cation exchange complex, which in combination with low salinity soil waters can lead to clay swelling and dispersion. This decreases the hydraulic conductivity of the soil and decreases the air and water flow in the soil. This process is costly and difficult to reverse, and therefore sodification should be prevented instead. This can be done by using models to predict when soil sodification occurs.

This topic will be modelled on the pore-scale, using pore-network modelling. The soil is simplified as a number of connected pores (Figure 1). The pores have simple shapes, e.g. spheres, cylinders, rectangles, so that the governing equations can be simplified. Flow and transport can then be simulated across the pore network, and the results can be averaged to obtain useful information for the larger scales.