Current freshwater sources are under pressure, while the demand is expected to increase further due to climate change and the growing world population. The pressure on existing water sources may be alleviated by using alternative water sources when possible, of which wastewater irrigation is an important example. However, the presence of contaminants in wastewater may pose a risk to the public health and the environment. For many chemicals found in wastewater, regulations have not yet been formed. Within this thesis, we model several aspects related to contaminant transport in the context of wastewater irrigation. Specific attention is given to the detrimental effect of excess sodium on the structure of clay soils. The results of this thesis may aid in developing regulations for irrigation water quality in case of wastewater irrigation.