A review of the effects of wastewater sodium on soil physical properties and their implications for irrigation systems

This paper reviews the effects of wastewater sodium on soil physical properties, particularly with respect to irrigation systems. Fundamental sodicity concepts are examined including (i) sodicity definitions, (ii) the effects of sodium on soil properties, (iii) a discussion of factors that impede the infiltration rate and hydraulic conductivity, (iv) the changes that occur in ionic strength of percolating water in soil, and (v) consideration of the wastewater and soil constituents that modify the effective sodium adsorption ratio. Importantly, the ability for soils to assimilate wastewater over time changes, but these changes are not often considered prior to the planning of such irrigation systems, or after the irrigation systems are operating. The general lack of understanding of sodicity is in part due to the considerable variation in sodicity definitions. Exchangeable sodium percentage (ESP) values that are reported to pose a sodicity problem vary around the world due to the different mineralogy of the soils investigated, but variations in threshold ESP values have also been caused by a lack of consideration of the solution electrolyte concentration when determining ESP. In practice, the effects of sodicity may be evident in soils that are well under reported threshold values. When the effects of sodicity are identified, the landholder at least has the opportunity to implement remediation practices. However, more often than not, the effects of sodium from irrigation water are latent, leading to considerable problems following the cessation of effluent irrigation and changed land use.