Long term irrigation with treated wastewater (TWW) and soil sodification

• Treated wastewater is more sodic than its fresh water of origin.
• Irrigation with treated wastewater leads to soil subsurface sodification.
• Exchangeable Na % cannot be predicted by Na adsorption ratio of treated wastewater.

Use of treated wastewater (TWW) for irrigation has grown noticeably in recent years, especially in arid and semi-arid regions. The sodium adsorption ratio (SAR) in TWW is considerably higher than that in its fresh water of origin. Recently, there is evidence showing that subsurface (depth >30 cm) exchangeable sodium percentage (ESP) levels in Israeli orchards may reach 6–9 which is higher than expected following long-term irrigation with TWW having SAR <5. Our objectives were to (i) determine the ESP in soil profiles of orchards exposed to irrigation with TWW, and (ii) examine the relationships between the SAR of the irrigation water, the SAR of the soil solution and the ESP of the studied soils. Soil samples were taken from different depths (up to 120 cm) in orchards grown on two different soil types that had been irrigated for >10 years with TWW. In each soil sample non-adjusted SAR and adjusted SAR (SARadj) of the saturated paste and ESP were determined. In all sampled sites except one, accumulation of adsorbed sodium in the soil subsurface was noted. The obtained ESP levels were higher than those expected based on the SAR of the TWW. A satisfactory agreement was noted between ESP data and the non-adjusted SAR and SARadj of the soil solution. These observations suggest that a chemical equilibrium exists between the soil exchange phase and the soil solution and that the properties of the latter were not always dictated by those of the irrigation water.