Effect of Long-Term Irrigation with Treated Wastewater on the Root Zone Environment

The increasing demand for freshwater (FW) for domestic use turns treated wastewater (WW) into an attractive source of water for irrigated agriculture. The main goal of this study was to evaluate the impact of 16 yr of irrigation with WW on the conditions that developed in the root zone of avocado trees planted on clayey soil and compare with FW use. High-resolution field sampling determined the spatial distribution of chloride, exchangeable sodium percentage, and dissolved organic content below the dripper, revealing higher salinity and sodicity, lower hydraulic conductivity, and possible preferential flow pattern linked to wettability in WW-irrigated soils. Laboratory measurements on disturbed samples showed that higher swelling pressure developed in the 20- to 40-cm and 40- to 60-cm layers of the WW-irrigated soil. Finally, continuous monitoring of oxygen concentration at the 10-, 20-, and 30-cm depths in the root zone near the trees and halfway between adjacent trees revealed that the oxygen level at the 20-cm depth was the most affected by WW irrigation. During the rainfall season, this layer could experience relatively long periods with minimal oxygen concentrations. During the irrigation season, less oxygen is available in that layer than in the FW-irrigated one. Dynamics of oxygen concentration at the 30-cm depth show a clear event of wetting and drainage in the FW-irrigated plots, while the relatively stable high oxygen level in that depth in the WW-irrigated plots might reveal nonuniform wetting, insufficient water percolation due to low hydraulic conductivity, and related low leaching efficiency of the soil profile.