Temporal impact of treated wastewater irrigation on field hydraulic conductivity and water-conducting macroporosity

Hydraulic properties of agricultural soils fluctuate spatially and temporally due to factors like field management (e.g., tillage and irrigation) and weather change, particularly variations in precipitation and temperature. Changes in hydraulic properties may also stem from fluctuations of salinity and sodicity due to irrigation, especially when soil is exposed to rainwater, enhancing the soil’s sensitivity to swelling. One emerging irrigation strategy involves using treated wastewater (TWW), which can contain high salt and solute concentrations, potentially impacting soil hydraulic properties. Given that the temporal and spatial variabilities could overshadow the effects induced by different irrigation sources, it is crucial to discern between these influencing factors. This study compared the effects of irrigation using three alternative TWW sources (treated urban wastewater, treated wastewater from a vegetable processing company and treated wastewater from a potato processing company) with those using rainwater only, on soil variables. Minidisk infiltrometers were applied to assess field hydraulic conductivity, that is near-saturated hydraulic conductivity (K_h) and associated soil indicators in the top 20 cm of a Retisol. For each irrigation treatment, K_h was evaluated at distinct matric suctions on various dates. Per treatment and replicated plot, ten measurements were conducted in a wet (2021) year and another ten in a dry (2022) year. Findings revealed that the tillage operation was the predominant factor affecting K_h variability. Regardless of the irrigation type, K_h and macroporosity increased post-tillage, and then gradually decreased throughout the growing season. The temporal variation in K_h also followed the rainfall events. Rainwater alternated with TWW supplied in plots decreased K_h, as lower electrical conductivity in the soil solution enhances swelling pressure by widening the diffuse double layer, following a higher diffusion tendency of salts. Variability in K_h was also influenced by yearly weather differences, with the dry year resulting in a higher average K_h. Furthermore, the variations observed between the two years could also be attributed to the cumulative impact of TWW irrigation over time. No significant differences in K_h were observed between irrigation treatments or between the two crops. Although TWW irrigation elevated soil salinity and sodicity, it did not detrimentally impact K_h or other soil attributes in this study. Our results highlight the significant impact of tillage and rainfall events, and indicated that while the effects of TWW irrigation on hydraulic properties were minimal after three years of secondary or tertiary TWW irrigation, with tillage temporarily enhancing hydraulic conductivity and macroporosity.