Long-Term Irrigation with Treated Waste Water in Agriculture: Soil Responses, Benefits and Sustainability Risks

Abstract

Treated wastewater irrigation can reduce dependence on freshwater and simultaneously supply nutrients and organic matter to agricultural soils, but repeated application may alter soil properties through cumulative loading of salts, sodium, metals and persistent contaminants. This review critically evaluates the long-term soil consequences of treated wastewater irrigation, with emphasis on salinity and sodicity, nutrient and organic-matter dynamics, heavy-metal accumulation, hydraulic and structural properties, biological responses, and emerging contaminants. Evidence was synthesized from long-term field investigations together with shorter-term studies addressing crop response, contaminant uptake and microbial processes, while responses were interpreted in relation to wastewater quality, irrigation duration, soil characteristics, climate, drainage and management. The evidence shows that wastewater irrigation commonly increases soil electrical conductivity and sodium-related indicators, although the magnitude and functional significance vary substantially among sites. Long-term studies also report increases in soil N, organic matter and available metals, demonstrating that nutrient recovery and contaminant accumulation can occur simultaneously. Hydraulic responses were inconsistent: substantial reductions in hydraulic conductivity were reported in some long-term systems, whereas more than 50 years of irrigation at the humid Living Filter site produced no significant reduction in saturated hydraulic conductivity despite increased soil salinity and SAR. Biological responses were similarly heterogeneous, ranging from increased microbial biomass and enzyme activity to shifts in microbial community composition without significant loss of overall diversity. Selected pharmaceuticals were detected in wastewater-irrigated soils and crops, including diclofenac accumulation in tomato fruit, but available evidence remains insufficient to establish a general long-term health or ecological risk. Evidence of recovery following freshwater substitution further indicates that some salinity, sodicity and structural changes can be reversible, whereas heavy metals represent a more persistent cumulative concern. Overall, the sustainability of treated wastewater irrigation cannot be judged from crop productivity, wastewater treatment status or any single soil parameter. It depends on whether cumulative inputs remain within the capacity of the soil–water–climate–management system to accommodate, transform or remove them without progressive loss of essential soil functions.