Spatial Assessment of Water Quality Along a Collective Surface Irrigation Network Under Blending Water Reuse Conditions

Abstract Agricultural water reuse involving the blending of treated wastewater (TWW) with conventional irrigation water in open irrigation networks is not explicitly regulated under Regulation (EU) 2020/741, which may present exposure pathways comparable to unblended direct reuse. This study assessed water quality dynamics along a fully operational irrigation canal network in Northern Italy, under real-scale hydraulic and agronomic conditions characteristic of reuse systems involving blending, in which TWW discharged from a wastewater treatment plant (WWTP) accounted for 68% to 86% of total canal flow. Over two irrigation seasons (2023–2024), 11 physical, chemical, and microbiological parameters were monitored at five points: one at the WWTP outlet, one upstream representing the conventional irrigation source, and three downstream of the confluence across 30 campaign days. Data were analyzed using generalized linear mixed models with sampling point as a fixed effect and sampling date as a block-level random intercept. The contribution of TWW to conventional irrigation water was primarily reflected in nutrient enrichment, with concentrations downstream of the confluence exceeding those of the conventional source by 48% to 59% for total nitrogen and 139% to 195% for total phosphorus, confirming the WWTP as the main nutrient source. In contrast, the effluent introduced no additional load for the parameters directly regulated under Regulation (EU) 2020/741 ( E. coli , biochemical oxygen demand (BOD₅), and total suspended solids (TSS)). TSS were significantly lower in TWW than in conventional irrigation water, leading to a 29% to 43% reduction in TSS after mixing, BOD₅ at the WWTP outlet did not differ significantly from that in conventional irrigation water, and for E. coli no significant effects were detected. However, BOD₅ increased significantly at one point after the confluence relative to the conventional source, from an input that could not be attributed to the effluent. Overall, these results demonstrate that although TWW accounted for the majority of canal flow in volumetric terms, it did not lead to generalized deterioration of irrigation water quality. Instead, it provided nutrient inputs that may be harnessed for agricultural reuse. This work presents one of the first systematic, real-scale assessments of TWW reuse through blending in a fully operational European irrigation district, illustrating the regulatory gap in Regulation (EU) 2020/741 and informing risk-based monitoring frameworks for reuse systems where blending occurs.