Human and environmental risk monitoring of indirect potable water reuse supplied to Barcelona under a severe long-term drought period (2021–2024): a successful water resilience strategy

ABSTRACT Schematic diagram of the Llobregat River showing monitoring sites 1 (river), 2 (effluent from the water reuse plant), 3 (river after discharge), and 4 (final drinking water). The figure also includes bar‑chart risk diagrams for the four sites. Catalonia (NE Spain) underwent an unprecedented 41-month drought (2021–2024), which triggered the implementation of an indirect potable reuse (IPR) to ensure drinking water security for Barcelona. This study assesses the human-health and environmental safety of the El Prat IPR system, which releases up to 2 m3/s of advanced-treated reclaimed water into the Llobregat River, 16.6 km upstream of the drinking water intake. A comprehensive 28-month monitoring programme quantified 260 regulated and unregulated chemical compounds across four sites, from reclaimed water discharge to final treated drinking water. Human-health and environmental risks were evaluated by comparing measured environmental concentrations with compound-specific quality standards and guide values (GVs) agreed between the Catalan Public Health Authority and the Catalan Water Agency. Pharmaceuticals were dominant in reclaimed water (5–30 μg/L total) but were reduced to <0.1 μg/L in drinking water. PFAS, particularly PFOS (6–15 ng/L), and AMPA persisted in the river but remained below GVs after treatment. The findings demonstrate that the Barcelona IPR scheme is safe even when reclaimed water is discharged only a few kilometers upstream of the drinking water intake and under very low dilution conditions, confirming its viability as a drought-resilient potable supply strategy.