From Quality to Purpose: Rethinking Groundwater Microbiological Standards for Emergency Urban Water Use

Climate change and increasing water scarcity are driving the need for resilient and fit-for-purpose urban water management. This study presents a case from Lisbon, Portugal, where twenty-one groundwater sources were evaluated as potential alternative supplies for emergency drinking and non-potable uses. Between 2018 and 2022, 127 samples were analyzed for microbiological (Escherichia coli, enterococci, fecal coliforms, heterotrophic plate count, Pseudomonas aeruginosa and Legionella pneumophila, physicochemical and fungal parameters (filamentous and yeast), alongside with microbial source tracking (MST) to determine contamination origins. Most sites showed exceedances of fecal indicators and heterotrophic bacteria, making water unsuitable for direct consumption without treatment, while fungi were ubiquitous and often above proposed guidance levels, highlighting a major regulatory gap. MST results indicated that human-derived contamination was rare and highly localized. Physicochemical parameters generally met legal thresholds, although occasional nitrate or salinity elevations reflected agricultural or coastal influences. Several sources were considered suitable for irrigation (EF, CC, AB, VF, and BS) whilst a subset met the criteria for potable supply with minimal treatment for risk management (CG, MM, CC, QC, EB, GR, PO, and MS). The findings of this study demonstrate that systematic, multiparametric assessment supports adaptive water allocation and emergency planning, aligning with EU regulations and advancing Sustainable Development Goal 6. The study argues for reconsideration of current microbiological standards, to improve public health protection in urban water reuse and emergency supply strategies.