Analysis of a drinking water contamination incident: Hydraulic intrusion pathways, preparedness gaps, and lessons for water safety management

Drinking water contamination events are rare in high-income countries, yet distribution-system failures can still cause significant public health impacts. This study reconstructs a microbial contamination incident caused by wastewater intrusion into the drinking water distribution system in Kuressaare, Estonia, in May 2023, using an integrated post-event review framework for water systems that complements the after-action review approach with a technical layer combining baseline assessment, operational and infrastructure documentation, water quality and hydraulic monitoring data, and extended-period and transient hydraulic modeling. The reconstruction identified transient low-pressure conditions and flow reversals as key mechanisms enabling contamination intrusion and propagation within the distribution network. The incident was associated with 135 emergency department visits for acute gastrointestinal illness, although the total number of affected individuals remains unknown. Clinical testing detected enteropathogenic bacteria, enteric viruses, or both in 39 of 40 analyzed patient samples, and healthcare contacts from multiple districts indicated broad exposure across the distribution area. The review of response actions revealed deficiencies in preparedness and crisis management, including incomplete risk assessment and project documentation, delayed recognition of contamination, and unclear communication and operational procedures. The findings demonstrate the value of integrated post-event reconstruction for identifying contamination mechanisms, system vulnerabilities and preparedness gaps, while highlighting the importance of systemic use of routinely collected operational data. Strengthening risk-based management through improved data integration, hydraulic analysis, and preparedness planning is essential for reducing the likelihood and impact of future contamination events.