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The extensive school closures due to the unprecedented COVID-19 pandemic resulted in prolonged water stagnation within schools' plumbing for longer durations than routine schools' holidays and summer breaks. With many of the U.S. schools suffering from problems of lead (Pb) in potable water for decades, the extended water stagnation caused by schools' closure has raised significant concerns regarding the schools' water safety. Thus, this research was conducted to evaluate the resiliency of schools' potable water plumbing toward the interruption caused by the COVID-19 pandemic. For this purpose, the impact of extended water stagnation on heavy metal release into water samples collected from fixtures with and without known lead problems in 25 schools within a school district in Tennessee was investigated. The results revealed a significant increase in the median Pb concentration due to the extended water stagnation. Furthermore, elevated levels of Fe, Zn, and Cu were released from both problematic and nonproblematic fixtures into tap water. Estimation of children's blood lead level (BLL), assuming the consumption of prolonged stagnated water, revealed an increased risk of elevated BLLs (>5 μg dL −1 ). To better identify the potential sources of lead release within schools, a combination of plumbing investigation and sequential water sampling was conducted. The lead-containing fixtures, connecting plumbing, and interior plumbing were found as the possible sources contributing to the lead release into water. Implementation of remediation actions reduced the lead release into tap water to less than 3.4 μg L −1 in the target fixtures. 

Extensive building closures due to the unprecedented COVID-19 pandemic resulted in long-term water stagnation within the plumbing of large buildings. This study examined water chemical quality deterioration in ten large buildings after prolonged stagnation caused by the closure of a university campus in response to the COVID-19 pandemic. Volume-based and constant-duration flushing protocols were implemented to replace stagnant water with fresh drinking water. The effectiveness of the developed water flushing protocols was examined by monitoring the disinfectant residuals, heavy metal concentrations and temperature for water samples collected from the buildings' point of entry (POE) and select water fixtures. More than 14 m 3 of water were flushed in all ten large buildings. The results demonstrated a significantly greater average total chlorine residual concentration in POE water samples collected after flushing (1.1 mg L −1 ) compared to the stagnant condition (0.6 mg L −1 ). For water samples collected from fixtures during the extended stagnation, chlorine was absent in 71% of samples from academic buildings and 69% of samples from athletic buildings. The effectiveness of flushing practices is underscored by increasing the median total chlorine concentration from <0.1 to 1.0 mg L −1 in academic buildings and from <0.1 to 0.75 mg L −1 in athletic buildings. Furthermore, the concentrations of Pb, Zn, and Cu had decreased following the water flushing, but the concentration of Fe had increased in some buildings. This study could be beneficial to prepare for prolonged water stagnation events including but not limited to pandemics.