Water quality during extended stagnation and flushing in a college residential hall

Abstract

Water quality can change drastically within a building during periods of little to no water use: residual disinfectant can decay, leading to microbial growth, and metals can leach into the water. This study aimed to understand the change in water quality within a 4-story, 10 000 ft² residential building over five months of building closures driven by the coronavirus (COVID-19) pandemic. Sampling events occurred seven times between April and September 2020. Complete flushing of cold and hot water was performed; samples were collected immediately after flushing, then one and four weeks after flushing, with building re-occupancy occurring before four weeks. A total of 90 full samples were collected. Chlorine residual was not detectable (>0.2 mg L⁻¹) for any sample collected during the five months before flushing. Flushing refreshed chlorine concentrations, and a minimum of 0.3 mg L⁻¹ total chlorine was brought to all cold water outlets. Flushing reduced the average lead concentration in cold water from 2.4 ppb to 1.0 ppb, however, the concentration rebounded to an average of 2.6 ppb just one week after flushing. Additionally, during flushing lead was measured at a maximum of 150 ppb as discolored water slugs moved through the system. Flushing reduced Legionella spp. concentrations in cold and hot water, and these concentrations remained reduced one week later. Levels of Legionella spp. were further reduced when normal use was resumed. Results suggest that flushing can refresh water quality in the short term, but its efficacy over a longer period is uncertain.