Accounting for fecal contamination of drinking water sources is an important step in improving monitoring of
global access to safe drinking water. Fecal contamination varies with time while its monitoring is often infrequent.
Wesought to understand seasonal trends in fecal contamination to guide best practices to capture seasonal
variation and ascertain the extent to which the results of a single sample may overestimate compliance with
health guidelines. The findings from 22 studies from developing countries written in English and identified
through a systematic review were analyzed. Fecal contamination in improved drinking water sources was
shown to follow a statistically significant seasonal trend of greater contamination during the wet season (p b
0.001). This trendwas consistent across fecal indicator bacteria, five source types, twelve Köppen–Geiger climate
zones, and across both rural and urban areas. Guidance on seasonally representativewater quality monitoring by
theWorld Health Organization and nationalwater quality agencies could lead to improved assessments of access
to safe drinking water.