Water quality monitoring that accounts for seasonal variability is crucial to ensure safe water services at all times, including groundwater self-supply, which provides drinking water for more than 40 million people in urban Indonesia. Seasonal variation of self-supply water quality remains a key evidence gap in Indonesia and elsewhere; therefore, this study investigated the associations between seasonality and faecal contamination of groundwater self-supply in the Indonesian cities of Bekasi and Metro. The study demonstrated mixed results in terms of associations between seasonality and microbial water quality. McNemar's test showed that high concentrations of Escherichia coli (E. coli) (≥100 MPN per 100 mL) were significantly more likely during the wet season than during the dry season in Bekasi (p = 0.050), but not in Metro (p = 0.694). There was no statistically significant association between the season and the presence of E. coli in self-supply sources for both study sites, nor was there a significant association between the season and the presence or high concentrations of E. coli at the point-of-use. At both study sites, presence and high concentrations of E. coli during the dry season significantly increased the risk of contamination in the wet season, but the predictive power was weak. Regular water quality testing complemented by sanitary inspection is required to understand the contamination risks of self-supply sources.

  • Insights into the relationship between seasonality and water quality of self-supply.

  • Implications for self-supply water quality monitoring in urban Indonesia.

  • Highlighting the need for regular water quality testing, complemented with sanitary inspections.

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