Risk factors for well contamination in urban Indonesia: evidence to inform siting of wells and sanitation systems

In Indonesia and many urban areas, the coexistence of on-site sanitation and groundwater supply poses faecal contamination risks. Indonesian standards recommend a minimum 10-m horizontal separation and 2-m groundwater depth for siting sanitation systems. This study evaluated the effectiveness of these criteria in Metro City by mapping wells and sanitation systems, controlling for other risk factors, and repeat measurements of groundwater depth and well contamination. E. coli was detected in 70% of wells, with a median concentration of positive samples of 47 MPN/100 mL (interquartile range 6 -727 MPN/100 mL). Although 60% of wells were within 10-m of a sanitation system, horizontal separation was not significantly associated with contamination. Shallower groundwater was significantly associated with an increased presence and high concentrations of <i>E. coli</i>. The 2-m threshold was associated with high contamination but not <i>E. coli</i> presence. Water quality and groundwater depth varied over the 2-month dry season sampling period, and associations with risk factors varied between repeat and single sample analyses. Other factors also contributed to contamination, including uncovered wells, presence of livestock and rainfall. The findings highlight the limitations of standardised siting criteria, suggesting that site-specific risk assessments may be more effective in managing water and sanitation risks.