On-site sanitation system and fecal contamination in shallow groundwater in urban Indonesia: assessing influence of distance and rainfall variables

Over 41 million people in urban Indonesia self-supply drinking water from private wells. However, unsealed on-site sanitation systems (OSSs) located near these groundwater sources raise concerns about fecal contamination. While previous research has discussed on-site sanitation system (OSS)-related risks, limited studies have quantitatively examined how lateral and vertical distance between OSS and groundwater sources influence contamination risk using a clear source-pathway-receptor model. The effects of climatic variability, particularly rainfall, also remain poorly understood. Given the widespread use of wells and OSSs in Indonesia, understanding these factors is essential for developing evidence-based water and sanitation policies. A year-long field study was conducted in Metro, Indonesia, to investigate how fecal bacteria from OSS migrate into groundwater. Seventeen piezometers, each 10 meters deep, were constructed at three sites. At each site, piezometers were arranged in cruciform pattern, located at two distances (2 m; 5 m) from the existing OSSs. Water samples were collected monthly, with more frequent sampling during one month of the wet season. Results showed that Escherichia coli (E. coli) contamination exhibited temporal variation, increasing during the rainy season. A seasonal flush was observed at its onset of the rainy season. This was marked by an approximate 2-log increase in E. coli levels, rising from 3 to 199 MPN/100 ml in the nearest piezometer and from 5 to 461 MPN/100 ml in the farthest piezometers. The number of heavy rain days and cumulative rainfall over a 3-day lag period significantly increased the odds of E. coli contamination. Over a 7-day lag period, only total rainfall remained significant. Greater horizontal and vertical distance between the OSS and groundwater significantly reduced the likelihood of E. coli contamination, with the impact of horizontal distance being more pronounced during wet months. These findings highlight the importance of vertical distance in sanitation-groundwater interaction, especially during wetter months. Incorporating vertical distance into latrine siting guidelines is recommended to reduce microbial contamination risk in groundwater and improve public health for millions relying on self-supplied water.