Tracing contamination from unsealed on-site sanitation systems: evidence from a dye tracer study in Metro city, Indonesia

Figure showing injection of fluorescein dye in unsealed OSS (left); tracer monitoring and groundwater sampling process (middle); sample of breakthrough result (right). On-site sanitation (OSS) systems can introduce faecal pathogens into groundwater, commonly indicated by Escherichia coli (E. coli) presence. However, surface activities that introduce faecal matter can also contribute to E. coli counts, as E. coli is not exclusively of human origin. To investigate the hydraulic connectivity between a subset of unsealed OSS systems and the shallow groundwater, and to assess whether these systems are likely contributors to faecal contamination at the study sites, tracer monitoring was conducted in two households in Metro, Indonesia. Fluorescein dye was injected into the OSS systems and continuously monitored using loggers and probes. Monitoring points were established in four piezometers, located 2 metres from each unsealed OSS system. Groundwater samples were analysed for E. coli twice a week. The monitoring results strongly suggested hydraulic connectivity between unsealed OSS systems and shallow groundwater, providing a plausible contamination pathway. This indicates that the unsealed OSS systems are likely contributors to faecal contamination in both households. Detection of dye in multiple piezometers suggested that the risk of exposure to contamination extended beyond the households located along the dominant groundwater flow direction. These findings emphasize the importance of maintaining a safe distance between OSS systems and groundwater sources.