Effect of separation distance between on-site sanitation systems and groundwater sources on faecal contamination: A...

Around 3.8 billion people globally rely on on-site sanitation (OSS) systems, despite concerns about the potential to cause faecal contamination of groundwater. Many studies have investigated whether the distance between OSS systems and groundwater sources affects contamination risk, but quantitative synthesis remains limited. Therefore, this systematic review and meta-analysis aim to quantitatively synthesise how distance and moderating factors affect groundwater contamination from OSS systems. Criteria for eligible studies included publication in English between 1900 and September 2025 and the provision of extractable data on faecal indicator bacteria (FIB) levels and the distance between OSS systems and groundwater. In total, 59 studies were included. Risk of bias was investigated using funnel plots, Egger's test, and by comparing study designs and quality scores. Data were analysed using random effects meta-analyses and forest plots. Meta-analyses consistently found greater horizontal distance is associated with lower FIB contamination. A pooled effect of -0.25 and very high heterogeneity (I² = 93%) was identified from the most robust model in between study analysis. Within study analysis showed ∼92% attenuation in FIB concentrations per 10 m increase in horizontal distance. Eligible studies were limited for vertical distance analysis; therefore, further research is needed. The results are robust based on the assessed risk of bias. This study suggests that horizontal setback distance should be understood as a risk gradient rather than a universal safe threshold. As reflected by the high heterogeneity, a single generic horizontal setback distance is unlikely to provide the same protection across different contexts.