Onsite sanitation systems and contamination of groundwater: A systematic review of the evidence for risk using the source-pathway receptor model

The level of risk that onsite sanitation systems (OSS) pose to groundwater quality remains uncertain. The link between contamination and OSS can only be proved if the source, pathway, and receptor are investigated and confirmed when assessing contamination. The literature on the connection between OSS and groundwater contamination has been reviewed several times but with limited assessments of the extent to which the literature confirms that the source of contamination is an OSS, that a pathway has been identified, and that the receptor is groundwater. A systematic review was conducted on published studies and supports previous work that concluded that the removal and transport of contaminants from OSS to groundwater is complex and varies significantly according to local conditions. This variability means simple siting guidelines based on horizontal separation are not reliable. Though not always recognised in the literature, formation of a biological layer is important for removal of microbial pathogens. This layer takes months to form which impacts the performance of OSS that are new or subject to highly variable loading. Under ideal conditions of an unsaturated zone comprising fine material, faecal indicator bacteria can be reduced to detection limits within 10 metres distance. However, ideal conditions are very often not present. Multiple studies showed the presence of viruses in the absence of faecal indicator bacteria. Ingress of contaminated surface water into faulty boreholes/wells and contamination of wells from users are both often significant yet not adequately covered in the included literature. The review established that it is extremely difficult to eliminate the risk of groundwater contamination when OSS effluent is discharged into the subsurface. It is unsafe to assume that contamination can be prevented in areas with high OSS density, and further research is needed to determine the critical density threshold for different ground conditions.