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 (SPR) are investigated and confirmed when assessing contamination. The literature on the connection between OSS and groundwater contamination has been reviewed several times. However, previous reviews have made 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, mainly due to uncertainties or insufficiencies in reported methodologies.A systematic review was conducted on published studies with the aim of establishing the state of knowledge of groundwater contamination from onsite sanitation systems that infiltrate wastewater into ground, such as pit latrines and septic tank systems. The quality of the methods used to establish the source, pathway and receptor was assessed. The review focussed on three main contaminants: bacteria, viruses and nitrates.Scopus, PubMed, and Web of Science databases were searched, screening criterion developed and applied, and 60 eligible papers were identified. 35 of these were rated as having a “strong methodology”. The selected studies were assessed to identify areas of knowledge regarding the interaction of OSS with groundwater where there is strong supportive evidence and where there are knowledge gaps.The review supports previous work that has concluded that the removal and transport of contaminants from OSS to groundwater is complex and varies significantly according to local conditions, including the nature of the ground materials, groundwater levels, ground moisture and other factors. 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 or sudden increases in hydraulic load.Under ideal conditions of an unsaturated zone comprising fine material (sands, silts, loams with some clay content, 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. Contamination can also occur via localised pathways, but these have not been thoroughly investigated. Contribution of the ingress of contaminated surface water into faulty boreholes/wells and contamination of wells through the spigot or spout 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. Generally, it seems unsafe to assume that contamination can be prevented in areas where OSS occur with any frequency.