5.4.2 Contamination of source waters with enteric pathogens

Source water for drinking water supplies is susceptible to contamination with enteric pathogens via faecal material from animals or humans. The extent of contamination depends upon the number of faecal sources and the level of protection of source water catchments.

Surface water

Surface waters will be contaminated to some degree with enteric pathogens from wild animal faeces. Depending on the land uses in the catchment and access to water bodies, human and domesticated animal faeces may also be present to contaminate surface waters.

Human faecal contamination may result in human pathogens entering surface water sources from:

• not having effective sanitation systems available in the presence of human activities

• failing to use available sanitation systems

• leaking sewage infrastructure

• discharging sewage from sewage treatment plants and on-site sewage management systems (whether intentional or unintentional)

• applying inadequately treated biosolids

• having public access to water bodies used to supply drinking water (such as during recreational activities).

Both wildlife and domesticated animals can carry pathogens that can infect and cause disease in humans (‘zoonotic pathogens’). The presence of humans within drinking water catchments, even at relatively low densities, can contribute to the cycle of transmission by infecting wild populations of native and feral animals with zoonotic pathogens. Animals more likely to carry higher concentrations of pathogens include:

• animals living in close proximity to human populations, especially animals most closely associated with humans (e.g. similar gastrointestinal tracts)

• more intensively reared and dense animal populations

• younger animal populations including newborn livestock (particularly as high-risk sources of zoonotic protozoan pathogens).

Faecal waste deposited on land may end up in water sources during rainfall or flooding events. High levels of discharge of enteric pathogens from land-based sources even outside of rain events may occur from the failure of faecal storage and distribution systems, such as:

• effluent storage ponds

• on-site wastewater management systems

• wastewater plumbing

• transport infrastructure

• direct human and livestock access to source water reservoirs and inflows.

International studies reporting on the environmental monitoring of pathogens in surface waters show that the presence and concentration of enteric pathogens:

• vary between surface water systems

• vary dramatically over time (e.g. varying by several orders of magnitude within hours to days or exhibiting seasonal/ event-related peaks and troughs)

• generally reach peak concentrations with increasing human activity in the catchment

• increase with increasing average E. coli concentrations.

A summary of these international studies is provided in Appendix 3. Limited published and extensive unpublished data from Australian systems confirms these trends for a variety of pathogens and faecal indicators, particularly for Cryptosporidium spp. oocysts and E. coli (Petterson et al. 2015).

Groundwater

Groundwater sources can become contaminated with enteric pathogens by a range of events including:

• surface water contaminated with pathogens (as identified above) recharging the aquifer

• direct faecal contamination of the aquifer from sub-surface inputs (e.g. seepage from on-site sewage management systems and wastewater infrastructure)

• inter-aquifer transfers leading to contamination of deeper aquifers (incorrectly assumed to be protected) e.g., through:

  • other bores

  • works

  • erosion

  • fractures or faults

  • cutting through less permeable rock or sediment (aquitards) and impermeable rock (aquicludes)

  • connecting shallow aquifers to production depths

• wildlife carrying zoonotic pathogens into bores

• surface or shallow water flowing directly into bores through a failure to install and maintain watertight headworks and fittings (e.g. vents, pressure relief structures, cable entries and surface structures).

Groundwater sources are complex and require careful monitoring and understanding to manage the potential risks from microbial contamination (see Box 5.2).

Box 5.2

Understanding and managing bore water security

For a groundwater source such as a drinking water bore to be considered “protected”, all of the possible pathways for pathogen contamination must be ruled out. A drinking water bore might be shown to be secure at a specific point in time when an assessment is undertaken. However, it is often difficult to detect if the security of a bore has failed until after contamination has occurred.

Bore water security should be discussed with the relevant health authority or drinking water regulator.

A starting position is to always assume a groundwater resource is unprotected until objective credible evidence can conclusively demonstrate otherwise.

The following actions should be undertaken to determine the safety of a bore water source and to ensure its continued security:

• Evidence of bore security should be collected and evaluated on a timely, regular basis (ideally continuous).

• Routine operational monitoring should set defensible, valid targets or operational limits. These should be linked to timely (ideally automated) corrective actions as part of reliable and auditable operational procedures and process control systems. For example, inline bore monitoring of physiochemical parameters to detect changes in water quality may suggest security has been impacted. Refer to Section 9.3 for further guidance on developing monitoring programs.

• The assessment of bore security should be carried out at both the individual bore scale and aquifer scale. This is due to the complex transport processes that occur within and between aquifers and bores.

• Suitably qualified scientific, hydrogeological and engineering knowledge should be sought to demonstrate bore security.

• Water treatment should be designed assuming that the risks in the groundwater are the same as those in the surrounding and recharging surface water, unless there is evidence of aquifer protection.

A precautionary approach should be taken when determining bore water security—multiple waterborne disease outbreaks have arisen where bores were incorrectly assumed to be secure (Hrudey and Hrudey 2004).

Other drinking water sources

There are other water sources also used for drinking water or to augment drinking water supplies which are not considered in the context of this chapter. These include:

• Roof water: Roof-harvested rainwater is typically low risk but is not free from pathogens. Outbreaks of Salmonella spp. and Campylobacter spp. bacteria have been reported from roof-harvested rainwater within Australia and elsewhere. Information on management of rainwater tanks is provided in Guidance on the use of rainwater tanks (enHealth 2010).

• Storm water, greywater and sewage catchments: Reuse of water drawn from storm water, greywater or treated sewage is addressed under the Australian Guidelines for Water Recycling (Phase 2) (NRMMC, EHPC and NHMRC 2008).

• Ocean catchments: Seawater may be used as a potable water source provided the intake is selected to minimise potential sources of contamination (e.g. enteric pathogens due to runoff from land, marine activities and discharge of effluent water). Explicitly characterising the level of contamination in seawater is not covered in these Guidelines. It is assumed that the barriers implemented to remove salt followed by disinfection would be adequate for the removal of all relevant pathogens. Further information is provided in Safe Drinking-water from Desalination (WHO 2011b).