4.2.2 Preventive measures for drinking water quality management

Where there are hazards that represent high risks, preventive measures (described in detail in Section 3.3) will be required to remove the hazard or to reduce it to an acceptable level. The effectiveness of existing measures should be assessed, but if these are not sufficient, alternative measures will need to be identified. As with all systems, assessment of preventive measures should include consideration of the important principle of the multiple barrier approach. The types of barriers and the preventive measures required will depend on the characteristics of the source water and the associated catchment (the principles and approach for assessing the contamination of source waters and management of those risks are set out in Sections 5.4, 5.5 and 5.6).

Groundwater

The starting position is always to assume that a groundwater source is unprotected until objective credible evidence can conclusively demonstrate otherwise. More information on understanding and managing bore water safely can be found in Box 5.2.

In most cases, contamination of groundwater supplies can be prevented by a combination of simple measures to ensure all possible pathways for pathogen contamination are ruled out. Groundwater in confined or deep aquifers will generally be free of pathogenic microorganisms and, providing the water is protected during transport from the aquifer to consumers, microbial quality should be assured. The local vicinity of the borehead should be protected from livestock access, and buffer zones should be established between the bore and disposal or discharge of septic wastes. Bores should be encased to a reasonable depth and boreheads should be sealed to prevent ingress of surface water or shallow groundwater.

Once the groundwater is pumped out of the aquifer, protection can be achieved by delivering the water through enclosed water systems. Storage tanks should be roofed, pipelines should be intact and cross-connections should be protected by the installation of backflow prevention devices.

Rainwater

Rainwater systems, particularly those involving storage in above-ground tanks, generally provide a safe supply of water. The principal sources of contamination are birds, small animals and debris collected on roofs. The impact of these sources can be minimised by a few simple measures: guttering should be cleared regularly; overhanging branches should be kept to a minimum, because they can be a source of debris and can increase access to roof catchment areas by birds and small animals; and inlet pipes to tanks should include leaf litter strainers. First-flush diverters, which prevent the initial roof-cleaning wash of water (20–25 L) from entering tanks, are recommended. If first flush diverters are not available, a detachable downpipe can be used to provide the same result.

Further information on rainwater tanks is provided in Guidance on the use of rainwater tanks (enHealth 2010).

Surface water

Assurance of quality from surface water sources is more difficult than from most groundwater or rainwater systems. In general, surface waters will require at least disinfection, and in some cases filtration, to assure microbial safety. However, as for groundwater systems, the first barrier is to prevent contamination at source by minimising contamination from human waste, livestock and other hazards as discussed above. The greater the degree of protection of the water source, the less the reliance on treatment and disinfection. After treatment or disinfection, water should be protected during delivery to consumers in the same manner as groundwater, by ensuring that distribution systems are enclosed. See Section 5.4.2 for information on contamination of source waters with enteric pathogens.