1.3.4 Physical and chemical guideline values

The Guidelines include two different types of guideline value for physical and chemical characteristics:

• a health-based guideline value, which is the concentration or measure of a water quality characteristic that, based on present knowledge, does not result in any significant risk to the health of the consumer over a lifetime of consumption;

• an aesthetic guideline value, which is the concentration or measure of a water quality characteristic that is associated with acceptability of water to the consumer; for example, appearance, taste and odour.

The guideline values should be used in two separate but complementary ways: as action levels for the short-term verification of drinking water quality, and as a means to assess performance over the longer term (e.g. over a 12-month period). Using a guideline value for short-term verification entails assessing whether individual results conform to the requirements of good quality water. If a value is exceeded, some form of immediate corrective action will generally be initiated. For example, if a guideline value for a health-based characteristic is exceeded, the response should be to take immediate action to reduce the risk to consumers, and, if necessary, to advise the relevant health authority or drinking water regulator and consumers of the problem and the action taken. If the characteristic affects only aesthetic quality, the action may be to advise the community of deterioration in water quality.

When guideline values are used in assessing overall performance (e.g. as presented in an annual report), the aim is to assess whether management strategies are effective. The assessment is used to identify emerging problems and to determine priorities for improvement. Resulting actions will generally be applied in the longer term.

The guideline values relate to the quality of water at the point of use (e.g. kitchen or bathroom tap). They apply to reticulated water at the consumer’s tap, rainwater for drinking, and source water if it is to be used without prior treatment. This does not, however, imply that the drinking water supplier is responsible for water quality problems caused by plumbing or other factors within a consumer’s property. However, although it is not possible to control consumers’ actions, suppliers should consider how drinking water quality may be affected in private plumbing systems and provide appropriate information to consumers.

The drinking water supplier should ensure that the quality of water in the reticulation mains meets the guideline values or agreed levels of service. The drinking water supplier would normally monitor quality in a service pipeline directly off a water main selected to represent the quality of water in the system. This is not usually within a private consumer’s property. However, it may sometimes be necessary to check at the consumer’s tap, either to confirm that chosen distribution sampling points are representative for microbial monitoring, to investigate specific problems such as leaching of metals into water, or as a consumer service.

The guideline values define water that, based on current knowledge, is safe to drink over a lifetime; that is, it constitutes no significant risk to health. For most of the water quality characteristics discussed, there is a grey area between what is clearly safe and what is clearly unsafe. Often the latter has not been reliably demonstrated and the guideline values always err on the side of safety. Therefore, for most characteristics, occasional excursions beyond the guideline value are not necessarily an immediate threat to health. The amount by which and the duration for which any health-based guideline value can be exceeded without raising concerns for public health depends on the particular circumstances. Exceeding a guideline value should be a signal to investigate the cause and, if appropriate, to take remedial action. If the characteristic is health related, the relevant health authority or drinking water regulator should be consulted.

Nevertheless, the Guidelines provide the minimum requirements for drinking water of good quality, both aesthetically and from a public health viewpoint. Water suppliers should adopt a preventive risk management approach, as stipulated in the Guidelines, to maintain the supply of water at the highest practicable quality. The guideline values should never be seen as a licence to degrade the quality of a drinking water supply to that level.