6.2.2 Approach used in derivation of guidelines values for physical characteristics

In general, the physical characteristics of water are not of direct public health concern, but they do affect the aesthetic quality of the water, which largely determines whether or not people are prepared to drink it. If water is unpalatable or appears to be of poor quality, even though it may be quite safe to drink, the consumer may seek other water sources, and these may not be as safe.

Each guideline value is set at a level that ensures good quality water – that is, water that is aesthetically pleasing and safe, and that can be used without detriment to fixtures and fittings. The values are determined by considering water quality guidelines used by other countries and international bodies, assessing any health implications, and then deciding on a point beyond which the quality of the water might no longer be regarded as good. Factors taken into account include:

• taste and odour thresholds (i.e. the smallest concentration or amount that would be just detected by a trained group of people);

• the concentration or amount that would produce noticeable stains on laundry or corrosion and encrustation of pipes or fittings;

• the concentration or amount that would be just noticeable in a glass of water and lead to a perception that the water was not of good quality.

The physical guideline values are not absolute; they are value judgments determined from an often wide range of values that may be broadly classed as acceptable – that is, there is no one right answer. Consequently, small, short-term excursions beyond a physical guideline value do not necessarily mean that the water will be unacceptable. What is aesthetically acceptable or unacceptable depends on public expectations, and must ultimately be determined by water authorities in consultation with consumers, taking into account the costs and benefits of further treatment. The Australian Drinking Water Guidelines provide a starting point for this process.