The colour of drinking-water is usually due to the presence of coloured organic matter (primarily humic and fulvic acids) associated with the humus fraction of soil. Colour is strongly influenced by the presence of iron and other metals, either as natural impurities or as corrosion products. It may also result from the contamination of the water source with industrial effluents and may be the first indication of a hazardous situation. The source of colour in a water supply should be investigated, particularly if a substantial change takes place.

Colours above 15 TCU (true colour units) can be detected in a glass of water by most people. Colours below 15 TCU are usually acceptable to consumers, but acceptability may vary according to local circumstances.

No health-based guideline value is proposed for colour in drinking-water.

The presence of aluminium at concentrations in excess of 0.2 mg/litre often leads to consumer complaints as a result of deposition of aluminium hydroxide floc in distribution systems and the exacerbation of discoloration of water by iron; concentrations between 0.1 and 0.2 mg/litre may give rise to these problems in some circumstances.

Available evidence does not support the derivation of a health-based guideline value for aluminium in drinking-water (see summary statement).

The threshold odour concentration of ammonia at alkaline pH is approximately 1.5 mg/litre, and a taste threshold of 35 mg/litre has been proposed for the ammonium cation.

Ammonia is not of immediate health relevance, and no health-based guideline value has been proposed (see summary statement).

High concentrations of chloride give an undesirable taste to water and beverages. Taste thresholds for the chloride anion depend on the associated cation and are in the range of 200–300 mg/litre for sodium, potassium, and calcium chloride. Consumers can become accustomed to concentrations in excess of 250 mg/litre.

No health-based guideline value is proposed for chloride in drinking-water (see summary statement).

The presence of copper in a water supply may interfere with the intended domestic uses of the water. Copper in public water supplies increases the corrosion of galvanized iron and steel fittings. Staining of laundry and sanitary ware occurs at copper concentrations above 1 mg/litre. At levels above 5 mg/litre, it also imparts a colour and an undesirable bitter taste to water.

Although copper can give rise to taste problems, the taste should be acceptable at the health-based provisional guideline value (see summary statement).

Public acceptability of the degree of hardness of water may vary considerably from one community to another, depending on local conditions. The taste threshold for the calcium ion is in the range of 100–300 mg/litre, depending on the associated anion, and the taste threshold for magnesium is probably less than that for calcium. In some instances, a water hardness in excess of 500 mg/litre is tolerated by consumers.

Depending on the interaction of other factors, such as pH and alkalinity, water with a hardness above approximately 200 mg/litre may cause scale deposition in the distribution system and will result in excessive soap consumption and subsequent "scum" formation. On heating, hard waters form deposits of calcium carbonate scale. Soft water, with a hardness of less than 100 mg/litre, may, on the other hand, have a low buffer capacity and so be more corrosive for water pipes (see Protection and improvement of water quality).

No health-based guideline value has been proposed for hardness (see summary statement).

The taste and odour thresholds of hydrogen sulfide in water are estimated to be between 0.05 and 0.1 mg/litre. The "rotten eggs" odour of hydrogen sulfide is particularly noticeable in some ground waters and in stagnant drinking-water in the distribution system, as a result of oxygen depletion and the subsequent reduction of sulfate by bacterial activity.

Sulfide is oxidized rapidly to sulfate in well-aerated water, and hydrogen sulfide levels in oxygenated water supplies are normally very low. The presence of hydrogen sulfide in drinking-water can be easily detected by the consumer and requires immediate corrective action. It is unlikely that a person could consume a harmful dose of hydrogen sulfide from drinking-water, and hence a health-based guideline value has not been derived for this compound (see summary statement).

Anaerobic ground water may contain ferrous iron at concentrations of up to several milligrams per litre without discoloration or turbidity in the water when directly pumped from a well. On exposure to the atmosphere, however, the ferrous iron oxidizes to ferric iron, giving an objectionable reddish-brown colour to the water.

Iron also promotes the growth of "iron bacteria," which derive their energy from the oxidation of ferrous iron to ferric iron and in the process deposit a slimy coating on the piping.

At levels above 0.3 mg/litre, iron stains laundry and plumbing fixtures. There is usually no noticeable taste at iron concentrations below 0.3 mg/litre, although turbidity and colour may develop. Iron concentrations of 1–3 mg/litre can be acceptable for people drinking anaerobic well-water.

No health-based guideline value is proposed for iron (see summary statement).

Although manganese concentrations below 0.1 mg/litre are usually acceptable to consumers, this may vary with local circumstances. At levels exceeding 0.1 mg/litre, manganese in water supplies stains sanitary ware and laundry and causes an undesirable taste in beverages. The presence of manganese in drinking-water, like that of iron, may lead to the accumulation of deposits in the distribution system. Even at a concentration of 0.2 mg/litre, manganese will often form a coating on pipes, which may slough off as a black precipitate. In addition, certain nuisance organisms concentrate manganese and give rise to taste, odour, and turbidity problems in distributed water.

Although concentrations below 0.1 mg/litre are usually acceptable to consumers, this may vary with local circumstances. The provisional health-based guideline value for manganese is 5 times higher than this acceptability threshold of 0.1 mg/litre (see summary statement).

The dissolved oxygen content of water is influenced by the raw water temperature, composition, treatment, and any chemical or biological processes taking place in the distribution system. Depletion of dissolved oxygen in water supplies can encourage the microbial reduction of nitrate to nitrite and sulfate to sulfide, giving rise to odour problems. It can also cause an increase in the concentration of ferrous iron in solution.

No health-based guideline value has been recommended for dissolved oxygen.

Although pH usually has no direct impact on consumers, it is one of the most important operational water quality parameters. Careful attention to pH control is necessary at all stages of water treatment to ensure satisfactory water clarification and disinfection. For effective disinfection with chlorine, the pH should preferably be less than 8. The pH of the water entering the distribution system must be controlled to minimize the corrosion of water mains and pipes in household water systems (see Protection and improvement of water quality). Failure to do so can result in the contamination of drinking-water and in adverse effects on its taste, odour, and appearance.

The optimum pH required will vary in different supplies according to the composition of the water and the nature of the construction materials used in the distribution system, but it is often in the range 6.5–9.5. Extreme values of pH can result from accidental spills, treatment breakdowns, and insufficiently cured cement mortar pipe linings.

No health-based guideline value has been proposed for pH (see summary statement).

The taste threshold concentration of sodium in water depends on the associated anion and the temperature of the solution. At room temperature, the average taste threshold for sodium is about 200 mg/litre.

As no firm conclusions can be drawn regarding the health effects of sodium, no health-based guideline value has been derived (see summary statement).

The presence of sulfate in drinking-water can cause noticeable taste. Taste impairment varies with the nature of the associated cation; taste thresholds have been found to range from 250 mg/litre for sodium sulfate to 1000 mg/litre for calcium sulfate. It is generally considered that taste impairment is minimal at levels below 250 mg/litre.

It has also been found that addition of calcium and magnesium sulfate (but not sodium sulfate) to distilled water improves the taste; optimal taste was recorded at 270 and 90 mg/litre for the two compounds, respectively.

As sulfate is one of the least toxic anions, no health-based guideline value has been derived (see summary statement).

Total dissolved solids (TDS) can have an important effect on the taste of drinking-water. The palatability of water with a TDS level of less than 600 mg/litre is generally considered to be good; drinking-water becomes increasingly unpalatable at TDS levels greater than 1200 mg/litre. Water with extremely low concentrations of TDS may be unacceptable because of its flat, insipid taste.

The presence of high levels of TDS may also be objectionable to consumers owing to excessive scaling in water pipes, heaters, boilers, and household appliances. Water with concentrations of TDS below 1000 mg/litre is usually acceptable to consumers, although acceptability may vary according to local circumstances.

No health-based guideline value for TDS has been proposed (see summary statement).

Zinc imparts an undesirable astringent taste to water. Tests indicate a taste threshold concentration of 4 mg/litre (as zinc sulfate). Water containing zinc at concentrations in excess of 5 mg/litre may appear opalescent and develop a greasy film on boiling, although these effects may also be noticeable at concentrations as low as 3 mg/litre. Although drinking-water seldom contains zinc at concentrations above 0.1 mg/litre, levels in tapwater can be considerably higher because of the zinc used in plumbing materials.

No health-based guideline value has been proposed for zinc in drinking-water (see summary statement).

Toluene has a sweet, pungent, benzene-like odour. The reported taste threshold ranges from 40 to 120 µg/litre. The reported odour threshold for toluene in water ranges from 24 to 170 µg/litre. Toluene may therefore affect the acceptability of water at concentrations below its health-based guideline value (see summary statement).

Xylene concentrations in the range 300 µg/litre produce a detectable taste and odour.The odour threshold for xylene isomers in water has been reported to range from 20 to 1800 µg/litre. The lowest odour threshold is lower than the health-based guideline value derived for the compound (see summary statement).

Ethylbenzene has an aromatic odour. The reported odour threshold for ethylbenzene in water ranges from 2 to 130 µg/litre. The lowest reported odour threshold is 100-fold lower than the health-based guideline value (see summary statement). The taste threshold ranges from 72 to 200 µg/litre.

The average taste threshold reported for styrene in water at 40 °C is 120 µg/litre. Styrene has a sweet odour, and reported odour thresholds for styrene in water range from 4 to 2600 µg/litre, depending on temperature. Styrene may therefore be detected in water at concentrations below its health-based guideline value (see summary statement).

Taste and odour thresholds of 10–20 µg/litre and odour thresholds ranging from 40 to 120 µg/litre have been reported for monochlorobenzene. The health-based guideline value derived for monochlorobenzene (see summary statement) far exceeds the lowest reported taste and odour threshold in water.

Odour thresholds of 10, 5–30, and 50 µg/litre have been reported for 1,2,3-, 1,2,4-, and 1,3,5-trichlorobenzene, respectively. A taste and odour threshold concentration of 30 µg/litre has been reported for 1,2,4-trichloro

benzene. The health-based guideline value derived for total trichlorobenzenes (see summary statement) exceeds the lowest reported odour threshold in water of 5 ¼?/litre.

In many countries, the earlier, persistent types of anionic detergent have been replaced by others that are more easily biodegraded, and hence the levels found in water sources have decreased substantially. New types of cationic, anionic, and non-ionic detergent have also been introduced. The concentration of detergents in drinking-water should not be allowed to reach levels giving rise to either foaming or taste or odour problems.

Chlorophenols generally have very low organoleptic thresholds. The taste thresholds in water for 2-chlorophenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol are 0.1, 0.3, and 2 µg/litre, respectively. Odour thresholds are 10, 40, and 300 µg/litre, respectively. If water containing 2,4,6-trichlorophenol is free from taste, it is unlikely to present undue risk to health (see summary statement).