The appearance of colour in water is caused by the absorption of certain wavelengths of normal light by coloured substances ("true" colour) and by the scattering of light by suspended particles; combined, these constitute "apparent" colour (1–3). Treatment removes much of the suspended matter from drinking-water, and most of the remaining discoloration arises from true colour, which is generally substantially less than apparent colour (4). 

It has been suggested that the organic matter (primarily humic and fulvic acids) usually responsible for the colour of drinking-water give it an earthy smell and taste, but there is no conclusive evidence for this. Highly coloured polluted water will frequently have an objectionable taste, but the precise causal relationship is unknown. It is known that the organic colouring material in water stimulates the growth of many aquatic microorganisms, some of which are directly responsible for the production of odour in water (5).

As already mentioned, colour in natural waters is due mainly to organic matter, particularly dissolved humic and fulvic acids, which originate from soil, peat, and decaying vegetation. In addition, inorganic iron and manganese are present in some groundwaters and surface waters and may impart a red and black hue, respectively. Highly coloured wastewaters, in particular from the pulp, paper, dye, and textile industries, can also produce coloured waters. 

Discoloration of potable water may arise from the dissolution of iron (red) or copper (blue) in distribution pipes, which can be enhanced by bacteriological processes. Microbiological action can also produce "red water", resulting from the oxidation of iron(II) to iron(III) by "iron bacteria". Similarly, black discoloration may result from the action of bacteria capable of oxidizing dissolved manganese to give insoluble forms. 

A low-ash preparation of soil fulvic acid was supplied in drinking-water at concentrations of 10, 100, or 1000 mg/litre to male rats for periods of up to 90 days (Becking & Yagminas, 1978, unpublished data). No significant changes in body weight, food and water intake, organ/body weight ratios, or tissue histology were observed. The same fulvic acid preparation given daily for 14 days to rats by gavage at a dosage of 1000 mg/kg of body weight was not lethal but did reduce the rate of weight gain and cause slight changes in kidney enzyme concentrations. 

Drinking-water containing nonchlorinated (total organic carbon concentration (TOC) concentration 1.0 g/litre) and chlorinated humic substances (TOC 0.1, 0.5, 1.0 g/litre) was administered to groups of male Sprague-Dawley rats for a period of 90 days (7). The average body weight gain and terminal body weight were decreased significantly by 1.0 g/litre chlorinated humic substances and slightly by 0.5 g/litre chlorinated humic and 1.0 g/litre nonchlorinated humic substances. No significant differences were observed in food consumption. However, fluid consumption was significantly decreased by 1.0 g/litre nonchlorinated and 1.0 or 0.5 g/litre chlorinated humic substances, namely by 14%, 16%, and 17%, respectively. The most significant finding of this study was the increased incidence and severity of haematuria in the group receiving 1.0 g/litre chlorinated humic substances. These studies suggest that there is minimal risk from exposure to chlorination by products of humic acids as far as target organ effects are concerned. 

Chlorinated and nonchlorinated humic acids (TOC 1 g/litre in distilled water) were tested for mutagenic activity using both in vitro and in vivo assays. In the Ames test, the results showed positive mutagenic activity only after chlorination, whereas induction of sister chromatid exchange was observed with both chlorinated and nonchlorinated humic acids. In contrast, in the in vivo studies, no evidence was found of mutagenic activity for both chlorinated and nonchlorinated samples (8).