The importance and origins of each flavour are:

Flavour: ALMOND    Chemical name: BENZALDEHYDE

“Almond, like bitter almonds, marzipan or cherries”

IMPORTANCE: Benzaldehyde imparts a pleasant, but nevertheless atypical, sweet odour to water, reminiscent of almonds or marzipan.

ORIGINS: Benzaldehyde is derived from the breakdown of organic material and through the action of microorganisms, including Actinobacteria, Pseudomonads and algae.

Flavour: BROMOPHENOL    Chemical name: 2-BROMOPHENOL

“Bromophenol, like disinfectant or iodine, medicinal”

IMPORTANCE: 2-Bromophenol Imparts an unpleasant ‘tainted’ note to water. It is often described by consumers as ‘chemical’ or ‘contaminated’, even when present at low levels.

ORIGINS: Bromophenols are produced through reaction of phenolic compounds (for example from waters associated with peaty soils) and free bromine. They can also arise due to trace contamination of water with wood preservatives and fire retardants.

Flavour: TOBACCO-LIKE    Chemical name: BETA-CYCLOCITRAL

“Tobacco-like, like hay or sweet tobacco”

IMPORTANCE: Cyclocitral imparts an aromatic odour to water, reminiscent of hay and dry wood at low concentrations, which becomes more tobacco-like and fruity at higher concentrations.

ORIGINS: Beta-Cyclocitral is formed by Cyanobacteria in reservoirs or water distribution systems. It is produced by both actively-growing organisms and through cell lysis or decomposition.

Flavour: DMS    Chemical name: DIMETHYL SULPHIDE

“DMS, like boiled sweetcorn or tomato sauce”

IMPORTANCE: Dimethyl sulphide is associated with sulphury and ‘muddy’ odours notes. The presence of DMS can also be indicative of contamination of the water supply chain with faecal bacteria.

ORIGINS: DMS is formed by bacteria growing in water. It can also arise from decomposition of algae in surface waters following an episode of algal bloom.

Flavour: ONION    Chemical name: DIMETHYL TRISULPHIDE

“Onion, like fried onion or garlic”

IMPORTANCE: Imparts an unpleasant ‘swampy’ odour to water. The intensity of the odour can be increased by the presence of other sulphur compounds.

ORIGINS: Dimethyl trisulphide is formed through decay of grass and vegetation in reservoirs and lakes and by bacterial action. It can also arise from decomposition of algae following an episode of algal bloom.

Flavour: FRESHLY CUT GRASS    Chemical name: CIS-3-HEXENOL

“Freshly cut grass, like cut leaves or cut grass”

IMPORTANCE: cis-3-Hexenol imparts a pleasant, but nevertheless atypical, grassy note to water. The compound often occurs in association with cis-3-hexenyl acetate.

ORIGINS: cis-3-Hexenol is derived from leaching of chemicals from grass and vegetation, and from growth of algae. The compound is produced in plants through the action of the enzyme lipoxygenase.

Flavour: H₂S    Chemical name: HYDROGEN SULPHIDE

“H₂S, like boiled eggs or rotten eggs”

IMPORTANCE: Hydrogen sulphide imparts an unpleasant sulphury note to contaminated soft drinks. H₂S is easily oxidized to less odour-active species, so its presence is indicative of the failure of water treatment processes or contamination arising during production of soft drinks.

ORIGINS: H₂S is a contaminant of carbonated soft drinks which can be introduced through use of insufficiently-purified carbon dioxide. It can also arise through microbiological spoilage by bacteria, and through chemical reactions involving metal surfaces in canned products.

Flavour: INDOLE    Chemical name: INDOLE

“Indole, like pig faeces or jasmine”

IMPORTANCE: In addition to contributing an unpleasant farmyard note, the presence of indole can also indicate contamination of the water supply chain with faecal bacteria.

ORIGINS: Indole is formed by bacteria growing in water, or contributed, pre-formed, by ground water pollution.

Flavour: VIOLETS    Chemical name: BETA-IONONE

“Violets, like violets or rose water”

IMPORTANCE: Beta-ionone imparts a pleasant, but nevertheless atypical, sweet aromatic odour to water, reminiscent of violets. It is derived from the breakdown of plant-derived carotenoids.

ORIGINS: Beta-ionone is formed by algae or Cyanobacteria growing in water and through the biochemical decay of grass and vegetable matter in lakes and reservoirs.

Flavour: MERCAPTAN    Chemical name: METHANETHIOL

“Mercaptan, like drains or soft white cheese”

IMPORTANCE: Methanethiol imparts an unpleasant sulphury note to water. It is easily oxidized to less odour-active species, so its presence is indicative of the failure of water treatment processes or post-treatment contamination.

ORIGINS: Methanethiol is formed in water by anaerobic sulphate-reducing bacteria and Cyanobacteria or by decay of grass or algae in lakes or reservoirs. It can also arise from decomposition of algae following an episode of algal bloom.

Flavour: SKATOLE    Chemical name: SKATOLE

“Skatole, like animal faeces”

IMPORTANCE: In addition to contributing an unpleasant, somewhat nauseating, faecal note to water, the presence of skatole can also indicate contamination of the water supply chain with faecal bacteria.

ORIGINS: Skatole is formed by bacteria growing in water, or contributed, pre-formed, by ground water pollution. It is produced by bacteria from tryptophan.

Flavour: MUSTY-TCA    Chemical name: 2,4,6-TRICHLOROANISOLE

“Musty-TCA, like corked wine or a damp cellar”

IMPORTANCE: 2,4,6-Trichloroanisole imparts an unpleasant musty odour to water which is reminiscent of damp cellars or corked wine.

ORIGINS: 2,4,6-Trichloroanisole is derived from methylation of chlorophenols by moulds or Actinobacteria, primarily in water distribution systems.