The absence of distinctive tastes, colours and odours is a feature of good quality rainwater, but there is a range of factors and/or conditions that can cause deterioration of these characteristics during collection, storage and piping. Other than dead animals (see Section 4) the principal sources of taste, colour and odour are:
- sediments and slimes at the bottom of tanks or in pipework that can hold stagnant water
- soil and decaying vegetation that is allowed to accumulate in guttering
- algal growth in pipework or open tanks
- tannins from leaf materials.
Pipework that does not completely self drain (for example, u-bends or underground piping from roof catchments to tanks, between tanks or from tanks to buildings) can also be a source of off-tastes and odours, particularly where stagnant water can develop and be retained between rain events. In these environments, slimes and biofilms can be formed and in the same manner as for tank sediments, anaerobic growth can occur, leading to production of sulfides.
Decaying vegetation and soils accumulated in guttering can also release taste, colour and odour compounds into the water, particularly if the gutters are not kept clean and do not fully drain between rain events.
Open tanks are fairly uncommon but exposure of stored rainwater to light will lead to algal growth. Most algae are not a human health risk, but growth can adversely affect the taste, odour and appearance of rainwater. Piping that is not impervious to light can also support algal growth.
Some pollens have very distinctive tastes and odours and if allowed to accumulate on roof catchments or in gutters, they can affect the quality of stored rainwater.
Off-tastes associated with roof catchments that include flues from wood heaters have been reported. Deposits could occur if the heater is not operated efficiently and the flue is not installed and operated in accordance with Australian/New Zealand Standards (see SAA/SNZ HB170 Wood heating resource handbook). The burning of preservative-treated wood could exacerbate the problem, and in any event, such wood is not a suitable fuel due to health and environmental concerns associated with production of toxic fumes and ash.