5.7.3 Deposits due to iron and manganese bacteria

Nuisance iron-oxidising organisms may cause problems in groundwater sources by encrusting bore screens, causing loss of yield and impairing the aesthetic quality of the supply. The presence of these organisms may also indicate organic pollution of the aquifer and should trigger an investigation if populations are changing.

Manganese-oxidising organisms (bacteria, fungi and very rarely protozoa) may be responsible for deposits in aquifers, wells and water conduits. The deposits can:

• reduce yield

• clog slots in the bore pipes

• slow the flow in pipes by increasing turbulence

• damage equipment for measuring water flows

• produce black water that stains laundry and disrupts food-handling establishments.

Bacteria can attach to the deposits. If disturbed, these will increase the heterotrophic bacteria colony count of the water. These problems will generally not occur if the concentration of manganese is below 0.1 mg/L. (See fact sheet in Part V on Manganese).

In water containing ferrous or manganous salts, iron or manganese bacteria can oxidise these compounds to form rust-coloured or black deposits in tanks and on the walls of pipes in slow-flowing parts of the distribution system. Changes in water flow can then release the deposits into the supply system, staining laundry and plumbing fittings and adversely affecting the appearance of drinking water. The slurry may also contain organic deposits that can break down to cause odour problems (see fact sheets in Part V on Colour, Iron, and Manganese).

Although these nuisance organisms can impair water quality, it is not practicable to monitor for them routinely because of their diverse nature and unpredictable occurrence. Consumer complaints, together with local knowledge of the water supply system and source waters, should be a trigger for action.