Microbial contaminants are disease-causing organisms.

In aquatic environments, microbial contamination is one of the crucial issues with regard to the sanitary state of water bodies used for drinking water supply, recreational activities, and harvesting of food due to a potential contamination by pathogenic bacteria, protozoa or viruses which make us sick.

Where do microbes come from?

Common sources of microbial contamination are animal waste, stormwater run-off, and untreated human wastewater discharges.  As E. coli survives outside the body and can survive for up to six weeks in fresh water it a useful indicator of faecal presence and therefore of disease-causing organisms in a river or lake.  Faecal concentrations are typically higher in pastoral streams, but even near-pristine streams are not totally free from E. coli because of faecal deposits by birds and wild animals.

Sediment and microbes are deposited on and eroded from the soil surface and are, therefore, transported predominantly by surficial runoff (overland flow). However, artificial drainage can also act as a conduit for sediment and microbes to surface water bodies.

How is microbial contamination measured?

Microbial contamination is monitored using indicator species that are present in the faeces of warm-blooded mammals and birds. In freshwater, the indicator species is Escherichia coli (E. coli) and is measured as a count under a microscope as Colony Forming Units per 100ml. E. coli is used as an indicator species for the presence of more harmful bacteria due to its comparative ease to detect and quantify.

High concentrations of this bacteria exceeding water quality guidelines indicate faecal contamination which can be harmful to human health. Too much E. coli means that the water is unsafe to drink or swim in and can cause gastroenteritis, or infections of ears, eyes, nasal cavity, skin, and the upper respiratory tract. Water is only considered safe for drinking if there are very low concentrations of E. coli present.

For more information on water quality contaminants see Nitrogen, Phosphorus, and Sediment.