What are faecal indicator bacteria?
Faecal indicator bacteria (FIB) are bacteria that come from the gut of warm-blooded animals (including people). FIB are introduced to the environment through animal and bird droppings, effluent and waste water discharges, and stormwater run-off. The presence of them in our waterways and groundwater indicates that other pathogens harmful to humans may also be present.
To be safe from pathogens, any drinking water should be completely free of FIB. Some faecal pathogens can cause disease in people simply by coming in contact with them, so there are guidelines for acceptable amounts of FIB in recreational water. Pathogens can also accumulate in some types of shellfish, posing a risk to people if eaten.
Regional councils and unitary authorities monitor faecal contamination in water using faecal indicator bacteria. The two most commonly measured FIB are E. coli (in freshwater) and Enterococci (in coastal waters).
E. coli
What is E. coli?
E. coli (short for Escherichia coli) is a type of bacteria commonly found in the guts of warm-blooded mammals (including people) and birds. E. coli can survive for up to four to six weeks outside the body in fresh water, making it a useful indicator of faecal contamination and the presence of disease-causing organisms. Common sources of E. coli bacteria are human wastewater discharges, animal waste, bird droppings and stormwater run-off.
Which unit is it given in?
E. coli counts are reported as Colony Forming Units (cfu) or Most Probable Number (MPN) per 100 ml. The different units reflect different test methods – counting directly under a microscope versus a statistical presence/absence approach – but they give essentially equivalent results. Both represent the number of viable bacteria in a 100 millilitre sample of water.
Why is E. coli a problem?
The presence of E. coli indicates that the water has been contaminated with faecal matter, and it may therefore contain other pathogens that can cause illness.
E. coli in rivers and lakes for swimming and recreational purposes
In water used for swimming and other recreation, low counts of E. coli are acceptable, but too much faecal contamination can cause gastroenteritis or infections of ears, eyes, nasal cavity, skin, and the upper respiratory tract. When E. coli in rivers and lakes is detected above 540 per 100 mL, swimming is not recommended.
Water samples are collected weekly from popular swimming sites by regional, unitary or local councils during the swimming season (usually November – March), and these results are shown in the 'Can I Swim Here?' section on LAWA. More information in this factsheet.
E. coli in rivers and lakes for State of the Environment monitoring purposes
E. coli samples in rivers and lakes are generally collected year round, as part of council's State of the Environment (SOE) reporting, to evaluate long-term improvement of these water bodies. The sampling procedures may differ from shoreline swim site testing. For example, in lakes the samples are collected at the surface from the deepest part of the lake. Concentrations of E. coli are evaluated at water quality sites nationwide against attribute bands described in the National Policy Statement for Freshwater Management 2020. See factsheets for more information on calculating state in lakes and rivers.
E. coli in drinking water
Drinking water should be completely free of E. coli to ensure that it is safe. The Drinking Water Standards for New Zealand has a Maximum Acceptable Value (MAV) for E. coli at less than 1 cfu or MPN in 100 millilitres (mL) of sample. In other words, E. coli should not be detected at all in a 100 mL sample.
Note that for a sample to have an E. coli value less than the MAV, the test must have a detection limit of no more than 1 cfu or MPN per 100 mL. A few of the results shown on LAWA come from tests with higher detection limits, and the results might be recorded as “<2” or “<5” cfu or MPN per 100 mL. This means that E. coli was not detected in the sample, but the test could not rule out the possibility that the sample might contain E. coli above the MAV.
If no bacteria are found, the result is reported as <1 cfu or MPN per 100 mL. It would not be appropriate to report a result of zero, because it is not possible to give absolute assurance that an entire water supply is free of bacteria on the basis of a single 100 mL sample. Anyone using groundwater as a source of drinking water should make sure their well is sealed and have their water tested regularly. Drinking water is often treated to ensure it is safe to drink.
How to test for E. coli?
Water samples are collected in sterile bottles and sent to laboratories to be tested using nationally approved methods.
Many laboratories offer drinking water testing services and can provide guidance on the analysis of drinking water for well owners. Regular testing for E. coli is recommended. Testing is the responsibility of the well owner. Public water suppliers also test for E. coli and can be contacted regarding test results.
Enterococci
What are enterococci?
Enterococci are various types of bacteria that naturally occur in the gut of humans and animals. These are the preferred biological indicator for faecal contamination of coastal swimming sites.
Which unit is it given in?
Enterococci are counted under the microscope as Colony Forming Units (cfu) per 100 mL.
Why are enterococci a problem?
In water used for swimming and other recreation, low counts of enterococci are acceptable, but too much faecal contamination can cause gastroenteritis or infections of ears, eyes, nasal cavity, skin, and the upper respiratory tract. When enterococci are detected above 280 per 100 mL (in two consecutive samples within 24 hours), swimming is not recommended. More information in this factsheet.
Water samples are collected weekly from popular swimming sites by regional, unitary or local councils during the swimming season (usually November – March), and these results are shown in the 'Can I Swim Here?' section on LAWA.
How to test for enterococci?
Water samples are collected in sterile bottles and sent to laboratories to be tested using nationally approved methods.
FAQs
Why not monitor for pathogens?
It is simply not practical, or even possible, to test for every potential pathogen that might be present in water. Each type of pathogen would require a separate test, and there are many pathogens for which effective tests have not yet been developed. The most effective way to test for pathogens is to use indicators, FIBs, to let us know if faecal material is present in the water. We know when levels of FIBs may start to cause illness in people using the water, which makes them a robust, reliable indicator.
Why is there a delay in getting FIB results?
Water samples are collected and then sent to the laboratory. At the lab a sample of water is put onto a special substance (selective media) which encourages the growth of any FIB present in the sample. After 24 hours, any FIB organisms present will have formed a colony, which are counted. This is why there is a delay in taking a water sample and obtaining a water quality result.
How does E. coli get into groundwater?
If soil is contaminated with E. coli, for example from grazing animals or wastewater discharges, rain water percolating through the soil can carry the E. coli downward into the underlying groundwater. Alternatively, an aquifer in close connection with a river or stream may not filter out E. coli contamination before it reaches a water supply well. Aquifers act as natural filters and can reduce microbial contamination of groundwater, but their filtration ability varies widely. Many factors influence their filtration effectiveness, including permeability and the speed that groundwater travels through the aquifer. In places where water flows rapidly through the soil and aquifers, E. coli can be transported considerable distances and end up in aquifers that people use for water supply. A poorly sealed or open well can also provide a potential pathway for contaminated runoff from the land to directly enter a groundwater supply, bypassing the filtration provided by aquifer materials. If a homeowner has a private well, E. coli can be pumped directly to the tap in the home.
How do I avoid contamination of my groundwater supply?
All wells should be constructed to specifications set out in the Environmental Standard for Drilling of Soil and Rock (NZ4411:2001). The area immediately surrounding your well should be fenced off to prevent stock access and kept clear of any potential contaminants. Well casing should be elevated above ground and above stormwater and flood levels. The top of the well should be securely sealed or capped, and any hoses or cables should also be securely sealed. Backflow prevention devices should also be installed.
Where do I find more information?
Microbial Water Quality Guidelines for Marine and Freshwater Recreational Areas (2003). New Zealand Ministry for Environment and Ministry of Health.
Well Water Health and Safety Guide. Hawkes Bay Regional Council.