Rarotonga Water Monitoring Data

Potential hazard? Water storage tank - Turangi.

Rarotonga Water Monitoring Data, Cook Islands.
Compiled by: Andy Kirkwood, Justine Flanagan, 2019.

Contact: [firstName] @ islandbooth.com.

Masthead Potential hazard? Water storage tank – Turangi, Rarotonga.


This is a working draft. Final Proofing in progress. Updated 5 Dec 2019.


The Te Mato Vai Project to update Rarotonga’s intakes and water-main includes Step 4: Disinfection to kill or inactivate micro-organisms. It is proposed to dose water at the intakes with chlorine solution.

Protecting the Source

The results from two spot microbial test, one commissioned by a private company (Mar) and the other by water authority To Tatou Vai (May), have not established that there are illness-causing organisms in Rarotonga’s, water. The samples collected are also not indicative of water after new physical treatment processes that are part of the new Te Mato Vai system (scheduled for commissioning in Dec 2019).

Responsible water management is about investigating and protecting source water.

When monitoring and regulating drinking-water quality, E. coli is an indicator organism. It is not a pathogen, it does not directly cause illness. E. coli is naturally found in human gut microbiota; it is a beneficial bacterium.

An increase in E. coli in raw water should be expected after heavy tropical rain (wet weather or after rain) as soil and organic matter is washed into waterways. A well-designed treatment system will respond to local conditions.

Testing at village filling stations is not indicative of the quality of source water. Breaks in the network pipes and maintenance of filtration and UV irradiation systems can contribute to high indicator organism readings.

In normal (dry) conditions, detecting E. coli in disinfected water, lets you know that something has changed. Maybe a filter needs changed; maybe roadworks have cracked a water main. Investigate, do some maintenance.

The absence of E. coli does not mean water is safe to drink. Parasites such as Giardia and Cryptosporidium are difficult to detect and cannot be killed or inactivated by chlorine solution. Chemical, mineral and metal contaminants also have human-health impacts.

Incidence of mass-illness (outbreak) would suggest the presence of pathogens in the water; but would still require investigation as the symptoms are often the same as when illness has been transmitted by poor food-handling practices.

In nearly 60 years of public water supply (without disinfection), there have been no mass-illness outbreaks in Rarotonga attributed to water quality. The source of the water has not changed, the catchment areas have not changed.

For a summary of the Te Mato Vai Project see: Te Mato Vai Disinfection: Chlorination: why E. coli is not a suitable indicator of contamination in the tropics.


Evidence of Contamination?

At the May 2019 Disinfection Presentations, the Community asked questions about water quality test results and about any hospital records that might provide evidence of pathogens in Rarotonga’s source water. No test results or health surveillance data were presented at the meetings, however a representative of the Te Mato Vai PMU described the water quality as ‘bad’, and made reference to recent water quality tests.

In the same article where he dropped the T-bomb, DP Mark Brown was quoted as saying: ‘The new water tests revealed “unacceptable counts of e-coli in our water”…’

…A single sample is a snap shot — it does not provide information on what happens under different conditions or at other times of the year. Analysis, of the parameters targeted … is limited by the laboratory facilities on Rarotonga, and the time periods for shipping samples elsewhere.
Te Mato Vai: Water Supply Master Plan for Rarotonga (2014)


Watercare Sample Processing

Spot water samples were collected in March and May 2019, prior to the handover of the assets created by the Te Mato Vai project to To Tatou Vai: the new Water Authority for Rarotonga. A total of 46 water samples – including control bottled and grey water – were collected in March (26 samples) and May (20 samples) 2019.

“The purpose of this testing was simply for us to gain an understanding of the current water (bacteriological) quality before taking over responsibility for water supply and water treatment later this year.”
TTV CEO, 20 June 2019 (pers comm.)

Testing untreated water delivered through the old network is of limited value when it comes to understanding the bacterial quality of the source water; or the effectiveness of the new Te Mato Vai treatment system. The tests that have been undertaken are not genuinely investigative; and have not identified the presence of any pathogens.

Samples were taken from catchments and from village water filling stations located on Rarotonga’s coastal ring road (Ara Tapu).

As the Te Mato Vai Stage 2 construction works had not been completed the data collected is not indicative of the physical (non-chemical) methods of water treatment. For example, filtration systems including the new settlement tanks and AVG sand filters that remove sediment were not operational. Nor was water flowing through the new network pipes.

  • Results from the catchments represent (untreated) source waters (often referred to as ‘raw water’).
  • Water at the village filling stations has been treated and disinfected: filtered and exposed to UV light.
  • All water samples were sent by air in the custody of an accredited water scientist and processed by Watercare Laboratory Services (New Zealand).
Map of Rarotonga showing water quality monitoring sample points, Mar, May 2019.

Above: Map of Rarotonga showing catchments and village filling stations sampled in Mar and May 2019. Unmarked are presumed private residences ‘BB House’ and ‘Tap 33’. Rarotonga International Airport is top-left. Rarotonga is approximately 32km in circumference with a maximum speed of 50km/h, slowing to 30km in more developed areas (Muri and Avarua/Town). Source: Adapted from Nath et al. (2006).

Data Source

Watercare source data has been adapted below:

  • One water sample per row.
  • Data entry errors have been corrected; e.g. ‘Muri’ not ‘Mun’, and ‘Avana Nui Water Station’, rather than ‘Avana Nui Intake’.
  • Only the values for E. coli and Total coliforms have been translated.
  • Water Sample IDs are assigned by the laboratory based on date of receipt.

The source data is also provided in the release by To Tatou Vai below.

Download To Tatou Vai Report - 20 July 2019 (1.1MB)

Rarotonga Water Monitoring: 16 March 2019 (26 Samples)

Report: 3310138-0 / Received date: 18 Mar 2019 / Report Issued: 20 Mar 2019
All water samples collected Saturday 16 March 2019 (Cook Islands Time).

Sample ID Location Time E. coli T. coliforms
190318-145-1 Avatiu-A 8:30am >200 >200
190318-145-2 Ngatoe-B 9:00am 200 >200
190318-145-3 Papua-C 9:25am >200 >200
190318-145-4 Taipara-D 9:45am >200 >200
190318-145-5 Totokoitu-E 10:15am >200 >200
190318-145-6 Papa‘aroa Prefilter S1 11:00am >200 >200
190318-145-7 Papa‘aroa Postfilter S2 11:02am >200 >200
190318-145-8 Papa‘aroa School Postfilter S3 11:05am <1.0 <1.0
190318-145-9 Avana Nui Water Station Prefilter S4 11:10am >200 >200
190318-145-10 George Maggie Prefilter S5 11:15am >200 >200
190318-145-11 George Maggie Postfilter S6 11:16am >200 >200
190318-145-12 Nukutere School Prefilter S7 12:15pm >200 >200
190318-145-13 Takuvaine Prefilter 2 8:30am >200 >200
190318-145-14 Tupapa Prefilter 3 9:00am >200 >200
190318-145-15 Matavera Prefilter 4 9:15am >200 >200
190318-145-16 Turangi Prefilter 5 10:00am 170 >200
190318-145-17 Avana Prefilter 6 10:15am >200 >200
190318-145-18 RSA Tap Reticulation 11:00am 130 >200
190318-145-19 BB House 11:30am >200 >200
190318-145-20 Arorangi Water Station Prefilter 11:45am 24 >200
190318-145-21 Arorangi Church Prefilter 11:20am >200 >200
190318-145-22 Rima’s A (Muri) 11:00am >200 >200
190318-145-23 Grey Water PR 12:45pm 650,000
190318-145-24 Grey Water Poly 13:00pm 640
190318-145-25 Bottle Water PR 12:20pm <1.0 140
190318-145-26 Tap 33 - 24 200

Data Source: Watercare — supplied by To Tatou Vai (Rarotonga Water Authority).

Total sample collection time: Saturday 16 March 2019 (CIT) 8:30am—13:00pm / 4.5 h.

Processing Methods: Report 310138-0 (March 2019)
The above samples were analysed for Microbiology using the Colilert-18/Quanti-Tray method.
Bacteria colony counting range 1—200, where >200 indicates a higher number of colonies.
Membrane filtration was used to process samples 23 and 24 (control: bottled water and grey water samples from Prime Foods).


Rarotonga Water Monitoring: 18 May 2019 (20 Samples)

Report: 319043-0 (Replaces Report: 319042-0) / Received date: 20 May 2019 / Report Issued: 24 May 2019
All water samples collected Saturday 18 May 2019 (Cook Islands Time).

Sample ID Location Time E. coli Total coliforms
190520-137-1 Avana Intake 10:00am 52 >2400
190520-137-2 Avana Nui [Water Station] 10:10am <1.0 2.0
190520-137-3 Turangi Water Station 10:16am <1.0 1.0
190520-137-4 Turangi Intake 10:34am 22 >2400
190520-137-5 Matavera Intake 11:05am 38 >2400
190520-137-6 Tupapa Intake 11:32am 23 >2400
190520-137-7 George Maggie Water Station 11:54am 77 >2400
190520-137-8 Avatiu Water Station 12:22pm <1.0 15
190520-137-9 RSA 12:31pm 16 >2400
190520-137-10 Takuvaine Intake 12:10pm 490 >2400
190520-137-11 Papa’aroa Water Station 9:30am 18 2400
190520-137-12 Tokokoitu Intake 10:00am 83 >2400
190520-137-13 Taipara Intake 9:54am 88 >2400
190520-137-14 Papua Intake 10:09am 120 >2400
190520-137-15 Ngatoe Intake 10:36am 73 >2400
190520-137-16 Arorangi Water Station 11:06am 5.2 190
190520-137-17 Nukutere College 11:40am <1.0 <1.0
190520-137-18 Takuvaine Water Station 11:50am 23 >2400
190520-137-19 Avatiu Intake 11:26am 60 >2400
190520-137-20 CICC Sunday School 12:08pm 9.7 730

Data Source: Watercare — supplied by To Tatou Vai (Rarotonga Water Authority).

Total sample collection time: Saturday 18 May 2019 (CIT), 9:30am—12:31pm / 3h 1m.

Processing Methods: Report 319043-0 (May 2019)
The above samples were analysed for Microbiology are noted in the Watercare Report as using the Colilert-18. However, total coliform values indicate Quanti-Tray/2000 system was used.
Bacteria colony counting range 1—2,419, where >2400 indicates a higher number of colonies.


Environmental Conditions: Normal and Extreme Weather

Factors that affect the microbial quality of water include weather conditions: rainfall, wind direction, temperature, seasonal variation, etc.

Q: What were the weather conditions when TTV collected the samples?
A: The weather preceding the March sampling round was heavy rain, intermittently widespread across Rarotonga for a few days, including on the morning of the tests. The weather for the second (May) tests was generally settled, but included some rain showers in the week before the sampling was done.
To Tatou Vai Release, 20 July 2019

Rarotonga rainfall Feb-Jul 2019.

Above average rainfall was recorded in March and May when samples were collected. Surface flooding in coastal low-lying areas was recorded in March.
Source: Cook Islands Met Service.

The intake/catchment samples collected in Mar and May are a spot sample indicative of microbial quality of source water in wet weather or after rain. A balanced assessment of source water quality would require sustained data recording and include samples collected in dry weather conditions.

E. coli is only an indicator organism (it is not a pathogen), and detection standards are:

…maximum acceptable values for regulatory purposes. They do not represent a dose/response relationship that can be used as the basis for determining acceptable concentrations of pathogens in drinking-water.
Drinking Water Standard for New Zealand (DWSNZ)

When monitoring water that has been disinfected, detection of E. coli is meaningful. However detection of E. coli in raw water, in the tropics, and in wet weather is to be expected, and may have not bearing on the safety of the raw water source.

Designing a water supply system for the tropics, flow variablity is a significant consideration and treatment methods should respond to the local conditions. In response to the proposed Te Mato Vai system community group Te Vai Ora Maori have proposed monitoring and control of streamwater collection; collect water in dry conditions (when the stream is clear and has low microbial content), and stop collection when the water is ‘muddy’.


Water Sampling and Processing Protocols

Also of significance are the sample collection, storage and processing protocols. If protocols are not observed, then the results may be inaccurate – as the indicator organisms will have continued to multiply in the water sample.

The Colilert-18 system for microbial analysis is detailed in supporting literature.

Above Colilert-18 test method. A more convenient version of the H2S Presence-Absence test. Source Idexx / YouTube.

[Colilert-18] 8.0 Sample Collection, Preservation and Storage
8.1 Sampling procedures as described in detail in Standard Methods for the Examination of Water and Wastewater.
8.1.1 Storage Temperature and Handling Conditions: Ice or refrigerate bacteriological samples at a temperature less than 10C (2-10C) during transit to the laboratory. Use insulated containers to assure proper maintenance of storage temperature. Ensure the sample vessels are not totally immersed in water during transit.
8.1.2 Holding Time Limitations: Examine samples as soon as possible after collection. The required hold time of samples is 6 hours from collection to initiation of testing.
IDEXX Colilert-18 and Quanti-Tray Test Method for the Detection of Fecal Coliforms…

Total sample collection times were 16 March: 4.5h and 18 May: 3h.

  • Check-in to Rarotonga International Airport: 30min.
  • Flight: RAR -> AKL: 4.5h
  • Customs clearance: 30min.
  • Transfer to laboratory: 10min.

Approximate total holding times (minimum-best): 10hr, 8.5hr; average 9hr 15min.

Flying from Rarotonga to Auckland (New Zealand) crosses the international dateline. Leaving Rarotonga on Saturday, the flight arrives in New Zealand on Sunday. The Laboratory Reports show that the samples were receipted by the Watercare Laboratory, at the earliest 8am Monday (NZT).

The Auckland laboratory is open to receive samples between 8am and 5pm Monday to Friday and by arrangement from 9am to 4pm during the weekend.
Watercare New Zealand - Accessed 21 July 2019.

The time taken to initiate processing of the samples was not according to the Colilert-18 Sample Collection, Preservation and Storage guidelines.

Above 6hr time-lapse video of E. coli colony growing on microscope slide. Source Wikipedia.

As observed in the Te Mato Vai Water Supply Master Plan for Rarotonga:

…Analysis, of the parameters targeted … is limited by the laboratory facilities on Rarotonga, and the time periods for shipping samples elsewhere.
Te Mato Vai: Water Supply Master Plan for Rarotonga (2014)


Interpreting the Data

Laboratories return raw data — values and numbers that have been determined by carrying out standard processing methods. It is up to the client to interpret the results.

“E.coli is excreted from the gut of humans, animals or birds. Therefore it is an indicator that there is faecal matter (faeces) in the water supplied. If there are faeces then it is likely or even highly likely that there will be pathogens in the water as well (the ‘normal’ strain of E.coli is not a pathogen as it exists in our gut).”
TTV CEO, 20 June 2019 (pers comm.)

The limitations of the analysis method also need to be acknowledged. The Colilert-18 method uses detection methods that are equivalent to a series of H2S tests.

Above Marking samples under UV light, Colilert-18 test method. Source Idexx / YouTube.

The (Colilert-18) test does not count individual colonies…it counts the number of wells that show evidence of the enzyme produced by E. coli.
Quantifying Fecal Coliforms – Ecoli In The Field Kim Clarkin, USDA Forest Service. Accessed 20 July 2019.


Drinking Water (Monitoring) Standard Cook Islands

The Cook Islands National Water Policy (2016) defines the regulatory framework for water resource management including water standards. Also that drinking-water standards for the Cook Islands are still in draft and have yet to be finalised. (As at July 2019).

Using the World Health Organisation Guidelines as a reference point, standards for drinking-water will be established that will be specific to and appropriate for the Cook Islands situation. These standards will be regulated and enforceable, with penalties for non-compliance.

From 1 July 2019 the new Rarotonga Water Authority: To Tatou Vai (TTV) assumes ownership of the water supply system and responsibility for the maintenance and monitoring water distributed through that system.

”…in the absence of a Cook Islands Drinking Water Standard the maximum acceptable value (MAV) defaults to the Drinking Water Standard for New Zealand (DWSNZ) for which the MAV for E.coli is 0.”
TTV CEO, 29 June 2019 (pers comm.)

The Drinking-Water Standards for New Zealand (DWSNZ) define the minimum quality regulatory standards for drinking-water in New Zealand. This includes standards for the number of microrganisms permitted in a water sample taken to monitor treated drinking water quality (where treatment is inclusive of physical methods such as filtration as well as chemical coagulation and disinfection.)

The standard identifies that the water supplier should take action on detecting the presence of indicator organisms (E. coli), however the microorganisms themselves are harmless, in most cases they to do not cause illness. The strain of E. coli commonly associated with gastro illness is more commonly associated with food; raw meat, raw milk and unwashed leafy greens that may have been fertilised with animal manure.

Detailing the standard starts off well, with a clear procedural statement…

A1.2.1 Bacterial results
To establish whether a transgression has occurred, the test result (measurement) must be compared with the maximum acceptable value (MAV).

…and is supported by a table of relevant monitoring values.

Table 2.1: Maximum acceptable values for microbial determinands

Micro-organism Maximum Acceptable Value [*1]
Escherichia coli [*2] Less than one in 100 mL of sample
viruses No values have been set due to lack of reliable evidence
total pathogenic protozoa Less than one infectious (oo)cyst per 100 L of sample [*3]

However, following the references, there is less certainty…


  1. These are maximum acceptable values for regulatory purposes. They do not represent a dose/response relationship that can be used as the basis for determining acceptable concentrations of pathogens in drinking-water.
  2. Indicator organism.
  3. The methods available for enumerating pathogenic protozoa are becoming less expensive and more reliable, but they are not yet suitable for routine monitoring of treated water quality. Although new methods of assessing the infectiousness of protozoa by using human cell cultures have been developed, they are not yet suitable for routine monitoring of Cryptosporidium contamination of drinking-water. The referee method cannot identify the species of Giardia or Cryptosporidium; nor can it determine the viability or infectivity of detected cysts or oocysts (ie,(oo)cysts). Until the methodology improves, results are to be reported as verified (oo)cysts.
  4. IBID

It gets really shakey, when dealing with transgressions. E. coli values have shifted from a firm ‘less than one’ to ‘more than 10’. The reliable detectability poses problems - if viral pathogens cannot be detected (above), than what level is ‘of concern’? Data sourced from public health surveillance has also been introduced.

A major transgression is an occurrence that immediately threatens the safety of the consumers of the drinking-water. Most major transgressions are likely to result from inadequate control of a treatment process or a failure to protect the distribution system.…Major transgressions can be identified by any of the following.

  • the presence in the treated drinking-water of:
    • excessive concentrations of E. coli (more than 10 per 100 mL)
    • pathogens at a level of concern
    • cyanotoxins or chemical determinands at a concentration sufficient to cause acute adverse health effects (ie, much higher than the MAV)
  • high levels of illness in the community indicating a potential waterborne disease outbreak.


Major transgressions are serious. The water supplier must carry out the actions specified in the DWSNZ immediately, which includes informing the DWA so the DWA can help to identify the steps needed to protect consumers. In the case of a major transgression, a medical officer of health may issue a water supplier with a compliance order to take appropriate action to protect public health under section 69ZZH of the Act.


Indicator Organisms: E. coli et al

Testing water quality relies on the detection of indicator organisms.

Testing drinking water for all possible pathogens is complex, time-consuming and expensive. It is easy and inexpensive to test for [types of] coliform bacteria.
Coliform Bacteria and Drinking Water (2016)

Coliform bacteria are present in the environment and are unlikely to directly cause illness — the majority of strains of coloform bacteria are not pathogenic.

Total coliform bacteria are not acceptable indicators of the sanitary quality of rural water supplies, particularly in tropical areas where many bacteria of no sanitary significance occur in almost all untreated supplies.
Guidelines for Drinking-Water Quality, World Health Organisation (1997)

The term ‘fecal coliform’ has no clear definition but is generally accepted to be that part of the total coliform group that is thermotolerant and largely composed of the genera Escherichia, Klebsiella, Enterobacter, and Citrobacter.
(PDF) Evaluation of Colilert-18 for Detection and Enumeration of Fecal Coliform Bacteria… (2011)

Escherichia coli (commonly abbreviated to E. coli), is a type of coliform bacteria found in the lower intestine of warm-blooded animals. Most strains of E. coli are harmless. Beneficial strains of E. coli are part of healthy human gut-microbiota, producing vitamin K, and defending against pathogenic bacteria.

One specific strain of E. coliO157:H7 — is known to directly causes illness, but is more commonly a food-borne illness associated with ‘consumption of contaminated raw food, including raw milk, leafy green vegetables, and undercooked ground beef’.

Detecting E. coli within a water sample increases the liklihood that pathogens may also be present — digestive systems process fluid and nutrients and expell wastes: including pathogens. E. coli cannot live long outside the host, their presence in water is considered evidence that water was recently contaminated by feces. However fecal coliforms — including E. coli — may not a suitable indicator of fecal contamination in the tropics.

Although E. coli is useful [as an indicator of microbial water quality], it has limitations. Enteric viruses and protozoa are more resistant to disinfection; consequently, the absence of E. coli will not necessarily indicate freedom from these organisms.
WHO Guidelines for Drinking Water Quality (2017)

  • Total coliform: a large collection of different types of bacteria.
  • Fecal coliform: a subset of total coliforms that live in digestive systems and are also present in feces.
  • E. coli: indicates possible, recent fecal contamination.
  • E. coli O157:H7 [typically food-borne] causes gastro-intestinal illness.

Laboratory testing is required to identify the type of coliform; if E. coli are present it may also be possible to identify the source of the contamination (animal).


Public Health

Receiving the Watercare data, Community Group Te Vai Ora Maori submitted the below questions to Te Mato Vai.

Q: What pathogens did you find in your test results? (e.g. which specific strains of E. coli?)
A: …We did not discern the type or source of the E. Coli as that requires a DNA marker test.
TTV CEO, 20 June 2019 (pers comm)

Similar questions regarding records of waterborne illness have been posed to Te Marae Ora – The Cook Islands Ministry of Health. Despite the media headlines, the official line in July 2019 was:

…there has been no bacteriologically confirmed water-borne bacterial or viral illnesses reported to Te Marae Ora…
Release, Te Marae Ora, July 2019

E. coli is not the only bacterium that may be detected by standard analysis methods.

The source of C. perfringens and especially the spores in environmental samples has been a disputed topic. Some consider the organism to be of exclusively fecal origin but others consider the spores to be ubiquitous in soils, sediments, and other environmental media. Because the spores can survive for decades, their presence in environmental media can be difficult to interpret in the absence of a known source of fecal contamination. That is, they could either be natural environmental inhabitants or represent an historical source of fecal contamination.
Indicators for Waterborne Pathogens (2004), National Academies Press.


Identifying Contamination Sources

Recommendations made on watershed management by Hajkowicz and Okotai (2006) include identifying how contaminants enter the water system:

[Recommendation] 5. Identify pathways and target sites
The pathways for pollutants entering the lagoon, streams and drinking water supplies should be identified. This should be done to identify target sites, where improvement of land management or other practices will have the most significant impact on water quality.

Sources of contamination at the catchments may be a result of:

  • Soil erosion and stream sedimentation
  • Herbicide and pesticide runoff
  • Fertiliser runoff
  • Livestock and animal waste

A spot test for agricultural chemicals/trace metals (Ngatoe Intake: Sample 15 from the May report) suggest that agricultural chemicals (herbicides and pesticide and synthetic fertiliser) are not present.



  • The March and May water samples are not representative of the new water supply system, as Te Mato Vai construction works had not been completed.
  • Water samples were not processed according to the Colilert-18 Sample Collection, Preservation and Storage guidelines, holding time was in excess of the recommended 6hr.
  • When it rains, surface water gets muddy. The steep interior of the island accelerates soil erosion and transport of organic matter into waterways.
  • Samples collected after heavy rainfall are indicative of extreme rather than normal conditions. In other countries extreme conditions would result in Public Health Authorities issuing a ‘boil water notice’.
  • The To Tatou Vai test results are not conclusive, they do not establish that there are illness-causing organisms in our water (most gastro illness is due to food handling, and not contaminated water).

See also


Social Media


Meitaki Ma'ata / Acknowledgements

Meitaki to Dr Teina Rongo (Rarotonga) and Dr Ian Calhaem (New Zealand) for help with interpreting the water quality data. Meitaki to Tisa Bates, Charlene Hoff and Riannon Tara for putting our local water stations on the map.




Classification/subjects: Te Mato Vai, To Tatou Vai, chlorination, drinking water standards, monitoring water quality, Rarotonga, Cook Islands, South Pacifc, water monitoring, Colilert-18, hydrogen sulphide test, E. coli, Presence-Absence test, coliform bacteria, faecal/fecal bacteria, testing in the tropics.

Working Draft. Updated: 27 Aug 2019.


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