The Basin – Victoria – E.coli
The second notification was for a sample taken from The Basin High Level tank on 16 March 2016 recording an E. coli result of 1 org/100mL. The sample tap is located on the outlet main, approximately 45 meters from The Basin High Level Tank. The supply to and from the tank is through the same water main.
The water supply to The Basin High Level Tank is The Basin No. 2. The Basin High Level Tank and The Basin No. 2 were inspected for signs of damage to the storages or ingress. No issues were identified at either tank site. There were no works in the area that affected the supply to or from The Basin High Level Tank.
The sample tap was checked and a fixed sample tap assembly identified. There was a build-up of dirt at the bottom of the tap which has been identified as the likely source of contamination of the sample. The fixed sample tap assembly has since been removed and replaced with a standard click-in sample tap assembly which can be covered with shrink wrap in between samples, thus adding extra protection from external contamination of the fitting.
Resampling of both The Basin High Level and The Basin No. 2 occurred on 17 March 2016, with the results indicating E. coli of 0 orgs/100mL for both samples.
On 18 March 2016 The Basin High Level Tank and The Basin No. 2 tank were dosed with sodium hypochlorite to a residual of 0.5mg/L. An additional sample was taken on 18 March 2016 from
a customer sample tap which is located on the inlet/outlet main supplying the basin. A South East Water representative was present at the time the sample was taken and observed serious sampler error. This subsequent sample indicated the presence of 1 org/100mL E. coli.
It is expected that random auditing of samplers will assist with preventing these errors in the future. Further resampling was then conducted at The Basin High Level Tank and the customer sample tap. Both of the samples contained 0 org/100mL E. coli.
Based on the results and investigation it was determined that the cause of the E. coli results was due to sample collection/handling and the fixed sample tap assembly within the cabinet. After consultation with the Department of Health and Human Services, it was agreed this positive result was not representative of the drinking water supplied, and met the definition of a false positive sample.
Escherichia coli should not be detected in any 100 mL sample of drinking water. If detected
in drinking water, immediate action should be taken including investigation of potential
sources of faecal contamination.
“Coliforms are Gram-negative, non-spore-forming, rod-shaped bacteria that are capable of aerobic and facultative anaerobic growth in the presence of bile salts or other surface active agents with similar growth-inhibiting properties. They are found in large numbers in the faeces of humans and other warm-blooded animals, but many species also occur in the environment.
Thermotolerant coliforms are a sub-group of coliforms that are able to grow at 44.5 ± 0.2°C. E. coli is the most common thermotolerant coliform present in faeces and is regarded as the most specific indicator of recent faecal contamination because generally it is not capable of growth in the environment. In contrast, some other thermotolerant coliforms (including strains of Klebsiella, Citrobacter and Enterobacter) are able to grow in the environment and their presence is not necessarily related to faecal contamination. While tests for thermotolerant coliforms can be simpler than for E. coli, E. coli is considered a superior indicator for detecting faecal contamination…” ADWG 2011