South Gisborne Tank, 10 December 2015 (Gisborne)
Issue – Routine sampling at the tank resulted in detection of 1org/100mL of E.coli in the presence of <0.05mg/L free chlorine and 0.17mg/L total chlorine.
Actions – The tank was isolated from supply and spot-dosed with chlorine to return a residual of 0.8mg/L free chlorine and 1.1mg/L total chlorine. The reticulation system was flushed to then draw freshly chlorinated water from the tank via pump station. Resampling was conducted for three consecutive days with all results clear of E.coli. The root cause was suspected to be an issue with the integrity of the sample collected (false-positive) as it was collected during a wet weather event. A possible alternative root cause is minor ingress of rainwater through the roof. Works to improve tank roof integrity following the trial of new technology is planned to commence across tanks in 2015/16.
South Gisborne Tank, 4 February 2016 (Gisborne)
Issue – Routine sampling at the tank resulted in a detection of 3org/100ml of E.coli in the presence of 0.10mg/L total chlorine. Actions – The tank was isolated from supply and spotdosed with chlorine to return a residual of 0.60mg/L total chlorine. The freshly chlorinated water was pumped into the reticulation system via pump station. Resampling was conducted for three consecutive days with all results clear of E.coli. The reported routine result was declared as a false positive. The tank was inspected and some ingress at the inspection hatch was suspected to be the root cause, however resampling did demonstrate no contaminated water was supplied to customers. Outcome – During early 2016, the tank was assessed by external consultants for refurbishment and a booster chlorinator will be installed in 2016.
Gisborne South tank, Gisborne (November 2017)
Issue – A routine sample from the South Gisborne tank in November 2017 reported a result of 1org/100ml E.coli (SDWR limit is 0orgs/100ml E.coli ).
Actions – The investigation showed the tank at the time of sampling had a good chlorine residual present and there were no downstream reports of E.coli on the same day of sampling at the tank. Resampling at the tank was performed on the day of reporting the initial exceedance, and the tank roof, hatch and sample tap were examined. The investigation identified the sample was likely contaminated by dust entering the bottle as it was filled. Based on the resample at the same location and all other network samples taken on the same day as the exceedance, it was found that the initial sample was not representative of water supplied to customers at the time
of sampling. Outcome – the tank has a booster chlorination system on site to maintain a good chlorine residual in the tank. Investigation determined ongoing maintenance of this system and regular inspections of the tank were recommended.
Gisborne Road Tank, Gisborne (March 2019)
Issue – A routine sample from the Gisborne Road Tank on 26 March 2019 reported a result of 2org/100ml E.coli (SDWR limit is 0orgs/100ml E.coli).
Actions – The investigation showed the tank at the time of sampling had a low chlorine residual present, and there was one downstream network report of 1org/100ml E.coli on the day of resampling on 27 March 2019 at the tank. Due to the verified report of water contamination, the issue was addressed in accordance with Section 18 Safe Drinking Water Act 2003.
Outcome – the tank has as upstream booster chlorination system to maintain a good chlorine residual in the tank. Investigation determined the importance of ongoing maintenance of this system and regular inspections of the tank and booster chlorination system were recommended.
“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