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TracWater discovers water quality issues in resevoirs

Problem statement

In common with many water utilities in Australia the City of the Gold Coast receives treated water into its water into its 3,500-kilometre water distribution network from a state-owned bulk water supplier who is responsible for operating the water treatment plant (WTP) which also distributes treated potable water to several other water utilities besides Gold Coast City.

For the past 6 years 31 TracWater self-powered water quality analysers have been making 10 physical and chemical water quality measurements every 5 minutes day and night. The TracWater analysers consist of a mix of cabinet units, portable units and in-ground units located at all main water reservoir sites measuring water quality before pipeline distribution and at strategic locations along the 3,500-kilometre water pipeline journey.

Over the last 6 years the TracWater cloud system has gathered and analysed a massive amount of water quality data and the level of free chlorine residual in all parts of the network at every TracWater water quality analyser is known and recorded. The level of free chlorine is normally between 1mg/L – 1.5 mg/L and never exceeds the Australian Drinking Water Guideline upper limit of 2mg/L.

In September 2022, spikes in free chlorine of up to 2.7 mg/L were observed by a TracWater analyser at one reservoir being supplied with bulk water from the water treatment plant. To complicate the issue the TracWater analyser measured spikes in free chlorine occurring every few days, with no set pattern but usually between 2am and 5am.

The bulk water provider provides water from its Water Treatment Plant with hypochlorite concentrations up to 2 mg/L maximum, so these peaks were inexplicable. A TracWater water quality analyser downstream from the water treatment plant on another trunk main confirmed that water leaving the treatment plant was within normal disinfection levels i.e below 2mg/L.

Figure 1 – TracWater Reservoir analyser free chlorine readings from 1st Sept – 17th Oct 2022

The reservoir is filled from two points:

  1. Via the trunk system that runs from the WTP through another reservoir, through the distribution area network being monitored by another TracWater analyser and finally to the reservoir.

  2. It is alternatively filled via the main Southern Regional Water Pipeline (SRWP) that is used to transport Bulk Water from the WTP located at the Gold Coast up to Brisbane. The SRWP has off-takes (blue lines) feeding 3 large reservoirs that are periodically used to keep the lines fresh

Because it was not possible to:

  • Have a free chlorine level at the reservoir higher than the level at the water treatment plant and

  • Because the water utility SCADA instrumentation was showing free chlorine levels in the normal operating range, it was suggested that the TracWater water quality analyser at the reservoir was giving an incorrect free chlorine reading.

TracWater technicians were dispatched to the site and performed multiple instrument calibrations over different days. Always the results were the same. The TracWater analyser was accurately calibrated and correctly measuring the level of free chlorine every 5 minutes and yet somehow recording an impossibly high free chlorine level at certain times during the night and only for very brief periods hence the “spike” in free chlorine measurements.

The water utility operations and water quality teams reviewed all SCADA control systems data searching for an operational reason why the free chlorine levels in one reservoir were so high and above the Australian Drinking Water Guideline levels at certain times.
Periodical off-take off from the main Southern Regional Water Pipeline to the reservoir was investigated as a possible reason for the free chlorine peaks. The observed peaks appeared to line up with some of the times when the reservoir received water from the main Southern Regional Water Pipeline, but not each time, and there was no obvious pattern.
The water utility suspected that there was some contamination affecting the first reservoir which had somehow then spread to the water in all reservoirs and that this contamination was somehow affecting the free chlorine sensors in TracWater analysers.

Figure 2 – Southern Regional Water Pipeline feeding water from water treatment plant to reservoirs.


TracWater developed an action plan:

  • We would test and prove measurement correlation at levels above 2mg/L between multiple water analysers using the same free chlorine sensors.

  • We would use our repository of data to closely examine water quality at each of the reservoirs allowing for water age and residual free chlorine decay.

  • Using our knowledge, we would develop our own theories as to why the events were occurring.

To examine correlation of high concentration free chlorine level measurements between TracWater analysers at the 3 different reservoir sites, a factory simulation was conducted. A “micro-water network”, consisting of the same models of TracWater analysers was established on small, pressurised ring main within a testing laboratory. The laboratory consisted of a 240L tank, pressure pump, ring main, and several TracWater portable water quality analysers. The analysers were all calibrated and correlated, so that free chlorine could be measured at 1 min intervals and chlorine dosing could occur in the tank, thus simulating a “micro-water network”.

Figure 3 –Testing of multiple TracWater analysers for high free chlorine concentration measurement correlation

Figure 4 – Map showing direction of water movement when Ormeau Reservoir is filled from SRWP.

Figure 5 – Free chlorine readings from TracWater analysers at the first and second reservoir for 16th – 26th Oct - before and after the operational change to address the main trunkline pipe break on 20th October.

When the pipe failed the TracWater analysers measured the same chlorine spike anomaly at 2 of the 3 reservoirs. There had never been any high free chlorine measurements in over half a million free chlorine measurements recorded by the TracWater analyser at the second reservoir site during the past 6 years.


TracWater cloud data demonstrated a clear relationship between high levels of free chlorine disinfection measured at two different reservoirs by TracWater water quality analysers. Because of this relationship TracWater formed the view that chlorine was being dosed into the water utilities reservoirs without the knowledge of the city’s water utility operations and water quality monitoring teams.

After further investigation it was discovered that an ancillary chlorine dosing station had been located on the main bulk water supply line. The bulk water supplier was disinfection dosing between 2mg/L to2.5mg/L to ensure that water quality disinfection residuals were being maintained along the bulk water delivery pipeline feeding to the city’s water utility reservoirs. The spikes in free chlorine levels were measured accurately by TracWater water quality analysers and observed on the TracWater cloud platform.

Unknown to both the city water utility and the bulk water supplier this water containing high free chlorine residual was back-feeding into the reservoirs from the main water pipeline when the pumps at the bulk water supply water treatment plant were turned off bringing with it undesirable higher levels of free chlorine into the city’s water storage reservoirs​

In the words of the Coordinator of Product Water Quality for the City of Gold Coast:

“We were not aware that our bulk water supplier had a dosing facility on the main water supply, and this was the missing piece of the puzzle. I would like to express my sincere thanks for the effort you and the team put in to investigate this with the information you have at your end. It is really appreciated. This is another great example of how powerful the information we receive from the TracWater analysers is and the value that they represent to the City of the Gold Coast in terms of water quality monitoring.
Thanks again.”

Results from Free Chlorine Dosing simulation.

The chart below represents over 1800 free chlorine test samples. The tank was allowed to dissipate all free chlorine for zeroing reference data. Then the tank was then dosed with 2mg/L of liquid chlorine and allowed to stabilise. After stabilisation, subsequent dosing of between 2 and 3mg/L occurred at approximately 30 minutes.

The rise in free chlorine levels measure by sensors occurred almost immediately after dosing the water supply, usually within 2-3 minutes. However, the decay in free chlorine concentration was gradual. The laboratory simulation proved that if high free chlorine levels were present at the reservoir sites for at least 5 minutes then the TracWater water quality analysers would have correctly measured and captured the high free chlorine concentration measurements.

Following the laboratory testing a desktop analysis was undertaken where approximately 2 million free chlorine measurements made at 5-minute intervals over 6 years were examined and compared for each of the 3 reservoir sites. Results exhibited the expected normal operations at each reservoir except for the unexplained high chlorine results at the one problem reservoir.

Armed with this information we then asked the water utility to request from the bulk water supplier the operational records for the water treatment plant and water quality testing records for the main southern water supply pipeline.
While this process was underway a trunk pipe break occurred, causing the problem reservoir to be supplied with water through the alternative SRWP line. Once full this reservoir was full it then back fed through the water network (below map) to where the next TracWater water quality analyser is located.​

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