2016-17: Koala Court – North Lakes (Queensland) – Trihalomethanes
In 2016-2017, there was one instance where the ADWG long-term health limit for Trihalomethanes – THM (0.25mg/L) was not met at a reticulation sampling site. Routine laboratory sampling identified the elevated result (0.28mg/L) at Koala Court, North Lakes. Further testing throughout the network confirmed there were no other elevated levels and this was an isolated event.
Disinfectant (i.e. chlorine) when added to water can react with naturally occurring organic material, to produce THMs. To mitigate this Unitywater has reviewed its secondary chlorine dosing strategy.
The Australian Drinking Water Guidelines states that THM concentrations fluctuating occasionally (for a day or two annually) up to 1 mg/L are unlikely to pose a significant health risk.
2017-18: Koala Court – North Lakes (Queensland) – Trihalomethanes
Incident Description: There were two detections of high THM’s from routine samples taken on 6/3/2018. The two locations were the Boundary Rd 32ML Reservoir Dakabin (PN02RE) and Koala Cl North Lakes (PN19DS) with respective results of 0.33mg/L and 0.34mg/L.
Corrective & Preventative Actions: Both sites were supplied with water produced from the Petrie WTP, which through the deterioration of the WTP recorded higher levels of total organic carbon leaving the plant. The Petrie WTP has been decommissioned and the Pine North scheme is now supplied water through the Southern NPI, which will decrease the risk of THM formation in this area.
Follow up sample results: Follow up samples taken on the 23/3/2018 with the two affected sites recording results of 0.9mg/L and 0.99mg/L respectively.
Trihalomethanes Australian Guideline Level 250μg/L (0.25mg/L)
Why and how are THMs formed?
“When chlorine is added to water with organic material, such as algae, river weeds, and decaying leaves, THMs are formed. Residual chlorine molecules react with this harmless organic material to form a group of chlorinated chemical compounds, THMs. They are tasteless and odourless, but harmful and potentially toxic. The quantity of by-products formed is determined by several factors, such as the amount and type of organic material present in water, temperature, pH, chlorine dosage, contact time available for chlorine, and bromide concentration in the water. The organic matter in water mainly consists of a) humic substance, which is the organic portion of soil that remains after prolonged microbial decomposition formed by the decay of leaves, wood, and other vegetable matter; and b) fulvic acid, which is a water soluble substance of low molecular weight that is derived from humus”. US EPA