2016/17 – North Lakes (Queensland) – Trihalomethanes

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.

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