Page 311 - Week 01 - Thursday, 10 February 2022

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b) Nearby Canberrans

A significant river flooding event was occurring at that time, and it is unlikely that people were in the downstream river (Molonglo or Murrumbidgee) during or soon after the spill. Downstream offtake of water is unlikely to have been affected due to the flooding significantly diluting the sewage overflow.

c) Freshwater native species spawning in (the affected rivers)

The additional impact on native fish due to partially treated sewage entering the river systems in the overflow event would have been negligible. Water temperature at the time of the overflow was less than 13oC and no native fish species of the ACT region would have commenced spawning under these conditions. The current sewage treatment process used by the Lower Molonglo Water Quality Control Centre (LMWQCC) does not remove potentially harmful chemicals such as pesticides and endocrine-disruptors, so risks from these to aquatic life would not have increased beyond background levels by the overflow event. While discharge of partially treated sewage with very high bacteria and nutrient levels (in addition to the higher salt and endocrine-disrupting chemicals of treated sewage) can reduce oxygen and result in algal spikes potentially causing fish kills, river flooding at the time of the event diluted the overflow to approximately 0.0002% of the flow. This would have almost immediately reduced any additional impact (beyond background levels) on native fish to negligible. Native fish already avoid the Molonglo River directly below the treated sewage outfall. Water quality monitoring stations downstream of the outfall gave no indication of a major water quality impact in the period.

d) Native fauna in and next to, the affected rivers

Some frogs are highly sensitive to endocrine-disrupting chemicals and other contaminants in sewerage which can affect growth and development rates, sexual development and physiology. As these are present in the ongoing normal discharge of treated sewage into the Molonglo River and not removed by the treatment processes used by LMWQCC, no additional risk would have been posed by release of partially treated sewage.

(6) Icon Water manages the sewerage services in the ACT and continues to renew and upgrade processes at LMWQCC to ensure reliability and capacity are maintained as Canberra grows and high-intensity and volatile wet-weather events become more common with climate change. These investment activities are supported by the Icon Water Climate Change Adaption Plan and Sewage System Strategy. Icon Water also has a routine program of preventative maintenance and asset replacement across the network and at its plants. Consideration of peak flow management is also supported through Icon Water’s modelling and planning of the network.

(7) Icon Water advises that there was no requirement for clean-up or cost following this event.

The partially treated effluent was 100 percent screened of coarse particles and solids resulting in only the liquid fraction being discharged from the dam. This liquid fraction mixed and was diluted by the elevated storm water flows within the river.

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