Christchurch City Council is developing a new pond management plan for the troubled Christchurch Wastewater Treatment Plant, as odour concerns continue in surrounding suburbs.

A pond management specialist has been on site today, and will remain tomorrow to work alongside council staff to create a more comprehensive monitoring and response framework.

The council said the plan would focus on tracking key indicators such as algae levels and dissolved oxygen, along with other early warning signs, and set clearer trigger points for when conditions begin to deteriorate.

“The aim is simple, reduce the chance of odour by identifying issues earlier, responding faster, and keeping the community better informed,” the council said.

It said the new plan built on extensive monitoring already under way, but would strengthen the approach by outlining earlier and more decisive actions when pond conditions start to shift.

Conditions at the ponds continue to trend in the right direction, with algae levels and dissolved oxygen slowly increasing, the council said.

“These are positive signs that the system is stabilising, but health indicators in ponds 1, 2A and 2B are still lower than ideal and we continue to monitor them closely,” the council said.

The council said the biological health of the ponds remains fragile and significant odour spikes are still being detected at community monitoring sites.

“Our teams are doing everything they can to address the current situation,” the council said.

The odour contains a mixture of highly odorous compounds, primarily hydrogen sulphide, which is detectable at much lower concentrations than those that can cause adverse human health effects.

Hydrogen sulphide is known for its strong rotten egg smell, even at low concentrations.

The council said it has undertaken an air monitoring programme to understand which gases are contributing to the odour, identify their source across the treatment plant site, and measure their impact at varying distances.

With the exception of hydrogen sulphide, community monitoring has not detected any odour emitting chemicals at concentrations that could pose a risk to human health.

Monitoring is now focused on measuring hydrogen sulphide levels in nearby residential areas to assess whether concentrations and exposure durations could potentially cause physiological symptoms.