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2 March 2022

Unprecedented flows to the Homebush Wastewater Treatment Plant, caused by high levels of groundwater after recent storms, have forced Masterton District Council to carry out an emergency discharge of treated wastewater to the Ruamahanga River.

The discharge will start today and is likely to continue for at least a week.

Because of the emergency situation, the discharges will occur with the river below the level at which treated wastewater is normally discharged.

Under its normal consent with Greater Wellington Regional Council, the Council is permitted to discharge treated wastewater when the river flow level is above 12.3 cubic metres per second in summer, with this reducing to 6.2 cubic metres per second in winter.

However, the extreme rain caused by ex-Cyclone Dovie last week led to high levels of groundwater, forcing water into the sewerage system and creating flows into the Homebush plant two and a half times what is normal for this time of year.

On Monday, the volume flowing into the plant was more than 24,500 cubic metres. The Council aims to irrigate much of its treated wastewater to Council land, but on Monday the irrigated volume reached its maximum, 14,000 cubic metres – already about twice what is normal.

Council Chief Executive David Hopman said the situation was highly unusual.

“It is unprecedented since the Homebush plan began operation in 2015. Because of the high level of groundwater, more water is coming into the wastewater system and we have reached the maximum level possible for irrigating to land.

“We have no option but to carry out an emergency discharge, but will only be discharging what we cannot irrigate to land. The river flow levels will be similar to the winter consent level.”

It is not clear when flows to the Homebush plant will reduce sufficiently to avoid the need for the discharge, and little substantial rain is forecast in the next week to lift river levels.

“This is certainly not a scenario we want to be in – we strive to limit discharges to the river. But this is an emergency situation and we will limit the discharge as much as we can,” Mr Hopman said.

The Council has an ongoing programme of sewer replacement to address the problem of leaks that lead to water entering the system, with 4.3km of sewer replaced in 2020/21, at a cost of just over $2.6 million.

ENDS