A new environmental report, to be published today, shows the need for a solution has never been more pressing: the report shows it could take years for the lagoon to recover to full health.
Some of the residual nutrients that cause algal blooms in the lagoon are from ground waters 15 to 30 years old, back when there was still run-off from intensive agriculture in Muri, the report reveals.
That means the legacy of today’s septic tanks, in accommodation and private homes, would take another 10 or more years to flush away.
The government’s Project Management Unit commissioned the in-depth independent scientific report into the health of Muri lagoon, which revealed that the currents cannot flush the nutrients away; the only solution is to decommission the septic tanks that continue to leach into the aquifers.
That will gradually reduce the amount of nutrients going into the lagoon, particularly in the sensitive, still areas where seaweed has become a problem.
New Zealand’s National Institute of Water and Atmospheric Research reviewed the report, and backed the PMU’s finding: that the report confirms the need for a centrally-operated wastewater treatment plant near Muri to dispose safely of wastewater.
Deputy project manager Tangianau Taoro said it could take time to see the benefits of reducing wastewater discharge – “sometimes up to years”.
“This is because it takes time for residual amounts of nutrients in groundwater, soil and sediments to be removed, naturally, from the environment. A wastewater reticulation system will assist with controlling the amount of nutrients entering the lagoon environment.”
Cleaning up the nutrients distributed between groundwater, soils, lagoon sediments, organic material and stream beds would take some time – but failing to address the source would guarantee conditions would worsen and take longer to clean up.
The government must now decide how to treat the sewage, and where to get rid of the treated wastewater: finding a big block of land to disperse it into the soil, or building a 300-metre sewage outfall pipe into the ocean.
Last year, the PMU called for expressions of interest from landowners on Rarotonga to provide parcels of land for construction of future wastewater and sanitation infrastructure – but the only land offered to them was for a treatment plant on the back road at Turangi.
They were almost reconciled to building an outfall pipe, despite widespread opposition – but now the family of leader Pa Ariki has signalled they may be willing to discuss leasing land to the government up Turangi valley.
Pa Ariki’s son, Sam Napa Jr, told Cook Islands News they had not yet begin talks but would do so in due course.
“I definitely want to avoid the environmental impact of pumping it into the ocean – I’d rather water my own garden with it,” he said.
“And I’m sceptical about the cost effectiveness of an ocean outfall anyway. I don’t think we’ll have enough population to justify it.”
Land-based disposal of treated wastewater is a preferred solution for engineers contracting to the PMU, who share the community’s distaste for a sewage outfall pipe.
But, unable to find the large 16 hectare block they needed for land-based disposal, they were reconciling themselves to an outfall pipe.
They have commissioned coastal engineers from the University of New South Wales to gather data on near-shore ocean currents, and mixing processes in the ocean off Rarotonga, to see if this would work. And they have reported a number of locations where ocean currents generally move along or away from the coast that make this a viable option.
Muri resident Lawrance Bailey said from what he had gathered from community meetings, people generally are against an ocean outfall and they have hoped science would bring out the best method.
Now, there is hope that land may be found after all, on Pa Ariki’s big, hilly block behind Turangi – the same land where David Bowie filmed the movie Merry Christmas Mr Lawrence, in 1982.
The PMU would say only that they’d had positive discussions with some landowners and community groups and were still looking for potentially suitable land for land-based disposal.
The environmental study has confirmed that septic tanks are having a negative effect on Muri lagoon and there is increased urgency to Install a centrally operated wastewater treatment plant in Muri. Other recommended changes include riparian planting, protection of wetlands and seaweed harvesting to achieve improvement in lagoon conditions.