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Investigation results confirm wastewater is the leading cause of Muri Lagoon issues

Updated: Nov 24, 2020

The Mei Te Vai Ki Te Vai project team (PMU) have today released the results of their extensive environmental investigations. The results confirmed that nutrients from on-site wastewater systems are harming the sensitive Muri Lagoon environment – and even well-managed systems are contributing to the problem.

Extensive environmental investigations have identified where the seaweed is growing in Muri Lagoon, why it’s growing there, where the nutrients feeding the seaweed come from, and how they get into the lagoon.

The seaweed in Muri Lagoon is growing in environmentally sensitive areas, where the water is relatively still. Nutrients from on-site wastewater systems are getting into shallow groundwater in the coral sands, flowing into these sensitive areas and feeding the seaweed growth. The seaweed is also flourishing in a ‘self-feeding’ cycle - when it dies back, it puts more nutrients into the sediment it’s growing in. So while the seaweed is not currently visible in the lagoon, it is still there and can be expected to regrow.

PMU lead environmental scientist Murray Wallis says: “The results enable us to conclude that the increase in wastewater systems over the years is most likely to have triggered the seaweed problem in Muri Lagoon. Wastewater is making its way into shallow groundwater, and out into the lagoon. The area of the lagoon it’s going into is sensitive, and the water is relatively still. This is why we’re seeing the heavy seaweed growth in these places.”

As well as identifying the cause of the seaweed problem, the PMU team also investigated a range of solutions, including some suggested by the local community through the consultation process.

Murray Wallis says: “Members of the community have asked us whether dredging might help. As part of our investigations, we tested three options – lowering the ridge in the main channel north of Oneroa, removing rubble along the inland side of the reef crest, at the outer edge of the lagoon, and removing the seaweed together with the sediment it grows in.”

“Unfortunately, none of these options improve the flow in the still water areas that are the problem. All three options would also cause adverse effects on the environment, and dredging the main channel could potentially worsen existing erosion issues.”

On a positive note, the investigations also identified three key changes that will help to restore the lagoon’s health. Firstly, a reticulated wastewater system to service the highly-developed areas along the coastline. Secondly, it will be important to protect and enhance swamps and the edges of streams. And the third action is to regularly harvest the seaweed, to disrupt its self-feeding process and remove it as a source of nutrients.

The investigations highlighted just how important swamps are to lagoon health, as they act as natural filters for nutrients and sediment. Likewise, planting along streams filters sediment, so helps prevent nutrients from getting into stream water and out into the lagoon.

In addition to identifying the cause and some solutions to the environmental problems in Muri Lagoon, the investigations have also contributed a significant amount of new knowledge about Rarotonga’s geology and Muri Lagoon’s flow patterns.

The PMU team will be sharing this knowledge with the community through a series of presentations this week. The PMU team has already been busy presenting to a range of community leaders including Cabinet, the House of Ariki and Muri Aronga Mana.

The summary report has now been published online at in the News and Publications section of our website and will be presented at a public meeting this Friday.

The full technical reports will also be published online, later this month.

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