Niwa has estimated the visual clarity of Te Waikoropupū Springs, near Takaka, to be about 76 metres, and at times approaches 81m - near the theoretical maximum for pure water of about 83m.

It produced the estimates after a three-month monitoring programme, during which nearly a million water clarity estimates were made.

Divers are not allowed into the spring, so Niwa used measuring instruments hung from a submerged line near one of the main vents of the springs.

NIWA Te Waikoropupū Springs: Niwa estimates that at times the water is so clear that at times it is just short of the theoretical maximum.

"What this study shows us is that the Te Waikoropupū Springs have exceptional visual clarity, at times close to that of pure water, and broadly comparable to that of Blue Lake which has a visual clarity of 70-80m," Niwa scientist Mark Gall said.

READ MORE:

* Te Waikoropupū Springs clarity improves over past 25 years

* Te Waikoropupū Springs flow highly modified, says TDC in Water Conservation Order application submission

* Protecting Te Waikoropupū Springs' purity holds deep spiritual significance to iwi

* Te Waikoropupū Springs Water Conservation order begins in Takaka

* Te Waikoropupū Springs hearing adjourned over challenge to evidence

The exceptional visual clarity appeared to result from extremely efficient natural filtering removing particles within the springs aquifer before re-emergence of the water.

CHRIS CONROY Extremely efficient natural filtering removes particles within the springs aquifer before re-emergence of the water.

Te Waikoropupū Springs, along with Blue Lake, were considered to have some of the clearest waters ever measured. Te Waikoropupū were also the largest cold water springs in the Southern Hemisphere.

Gall used a GoPro camera to film the deployment of the monitoring instruments and to check they were in the right place.



The underwater footage also provided some insight into how the springs work, and how the water clarity varies for short periods when water pushes out through layers of sand.

Niwa used measuring instruments hung from a submerged line near one of the main vents. The main instrument used was a beam transmissometer which captured 60 readings in one-minute bursts, every 10 minutes, resulting in the nearly one million water clarity estimates over the deployment period.

The high frequency monitoring detected small daily variations corresponding to about 1-2m in visual clarity, with the highest visibility at midnight, and lowest around midday.



"This is likely to be due to plants in the spring basin releasing light-scattering oxygen bubbles as they photosynthesise during the day", Gall said.

Underwater video demonstrated that "dancing"' white marble sands on the floor of the springs coincided with some short-term episodes of reduced visual clarity, lasting between a few hours and several days.

The last time water clarity measurements were made at Te Waikoropupū was in 1993 by the Department of Scientific and Industrial Research. At that time the visual clarity was measured by divers at 63m.



Since then, technology has advanced, theoretical maximum clarity measurements have been revised and divers are no longer permitted in the spring.

The Niwa monitoring was done for the Tasman District Council, which has identified maintaining the clarity of the springs as a key priority.

An application for a water conservation order for Te Waikoropupū Springs has been lodged with the Minister for the Environment, with hearings held in April and May.