Study proves exceptional visual clarity of Te Waikoropupū Springs

Friday 13 April 2018

Te Waikoropupū Springs has proven to be clearer than it was when first measured 25 years ago.

Using new methodology, NIWA scientists, contracted by Tasman District Council through an Envirolink Science grant, deployed instruments for three months between October and January at the Springs in Takaka to measure the clarity of the water.

Mayor Richard Kempthorne said with the level of community and wider interest in the Springs the Council felt it was important to see if it was possible to establish a recent clarity measure for the Springs.

“With the success NIWA achieved through the use of instruments instead of divers to assess visual clarity in Blue Lake in the Nelson Lakes National Park we felt there was such a possibility.”

The first optical measurements of the Springs were made by the Department of Scientific and Industrial Research in 1993. This work resulted in a visual clarity measure of 63m.

25 years later NIWA scientists have revised that figure upwards and now estimate the average visual clarity of the water to be about 75m. Pure water has a visual clarity of about 83m.The Springs are now broadly comparable to Blue Lake which has a visual clarity of 70-80m.

The clarity was measured during a three month deployment in the main Spring basin using a beam transmissometer which captured 60 measurements in one-minute bursts every 10 minutes, resulting in almost one million data-points. So there was a need for detailed data quality control and analysis. As this project ran over several months it has also provided a comprehensive data set to assess how visual clarity varies over different time scales, from hours to months.  “Given the time taken and number of data-points gathered we can have a high level of confidence in the result.”

With the success of the methodology and the ongoing interest in the health of the Springs Mayor Kempthorne suggested that this is likely to be something we would now look to repeat on a regular basis to give the community reassurance that nothing is changing, for example every five years to coincide with other State of the Environment Monitoring programmes.

In addition to the clarity measure additional water quality properties, including temperature, dissolved oxygen and turbidity, were also measured.

“NIWA will compare this to Council’s regular water quality sampling and that of the Friends of Golden Bay and reported back in a few weeks’ time,” said Kempthorne. “While giving further reassurance on the quality of the spring it should also prove the robustness of the monitoring undertaken by the Council.”

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.

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. Visual clarity was as low as 4m for a short time in mid-January following almost 240mm of rainfall that resulted in surface waters entering the springs’ basin from the surrounding bush reserve. These intense rainfall events are expected to have this type of impact in the valley floor with similar reductions in clarity occurring at Blue Lake from time to time following storms.

The exceptional visual clarity appears to result from extremely efficient natural filtering removing particles within the Springs aquifer before re-emergence of the water. Te Waikoropupū Springs, along with Blue Lake, are considered to have some of the clearest waters ever measured. Te Waikoropupū Springs are also the largest cold water springs in the Southern Hemisphere.

“It is great news that there is no evidence to indicate there has been any decline in visual clarity in Waikoropupū in the 25 years since the direct measurement of 63 m was made by the DSIR,” said Mayor Kempthorne. “The new report is very reassuring and has been keenly awaited by the community so I know it will be widely distributed.

“We are expecting a level of national and maybe even international interest given the iconic nature of the Spring and the present Water Conservation Order process occurring”.