Study proves exceptional visual clarity of Te Waikoropupū Springs
Te Waikoropupu 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. TeWaikoropupū 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”.
For the full report see: http://www.tasman.govt.nz/link/water-clarity-report-te-waikoropupu-springs