Springs still hold a surprise or two
Te Waikoropupu¯ Springs’ phenomenal output makes them this country’s largest freshwater springs. But many people are unaware that they are not the only exit for groundwater gushing out of the subterranean marble and limestone that underlies the Takaka Valley.
Local fishermen have long speculated about the existence of three large freshwater springs offshore in Golden Bay, their presence ‘‘felt’’ as a vessel moves over them. In completely calm conditions at low tide, they can even be seen on the surface as a slight upwelling.
As pressure in the Waikoropupu¯ goes down (and there’s absolutely no doubt in my mind that this is happening as the Tasman District Council grants more and more water extraction from the connected aquifers), then I am prepared to speculate that these marine springs must be correspondingly reducing their outpouring. They all come out of the same aquifer, after all.
These three marine springs are now well marked on the marine chart of Golden Bay, each with a pictorial representation of a whale spout. One lies halfway between Tata Islands and Taupo Point, just off Wainui Bay; another is
2.5 kilometres due north of Tarakohe; and the third is 5km northeast of Waitapu.
Their positions were accurately determined about 1956 by HMNZS Lachlan, which then added them to the chart.
The 2200-tonne River-class frigate would have made a grand sight going back and forth in Golden Bay at the time of its survey. The ship had been borrowed out of reserve from the Royal Australian Navy (RAN) in 1948 while a new survey ship was built for our navy in England.
Part of the ship’s company included hydrographic experts from the RAN and the Royal Navy, here to help us set up our hydrographic service.
The Lachlan’s first survey was of Wellington Harbour and Cook Strait, before it did Foveaux Strait and Bluff Harbour. During the next 24 years, over 50 per cent of our coastline as well as several Pacific islands were resurveyed by this vessel.
During the survey of Golden Bay, the Lachlan’s echo sounder suddenly showed what appeared to be a big protrusion of seabed right under the keel. She ‘‘slammed on
the brakes’’, and a closer examination by the hydrographic crew revealed that the seabed trace was actually caused by differing water densities, typical of fresh water vents in the sea.
With their existence authenticated, it didn’t take long for some local entrepreneurs to come up with the idea of profitably extracting the water. Maybe if a big pipe could be sunk into the vent, the upwelling would fill a tanker securely moored to a pontoon, with its watery cargo sold to arid countries, namely the emerging Middle East.
The idea caught on with managers of the Golden Bay Cement Company, who thought it could better utilise the company’s growing port facilities. They even authorised the Strathallan to locate the three springs again and mark each one with a big buoy.
This 14.6-metre tug and pilot boat had been purchased by the cement company from the Timaru Harbour Board to assist with berthage in the developing harbour, namely of the John Wilson and Ligar Bay cement boats coming in and out of Tarakohe at the time.
For a whole fortnight, the Strathallan plied up and down and across the bay, following a grid pattern, her echo sounder tickticking away, looking for the telltale ‘‘spike’’ to appear on the graph paper. The only hopeful hit was found just off Taupo Point in Wainui Bay, but when a scuba diver went down to investigate, he couldn’t find anything to indicate a vent. The whole venture proved a waste of time.
It’s amazing how little we still know about these remarkable marine vents. The discharge must be relatively large to have been revealed by surface disturbance from a depth of 13 metres to 15m (seven and eight fathoms), which is twice the depth of the main pool of Waikoropupu¯ .
The small tidal nature of Te Waikoropupu¯ Springs – four millimetres at Main Springs, and 16mm at Fish Creek Springs – was at first assumed to be a result of the
subterranean connection to the offshore freshwater vents, their frequency preceding the marine tides in Golden Bay by one hour.
But newer research shows that the double-amplitude pattern of the tidal cycles in the springs – in this case, each second tide being roughly twice as high as the preceding one – is more suggestive of earth tides than marine ones.
Earth tides result from the flexing of the planet, rather than the tidal movement of the seas. The gravitational pulls from the Sun and Moon cause Earth to distort, more so as Sun, Moon and Earth align. Enclosed bodies of water such as lakes are affected by this flexing, so it’s no wonder the 1.5-cubic kilometre reservoir of water beneath Te Waikoropupu¯ Springs is expressing itself with daily tides, just like the marine vents would be as well.
Saltiness is another hookup. Salinity in the Waikoropupu¯ has been determined at a constant 0.5 per cent, or a seawater to freshwater ratio of 1: 200. Initially, it was thought that the ever-soslight brackishness of Te Waikoropupu¯ Springs arose from backflow from the marine vents. But it’s more likely that a dense mass of seawater penetrates the cave systems under the Takaka Valley. Caught up in the milling freshwater currents, it mixes back by a venturi process to subtly contaminate the springs.
Without a doubt, our local springs are fascinating, but perhaps the last word should go to 14th-century Japanese Zen master Muso Soseki, who so succinctly said: ‘‘A wise man enjoys the purity of water.’’