Water From Taupo

Sun (Auckland), Volume I, Issue 288, 25 February 1928, Page 8

Water From Taupo

INTERESTING THEORY EVOLVED

Is There an Underground River?

THE interesting theory that Taupo water feeds some of the lakes and springs in the neighbourhood of Auckland is presented by Mr. F. R. Field, who suggests that from this mysterious subterranean source comes the reliability of the Lake Takapuna and Onehunga water-supply systems. Mr. Field, in the accompanying article, advances the further suggestion that investigation may help Auckland out of its water-supply difficulties.

TN the minds of those who have -* given the matter much consideration, writes Mr. Field, there can be little doubt that Auckland’s future water supply must be drawn from Taupo Lake, the only disputed poiDt. being as to how soon that supply shall be adopted. But, strangely enough, it never seems to have occurred to anyone that Taupo water is already in Auckland, and is being used all over tbe metropolitan area. Any investigator should know that Takapuna (Pupuke) Lake is fed by an underground stream, which is capable of raising the level of the lake till other outlets of the stream restrict its pressure. At the Western Springs, and at other places, notably upon the shores of Manukau Harbour, other large streams constantly debouch, all at about the same level. m Hi

SR The large Otahuhu supply, supplies in various other wells, and other springs now running to waste, all come from much the same level. And there are presumably other springs which discharge below sea level. The water in all cases looks and tastes the same. The evidence seems very clear that all these supplies come from the same source, and are connected. Presumably Lake St. John is fed from the same common source. If so, the lowering of that lake must have slightly reduced the pressure at the other outlets, and rendered them more liable to contamination. These supplies are not appreciably diminished by drought, nor increased by heavy rainfall, as is the case with any surface supply such as that upon which the city mainly depends. Where do they come from? The source must be inland, and a long way off. It must be such that a severe drought does not appreciably affect it. It must be at very considerable elevation, for the water to force its way through long, tortuous, and extremely irregular underground channels and maintain the flow it does at the multiplicity of outlets. The water almost certainly enters these channels from the rocky bed of a lake, not from a bed of a detritus-laden river. Taupo Lake is the most probable source, if not the only possible one. VOLCANIC RIFTS Between the central volcanic region of this island and the Auckland Isthmus there runs a great leading fault zone, part of the general world system. In it are probably from one 1o three original fractures, running roughly north and south, approximately parallel to one another, and extending right through the 30 miles or so of the earth’s crust. They were formed during the very early period of tropical glaciation upon the earth. Once they were formed, no condensation of vapours from below could ever permanently seal them, even

though their seams of quartz, etc., would be periodically formed in them —only to he pushed upwards with successive tilting of the crust segments, and even though the thickness of the crust would continue to increase. As time went on, earthquakes would inevitably shatter the crust upon both sides of a main fracture, not so much (if at all) at the inner surface of The crust, but for an increasing width toward its outer surface, thus producing the broad fault zones we iind today. In the immediate vicinity of voicanoes the subterranean rocks are shattered still the more. Euormous lateral pressure would compress the faces of all fractures, sometimes the more at one level, sometimes the more at another. Periodical disturbances, altering the levels of crust segments, would inevitably cause interstices to be left in all directions, in the upper part of the crust. But no lasting system short of an active volcanic vent could traverse the whole thickness of crust. Where and whilst interstices permit water to percolate to the comparatively shallow depth at which the heated rocks and the pressure permits of sufficient expansion or vaporisation, that water must rise again, and may gush forth in the form of constant hot springs or intermittent geysers such as we find in our thermal regions. In no case, however, can water pass completely through the earth’s crust, because of the intense heat and pressure from below. As a rule it filters through the network of fissures comparatively near the surface, forced sometimes upwards, sometimes downwards, in its very irregular course from a higher to a lower level. Taupo Lake occupies a great basin, in the high volcanic country, where subsidence of crust segments has taken place. It is fed by many streams and small rivers, and It has but the one surface outlet. Gauging would no doubt show that the amount of water flowing into the lake is far greater than that escaping from it by the visible outlet and through evaporation. The balance passes underground. Some of this almost certainly finds its way into the Waihou River. The bulk of it, however, presumably follows the fault zone toward Auckland; and It is quite on the cards that the damming of the Waikato will slightly augment it. WATER GOING TO WASTE? In the underground fissures about the volcanic cones of this isthmus there must be great stores of it, fed through meandering natural conduits from the southern interior, its escape locally being hindered by the surface lava flows. And these same lava flows, with their natural and numerous downward-sloping caves or tunnels, serve to carry off the surface drainage, which in any case would not readily mingle with the purer water held under considerable pressure in the rock-bound cisterns below. For example, a subterranean channel beneath the Waitemata Harbour would convey water to the open Takapuna Lake without any appreciable quantity of sea water entering through cracks which probably exist in the harbour bed, so long as comparatively slight pressure was maintained in that channel. That some or all of this inland water is subjected to high temperature at parts of its underground course is extremely probable—perhaps providential. Indeed, I have seen springs at Onehunga giving off a large amount of steam upon a cold morning, and I am assured that quite a hot spring exists, or existed, near Anne’s Bridge. Perhaps a comparison of chemical characteristics in water from various outlets and from Taupo Lake would definitely settle the question of source. Without a fairly comprehensive survey, including a water diviner’s search for the main channel beyond its entrance to the metropolitan area, no one can tell the full capacity of the supply, or the extent to which it might be developed. But it is plain that there is an immense amount of excellent water now going to waste that might at trifling cost be added to the city supply. Surely the matter is of such importance tltat it ought to be carefully looked into.