Art. VII.—Notes on the Proposition to Supply Auckland with Water from Mount Eden.
[Read before the Auckland Institute, 8th December, 1873.]
At last monthly meeting of this Society, Mr. Goodall read a paper in which the above proposition was pretty fully set forth, and a scheme for testing its feasibility stated in detail. This is not, by any means, a new idea, as about eleven or twelve years ago it was proposed as original, and advocated with other three schemes for the same purpose. One of the schemes was, in sober earnest, a proposal to impound the water flowing down the valley of Newton, from the cemeteries; and the advocacy of the Mount Eden one showed equally the absence of all engineering thought or study. Mr Goodall's paper dealt with it differently, and in a clear manner stated a method of testing the level and area of the supposed water basin. One grand point was, however, overlooked, just as the earlier propounder had done. While the existence of water in wells near the base of Mount Eden is undisputed, and that at a tolerably high level above the sea, there is no attempt to show that there is a source of supply at all adequate to the demand. It is not only necessary to show a reservoir of water, but how much may be daily and yearly drawn from it without failure must also be demonstrated. Two lines of evidence are required to show the latter point in this case. Firstly, the discharge of the
water from the presumed basin, and, secondly, the source from whence it is drawn. In ordinary gathering grounds, the first of these only would be pretty conclusive, although the second would always be required and given as corroborative evidence. There being no visible overflow which would be anything like an adequate supply within more than two miles, or nearer than the Western Springs, we must look to the source of supply, and compare it with that discharge.
The Mount Eden cone has been thrown up nearly in the centre of a tufaceous basin, which is now incomplete, although distinctly traceable on several parts of its circumference. Towards the east the tuff crater has been washed away, or broken through by the solid lava streams on which New-market stands; the clay is 12ft. to 18ft. below the level of that place, and it is important to note that it is at very nearly the level of that clay at which water is found in the wells at Messrs. Seccombes' brewery and at the gaol. We have no means of knowing the exact area of the annular space between the lip of the tuff crater and the central aperture from which the lava was, subsequently to the elevation of the tuff basin, discharged. But from the enormous masses of lava ejected in many successive eruptions, and in nearly all directions, the annular water-holding area must be very small, and cannot be looked on as being more than half a square mile. It is needless to remark that such an area, or double that area even, is wholly inadequate to serve as a gathering ground.
Thus far have we considered the supply from rainfall. It is true that, in dealing with these lava cones and their so-called mysterious springs, many do not look to local rainfall as the source, but boldly scan some distant lake, and, totally ignoring the laws of gravitation and those regulating the flow of water, as well as the seemingly insuperable difficulty of intervening seas, point to a probable subterranean connection and source of supply. Such a connection between Mount Eden and Takapuna Lake is hinted at in the paper calling forth these remarks, and not a few have expressed belief that the rainfall of Rangitoto is the source of the waterflow from that same lake. Now, it seems the result of an exceedingly strong imagination to conceive that water falling on a mountain like Rangitoto, composed of scoria extending into and below the level of low water, should find its way to any place but the sea; or why should the very limited overflow from the lake suggest any other source than the area of its surrounding basin?
But to return to the subject of enquiry, the outflow at the Western Springs represents with certainty that of several thousand acres, as at that locality only has the tertiary formation permitted the lava to reach the sea, which was ejected from Mounts Eden and Albert to the northward. The rainfall on that area, not evaporated or retained by soil and vegetation, must be that
overflow which wells up so grandly; and many underground rivulets, following ancient valleys in the tertiary clays, must be convergent to form those noble springs. And the farther inland at which water is sought, the smaller and further apart will be those rivulets, until, on reaching the summit of the watershed at Mount Eden, the minimum will be attained; and, although at that elevation a basin may be found containing many million cubic feet of water, it would only be a work of time to exhaust it if the all-important points of rainfall and gathering ground are inadequate to keeping up the supply.