Art. XIII.—On a Source of Water Supply for Invercargill.
[Read, before the Southland Institute, 4th May, 1881.]
It seems certain that the whole of the country between Invercargill and the Hokonuis consists of a comparatively level deposit of silts and clays, resting on a bed of sandstone sloping gently to the south. This deposit of gravel, etc., lying on the impervious subjacent rock, has been proved by actual experiment to be absolutely saturated with water; and that, as soon as we pass the first bed of clay, at a very considerable pressure indeed. Here then is what we want; an inexhaustible supply of well-filtered water, close at hand. Why go thirty miles for what can be had within less than thirty yards? Supposing for a moment that artesian springs will never be discovered strong enough to reach the surface, and that even the supply from a series of tube wells might not prove sufficient, why not sink an ordinary shaft, protected by suitable casing (say a bridge cylinder), as far as such a shaft could be sunk, and kept dry with a good pump and a ten horse-power engine? When the engine can no longer keep down the water, then you have got all you want, namely, a water supply which will suffice for many a year to come, and that at a very moderate cost. From what I have seen I should be much surprised if such a shaft ever reached the depth of 100 feet, the quantity of water is so great. Then as to the permanence of the supply. There seems no reason to suppose that the vast subterranean-stream which it is proposed to tap should be less permanent than any river flowing on the surface. Of the existence of this immense supply of water, all who witnessed the progress of the borings are firmly convinced; and it seems reasonable that others should accept their testimony. The water was in fact the one difficulty, and an ever-present difficulty, in the way of the boring. A recent bore put down at Clinton to a depth of 102 feet, presented precisely the same features, so far as the constant inrush of water was concerned. There is really no question as to the quantity. Then as to quality: the water from all the lower levels tasted perfectly pure and free from any mineral solution. As to its freedom from organic matter, twenty miles of natural gravel filter, from the base of the Hokonuis to Invercargill, is a sufficient guarantee for that. This is an advantage which no water collected from surface gathering-grounds can possibly possess; for where the water, or any considerable portion of it, passes over ground covered with herbage and the droppings of animals, organic matter in various stages of decomposition is always present; and where that is found, living 10
organic forms, chiefly of a low type of animal life, are found also. The most careful artificial filtering fails to free the water entirely from these organisms, some of which are now regarded as the probable source of many diseases. Filtering certainly fails to eliminate the germs; that is to say, such artificial filtering as is possible on a large scale, such as that of a water supply. But what art cannot do, nature does with unerring certainty, for microscopic examination shows that the water from deep strata is perfectly free from every form of organic life. It must be remembered also that the vast natural filter-beds which lie between Invercargill and the Hokonuis do their work without trouble or interference, and while perfect in their action, are not a source of expense. I therefore claim for the subterranean supply the double advantage of abundant quantity and perfect purity.
The quantity and purity being taken for granted, it has still been objected to this proposal that the expense of pumping, and the difficulty of obtaining a sufficient pressure for extinguishing fires, render it inferior to a gravitation scheme. Without now going into details, I may remark that the question of expense is one which can easily be settled by calculation; and I fearlessly assert that the expense of pumping will be but a mere fraction of the interest on cost and the maintenance of thirty miles of iron mains.
It should be noticed that the pumping scheme has the advantage of being capable of expansion, at no great expense, exactly as the wants of the town increase. When one shaft is no longer sufficient, another at a small distance can be added, and the supply doubled, and so on as required. Or should it ultimately be decided to adopt a gravitation scheme, this would not stand in the way; for, with the single exception of the pumping machinery and shaft, everything else in the town service is exactly the same as that required for the gravitation scheme, and could be utilized without the slightest loss or additional expense.
On all these grounds it appears that the subterranean sources of supply are worthy not only of more attention than has hitherto been bestowed upon them, but of a serious practical trial. The expense of a trial-shaft would not exceed £200, including hire of engine and pumps. The question of quantity would then be for ever settled.