CHAPTER IV. ARTESIAN WELLS.
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CHAPTER IV.
ARTESIAN WELLS.
A Report on the formation of the Canterbury Plains would be imperfect, did it not offer some observations on the causes which have led to the remarkable phenomenon of getting water in abundance, by artesian wells, in and near Christchurch, apparently in loose deposits; but, as this Report is already so much extended, I shall not dwell at length on this subject, the more so as I intend to treat of it fully in a Report on the geological features of the ground below and around Christchurch.
In the first chapter, I stated that the large lake between the mouths of the Rakaia and Waimakariri, by various causes at work, had been mostly filled up, and of which I enumerated, among others, the following:--the deposits of silt (glacial mud brought down by the rivers), the rivers forming deltas, and the invasion of drift sand from the shores of the sea. I further explained that by slow upheaval swamps had been formed, giving existence to extensive beds of peat, by which the ground gradually would rise still more, so as to offer the necessary conditions for the growth of kahikatea (white pine) and other forest trees, for which swampy ground is favourable, whilst, in intersecting channels between the dry land the deposits of silt would still be thrown down, so as to form beds of clay and loam.
The rise or oscillation of the country still continuing, rivers began to cross these lacustrine or peaty deposits, either by throwing an embankment over them or by excavating and afterwards filling their channels by raising their beds with shingle and sand. Thus, it happens that the most heterogeneous beds are met with, when sinking for wells in these old lacustrine beds, as, for instance, shingle river deposits, old vegetable soil with stumps of trees, peaty beds, leaves and twigs, strata of clays and loam, buried forests and driftsands, so as to baffle all attempt to trace their succession for any distance, every well presenting different features. But one peculiar and highly interesting fact was revealed, which in future years, when our plains are more settled, will prove of great value, namely, that the shingle beds, when reached at a certain depth and under peculiar conditions, become so compact, as to be impervious to running water, and that, as a consequence by boring to a certain depth, the water having percolated through the looser deposits on the top will rise and form artesian wells, in every way resembling those formed amongt strata of harder rocks.
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It is natural that water of our glacier torrents, when sinking through the shingle, being charged with finely triturated matter, will leave it behind in a very short time, and as soon as the necessary conditions present themselves, will rise again to the surface, but perfectly clear and tasteless, as the waters of the Heathcote and similar rivers, as well as that of the artesian wells in Christchurch.
But to account for the latter is not so easy as it appeared at first sight, from the fact that on the northern side of the river behind the residence of Mr. Gould, a natural spring of similar nature rises to the surface, and the question is at once suggested why we have on the southern side of the Avon to bore 60 or 80 feet before we reach the water ascending above the surface, whilst on the northern side, at a very short distance, it rises without any artificial means having been employed.
Three causes by which such a phenomenon could be explained are apparent, namely, either a basin sloping slightly down from Banks' Peninsula towards the north, with a very steep wing on the northern side of the River Avon; or, secondly, a ridge of rocks reaching nearly to the surface where the natural spring behind Mr. Gould's residence rises; or, thirdly, and what the surface of the localities in question indicates, an old river-bed of the Waimakariri, after having cut its channels in the lacustrine and similar deposits, forming a new embankment, and thus filling up its course with deposits of a more permeable nature.
Thus water from leakage on higher ground would, on both sides of that channel, rise when coming in contact with the impermeable argillaceous deposits, as soon as the necessary conditions were offered. Such an occurrence is the more easy to understand when we consider that the lacustrine beds will not only be deposited on slight slopes, but also around and over any accidental unevenness, by which the water running upon the former would be pressed upwards.
In the old channel of the river, which can easily be traced from the sources of the Avon to the main river, the water would sink to such a depth till it would reach impermeable strata on which it could flow.
That such changes have taken place, even repeatedly in this very same river-bed, is pointed out to us by the beds of peat, sand, or loam, on which the surface water collects, situated at a depth of ten to twenty feet, and met with in various localities. This water, by coming in contact with the humic acid and other decaying vegetable and animal matter, is sometimes of a dark colour, with a very offensive smell and taste.
Thus when the old river left its bed to form another course more
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towards the north, lacustrine, peat, or similar formations soon obliterated the abandoned channel, till, by a new change in its course, the Waimakariri once more entered it, as shown by the shingle, sand, and silt, near the present surface, and by the well-defined terraces along the banks of the Avon.
The conclusion drawn from the observation that fluviatile deposits become at a certain depth impervious, is one of great practical value, because many parts of our plains, at present waterless, or where the obtaining of water by means of ordinary wells offers greater difficulty than a settler with limited means could overcome, will thus be made available.
And if I might be allowed to suggest an experiment to test the truth of such an hypothesis, it would be the attempt to bore an artesian well on the Great South Road, between the Ashburton and Rakaia, which would be a great benefit for the public. If a constant large supply of good and wholesome water could thus be obtained, would it not at the same time give great assistance in settling that part of the country, and the expense incurred by the Province would thus be amply repaid.
In following the boundaries of the different fans, there is no doubt that many localities could be chosen which would present us with a fair chance of succeeding.
I owe to the courtesy of Messrs. E. Dobson, E. Moore the City Surveyor, and several professional gentlemen, a great deal of useful detail information on this subject, which I hope to lay before you, together with my own observations, at some future day.
