THE CHRISTCHURCH ARTESIANS AND THE CITY WATER SUPPLY.

Star (Christchurch), Issue 17343, 16 July 1924, Page 10

THE CHRISTCHURCH ARTESIANS AND THE CITY WATER SUPPLY.

BY CHAS. CHILTON (Rector and Professor of Biology, Canterbury College, N.Z.) XLVI. DEEP WELLS AND SHALLOW WELLS COMPARED. The idea that it was necessary to have a reservoir on a firm foundation on the Port Hills was one of the reasons fox' placing the pumping station at Cashmere, where is would be near the reservoir. For reasons that have been already given, the writer has contended that it is bad policy to maintain the Cashmere Station as the head, works for the growing city. As an additional reason he has urged that at Cashmere only the shallow stratum can be tapped, that it is less permanent and less abundant than the deeper strata, and is more liable to contamination. The apologists for the City Council's Scheme No. 2, submitted to the ratepayers in January, 1924, questioned the accuracy of these statements and, indeed, gave them a general denial. The ex-City Engineer is reported to have said: “ None of the deep wells will return so large a volume of water as that to be obtained from the 80 feet stratum at Cashmere . . . He knew of no greater supply that could possibly be obtained underground.” The Mayor was less general in his assertion, and gave definite figures, saying: “ It is sufficient to say that the wells at Cashmere Station, with a total sectional area of 371 square inches, give 2,500,000 gallons a day, or 6738 gallons a square inch daily. The deep wells at St. Albans, with an area of 191 square inches, give only 816,000 gallons, or 1272 gallons a square inch—less than two-thirds of the either yield.” These statements, made first in the daily newspapers, were repeated in substance, if not in words, at the public meeting held for the purpose of explaining the proposals to the citizens two days before, the poll. At the meeting it was added that the St. Albans Station already had not been a success, and the council did xxot want to have any more disappointments. Why a station supposed to be not a success was the one selected for the purpose of comparison was not explained, aud by some oversight no comparison was made with the other sub-station, i.e., the one at Sydenham. Here there are three deep wells only 4in. in diameter. The sectional area is therefore 37 square inches, and as the flow is given as about 800,000 gallons per day, the flow per square inch works out at 21,622 gallons per square inch as against the 6738 gallons at Cashmere. Really it is probably much more for, although 800,000 gallons a day is given as the amount that can be pumped per day, the City Council Handbook for 1924 gives the actual flow as 910 gallons per minute, that is 1,353,600 gallons per day; the flow per square inch would therefore be about 36,581 daily. This may be tested by another example. Since the rejection of the loan at the poll in January, the City Council has sunk a l lin. bore at Cashmere to a depth of about 80 feel, and this is reported to he yielding approximately 1,000,000 gallons daily. This comes to about 6486 gallons to the square, inch, that is, nearly the same as the flow given by the other wells at Cashmere. Compare this with the tin. "bore at the City. Tepid Baths, yielding, according to the Handbook, 300 gallons a minute, or 432,000 per day. This gives a flow of 34,354 gallons per square inch daily. Other figures could easily be given, jiut these are sufficient to show that the method of comparison of shallow with deep wells adopted by the City Council has something of the boomerang nature, about it. Undoubtedly, a very good supply of water is obtained at Cashmere, and, without exaggeration, it may be described as “ a vast supply ” —it would need to be if the, whole of the water supply for Greater Christchurch is to be obtained from it. But the flow at Cashmere is probably no greater than that from the saixxc stratum obtained at other parts of the city. The Christchurch Public Hospital, the Railway Works at Addington, and manyother public or private concerns get their supply from the same shallow stratum at about 80 feet depth, and find it quite sufficient for ail their wants. The ex-City Engineer has, however, sought to explain the copious flow at Cashmere by supposing it to be water from deeper stratum that makes its way to the surface at the foot of the Port Hills, and that pumping at Cashmere is like pumping from a beautiful lake of the purest possible water. Against this theory it is sufficient to point out that the shallow stratum can lie traced over the greater part of Christchurch, right down to near the Port Hills, it is found at depths from 70 feet to 100 feet, in all cases the water now rises to about the surface; in years past it rose two, three, or four feet above the surface, till the pressure was reduced by the sinking of too many wells; chemical analysis in 1907 of the water at Cashmere and another shallow well nearer the city showed that they were practically identical and therefore probably came from the same source. Moreover, experience has shown that pumping from the cluster of wells at Cashmere has undoubtedly partially drained the shallow wells in the neighbourhood—complaints to this effect were made within a year or two after the Cashmere Station was put into operation. If the water obtained at Cashmere is deep stratum water that by some means has made its way up through the impervious beds so must lie ail the water from the same stratum, and the difficulty of explaining why it rises only to the surface while that from the deeper strata when lapped rises to varying heights above the ground level is one that may be left to the City Council’s officers to explain. But while the ex-City Engineer accounts for the great flow at Cashmere by supposing it to be. really deep stratum'water that is being obtained from the shallow depth to which the wells are sunk, it is curious to note that Mr Stewart, the city’s wellsinker, is not satisfied to use this deep stratum water apparently ready to his hand, hut is firmly of opinion that there is a deep stratum existing at the main station and, for many weeks, with the permission of the City Council, he has been boring down below the 80 feet stratum—-at a cost lo the citizens of £1 per foot —in the hope of finding ibis deep stratum. When he finds a deep water-bearing stratum there the City Council will doubtless let the citizens know: (The next article will appear on Saturday, July 19.)