LAKE TAKAPUNA WATER

New Zealand Herald, Volume LX, Issue 18552, 9 November 1923, Page 11

LAKE TAKAPUNA WATER

NO ENTRY FROM THE SEA.

REPORT BY GEOLOGIST.

" TENTATIVE CONCLUSION."

The question of sea-water entering Lake Takapuna is dealt with in the report furnished to the Lake Takapuna Board of Control, by Mr. P. G. Morgan, Government geologist. * Lake Takapuna,** Mr. Morgan says, . is a' nearly circular basin, with a shallow bay at the north-oast corner. It has an area of 270 acres, and a maximum depth, when at its highest level, of about 200 ft. No streams run into it, but it receives the surface drainage of about 200 acres of land, and is evidently fed to some extent by springs. As is well known, the lake occupies an explosion crater. It has been used as a source of water supply since 1895. In ' that year 19,000,000 gallons of water were pumped from the lake; in 1922, approximately 201,000,000 gallons. The population of the district supplied has greatly increased since 1895, and is still rapidly increasing. In 1916 it wa? 14,406; in 1921 it was 17,223, and is now estimated at 18,400 or more. " Owing to variation in rainfall and rate of evaporation, the amount of water obtainable from Lake Takapuna in any one year is variable. When the lake is high there is considerable leakage; when it is low there may bo some leakage, but almost certainly not a great deal. The data and other information at my command do not enable me to make a very good estimate, but in an average year the available water supply seems to be somewhat over 150,000.000 gallons, and I will assume that it is 160,000,000 gallons. The daily supply to a population of 18,400 on this basis"would be between 24 and 25 gallons, which is insufficient for modern vequirements. Not less than fifty gallons per inhabitant per diem is required in a climate like Auckland's. Evidently ever since the population served reached 9000 or 10,000— is. for the last twelve years or more—the Takapuna supply has been insufficient, and the necessity for an additional supply urgent. Use of Lake as Reservoir. ** The subject hag been much discussed, and several reports on possible sources of water supplies have been obtained, but none of the schemes suggested has yet been adopted. The Rangitopuni stream is at present favoured for a permanent supply, but since it will be at least six years before its water can he brought into She Devonport-Takapuna district, those concerned hope that Lake Takapuna can be used as a reservoir, and lowered below sea level without an influx of sea-water taking place. "The surface rocks round Lake Takapuna are loosely compacted tuff and scoria, underlain in many places by a How of basalt. The popular view, very naturallyand one more or less held by myself until my recent examination— that if the lake were lowered below sea level, seawater would begin to flow in. On examining the lake and its surroundings, in company with members of the Board of Control, Mr. F. E. Powell, consulting engineer, and Mr. J. A. Bartrum, lecturer on geology at the Auckland University College. I ascertained that the basalt was underlain at many points by a watertight silt of clay. Lake a Watertight Hole. "This silt, almost certainly, is underlain by the Waitemata beds, which in the main are claystones with more or less fine sand, and are practically watertight. Thus, as suggested to me by Mr. Kartrum, the lake is probably not in communication with the sea by porous beds lying below sea level, but may be regarded as a watertight hole, which may be pumped below sea level without salt water flowing in. If this is so, the level of the lake can safely be reduced below that of the sea. "While the Rang'topuni or other scheme of water supply is being brought into operation, it is proposed, if necessary, to divert into the lake water frojrt the Wairau Stream, which drains the country to the west of the lake and flows into the sea not far to the north. The situation is such that I cannot take it on myself to advise whether it is necessary to obtain a temporary supply of water from the Wairau Stream wbiie the Rangitopuni or other permanent supply is being brought in; but I must urge the desirability of obtaining a supply of pure water without delay, from a source or sources sufficient for the needs of a largo population. No more reliance ought to be placed upon Lake Takapuna as a source of water than circumstances compel. Regular Tests of Salinity, "Boring would be useful in ascertaining whether the watertight silts previously mentioned reach above sea level in this locality, but cannot give absolute certainty* that no channel below sea level exists. ... I suggest that frequent regular tests of the salinity of the lake water should be made, the samples to be ; ta' en from several points, more especially the north-eastern side of the lake. A marked increase in chlorine contents would indicate influx of sea water. Fortunately chlorine determination can be made easily, quickly and rapidly. , . . The conclusion is that the hollow occupied by »Lake Takapuna below sea level is practically watertight, and therefore can probably be lowered below sea level without sea water entering. This conclusion is tentative. I trmt, however, that it will not be modified by further investigations."