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Art. XXVIII.—On the Percentage of Chlorine in Lake Takapuna.
[Read before the Auckland Institute, 25th September, 1899.]
On account of the proximity of Lake Takapuna to the sea, and its being considerably deeper than Rangitoto Channel, the question has been raised as to whether there may not be a substratum of sea-water in the lake at depths below the channel in question. To decide this matter I have made tests of the water taken from the lake at various depths.
Lake Takapuna, or Pupuki, is stated by Hochstetter to be a volcanic crater having a depth of 28 fathoms, or 168 ft. It has a superficial area of 268 acres, and its eastern edge is not more than 200 ft. from high tide in Rangitoto Channel, while the tidal waters of the Waitemata Harbour in Shoal Bay approach within 400 yards of its western edge.
The deepest part of Rangitoto Channel is given on the charts as 8 fathoms, or 48 ft., at low water: therefore the bottom of the lake is, according to Hochstetter's measurement, 100 ft. below the deepest part of the channel, estimating the surface of the lake at 10 ft. above high-tide mark, and the rise of tide at 10 ft.
From analyses we have made at various times the chlorine
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at the surface is found to be fairly constant, and the same remark applies to the total solids. As an instance of this, three samples were taken on the 2nd December, 1897, the first being from about 100 yards from the edge and from 14 ft. below the surface of the lake, the second being from the pump-well at the pumping-station, and the third from the Council Chambers at Devonport. These samples yielded chlorine 2.87 gr. per gallon in each instance, while the total solids at 105°C. were 8.68 gr. per gallon from the lake and Devonport samples, the pump-well giving 8.96 gr. per gallon.
To obtain the water from the bottom of the lake I used the apparatus advised by Fresenius, the results being satisfactory. The first sample was taken at 50 ft. below the surface, the water being clear and without sediment. The second sample was taken at a depth of 100 ft., the water being also clear and free from precipitable matter. The third sample was taken from the bottom of the lake, at a depth of 157 ft., the water containing a small amount of sediment. These samples were taken from midway between the pumping-station and the wooded shore opposite, this being stated as the deepest part. A fourth sample was then obtained about 100 yards nearer the wooded shore, the depth being 160 ft. This sample was very turbid, and contained a considerable amount of ooze from the bottom, which precipitated completely after twenty-four hours. A fifth cast was made at a depth of 163 ft., no water being obtained on this occasion, this being made as a test of the apparatus, which was found to work satisfactorily.
The results of analysis gave chlorine 2.87 gr. per gallon, equal to chloride of sodium 4.73 gr. in each sample, while the total solids gave 8.68 gr. per gallon in each instance, the water from the bottom of the lake being decanted from the precipitated matter. There is therefore not the slightest difference in these two factors of the water's purity at varying depths, and these agree with our analyses of samples taken at various periods.
The deposit from the lake-bottom was composed chiefly of vegetable débris and ferruginous clay, together with a large number and variety of diatoms, some amongst them being very beautiful objects.
The fact that Lake Takapuna is a water-supply of great purity, and that its present utilisation must be very largely increased in the not distant future, is, I think, more than sufficient apology for my bringing this subject before you.