Art. L.—On the Artesian Wells at Longburn.
[Read before the Wellington Philosophical Society, 4th March, 1899.]
The well I have particularly to describe is an artesian pipe 2in. in diameter recently put down by the Wellington and Manawatu Railway Company for the purpose of obtaining a supply of water for locomotive purposes. Unfortunately, I could not get a clean sample of the sand, as it had been removed, and was mixed with cinders and other extraneous matter.
The railway-well marked “A” on Plate LII:, fig. 1, is sunk to a depth of 358ft. below the surface of the ground. The water rises in a pipe to a height of about 43 ft. above the surface. The pressure was 20 lb. on the square inch, indicated on an ordinary pressure-gauge at the surface of the ground. The discharge at ground-level was 4,300 gallons per hour.
The following is a note of the depth of the Various Strata passed through in sinking :—
|Sand (water-bearing; good flow here)||6||285|
|Sand (timber in this)||4||325|
|Papa (water here)||16||356|
This well was successfully sunk by Mr. E. J. Martin, of Palmerston North.
I give below particulars of two other wells sunk in the vicinity of Longburn. Well marked “B” (Plate LII., fig. 1), 6 in. diameter, is sunk for the Longburn Freezing Company. It is situated about 20 chains south-east of the company's well, and passes through the following strata:—
|Ft. in.||Ft. in.|
|Clay||46 0||46 0|
|Shingle||57 0||103 0|
|Clay||17 6||120 6|
|Sand||44 0||164 6|
|Clay||5 0||169 6|
|Sand||12 6||182 0|
|Clay||11 0||193 0|
|Sand||5 0||198 0|
|Clay||7 0||205 0|
|Sand||65 0||270 0|
|Shingle||6 0||276 0|
This well discharges not more than 1,000 gallons per hour.
There is another 6 in. well and a 2 in. well at the freezing-works. The 6 in. goes to the same depth as well marked “B.” The 2 in. well goes to a depth of 330ft. approximate, but I have not yet obtained particulars of it. None of the Freezing Company's wells discharge nearly as much water as the Railway Company's well marked “A.”
Well marked “C” (Plate LII., fig. 1) is situated on Mr. Walker's property, about 100 chains south-east of the Rail-
way Company's well near Longburn. It is sunk in a natural depression of about 10 ft. below the general surface of this ground, and passes through the following strata:—
|Sand and timber||3||13|
The water rises about 16 ft. above ground-level in a pipe, or, say, 6 ft. above the average surface of the ground.
I may state that Mr. Martin is sinking other wells near Longburn, including one for Mr. Riddiford, between Pal-merston and Longburn, on the eastern side of the main road, between the road and the line. It passed through—
|Hard blue clay||20||95|
|Clay and shingle mixed||20||115|
|Black sand and timber||27||165|
|Hard blue clay||27||210|
|Blue sand and shingle||10||220|
Underneath is shingle carrying good soft water rising 32ft. above the surface of the ground, and running 72 gallons per minute, or 4,320 gallons per hour, to 2ft. above the surface of the ground.
It is important to ascertain by analysis the degree of suitability of this water for use in locomotive boilers. I use carbonate of soda and caustic soda in small quantities with all our boiler-water, with a view to neutralise any free acid, and to assist in precipitating any carbonate of lime, that may be present.
Mr. Skey submitted samples of this water to analysis, and reports: “This is a clear, colourless, and feebly alkaline water, containing 12.22gr. of fixed salts per gallon. These salts are principally sodic chloride and carbonate and calcic chloride. The proportion of lime in the water is 4–28gr., and of silica 3–01gr., per gallon. Only traces of sulphates and magnesia are present. As lime, magnesia, and silica are not
present in this water in proportions greater than are here stated, I consider this water is well adapted for use in the boilers of engines generally.”
Note By Sir J. Hector.
The artesian wells described by the author are certainly the most powerful and copious which have yet been discovered in New Zealand. The examination of the samples of strata passed through and the details of the various borings prove that the formation pierced is the ancient river-bed deposit of the present Manawatu. The old-valley of this river system must be very deep, and it has been filled up by successive layers of material carried forward by the river at a time when the land was at a considerably higher level above the sea than at present. These layers of gravel, sand, and clay were, deposited at steeper angles than the present slope of the surface. As shown in Section C, Ashurst, at the lower end of the Manawatu Gorge, is eleven miles from Longburn, and is. 238ft. above the sea; Palmerston is 103ft. and Longburn 62ft. above the sea. The general slope of the surface is therefore about 16ft. per mile. The information obtained by the borings that have been made, and from the inspection of sections along the river-terraces, indicates that the layers of river-deposit dip to seaward at 30ft. to the mile. All the absorbent beds in this ancient river-course must be saturated with soakage water from the higher levels, and when the non-absorbent tough layer that seals down a water-carrying bed is pierced by the pipe the water will rise to a height proportionate to the altitude of the outcrop of that layer further up the river.
At this point water absorbed into the porous strata near the junction of the Porangahau and Manawatu at Ashurst will, according to the foregoing estimates, be reached at a depth of 380ft. above Longburn, but, as the difference of surface-level between the two places is 176ft., when the water springs to the surface through the artesian-well pipe it must have a very considerable head of pressure notwithstanding that the percolation is through yielding material.
Explanation of Plate LII.
Fig. 1. Sketch-plan: Longburn to Manawatu River.
Fig. 2. Bough section of same.
Fig. 3. Section from Longburn to Ashurst.