Art. XLVII.—Notes on an Artesian Trial Bore, Westshore, Napier.
[Read before the Wellington Philosophical Society, 19th July, 1916; received by Editors, 30th December, 1916; issued separately, 10th December, 1917.]
The construction of the railway northward from Napier requires the building of a long bridge across a part of the Inner Harbour, while the proposed extension of the Inner Harbour works necessitates the removal of the present road bridge; and, as decay has rendered it unsafe, it has been decided to erect a combined road and railway bridge, and to build the structure of reinforced concrete. The length of the bridge is 1,232 ft.; the width, 41 ft.
In the mixing of the concrete a large quantity of fresh water is necessary, salt or brackish water not being permissible where concrete has to be reinforced with iron or steel. A large quantity of fresh water will also be required to supply the boilers of the machines used in the construction of the bridge, and of the locomotives which will be working on the construction of the line as far as the River Esk, the first source of supply after leaving Napier.
“Westshore” is an extension of the “Spit.” It consists of a number of dwellings, a meat-works, and ship-repair yards, &c., all of which require fresh water. The village, and also the railway, is situated on a ong
shingle spit extending from the entrance to the Inner Harbour to the valley of the River Esk. It is bounded on the east side by the sea (Hawke Bay), and on the west by the Inner Harbour (salt) and the low-river channel of the Esk River. During low river the proper mouth of the Esk
is usually blocked up with shingle thrown up by the sea. On the occurrence of a flood of sufficient volume the shingle bank is broken through, when the greater part of the water flows directly out to sea, the remainder finding its way into the Inner Harbour. Pure salt water thus occurs on both sides of the Spit for more than three miles.
Much consideration was given to the question of obtaining sufficient fresh water for the works, only two sources apparently being available—viz., from the Napier Borough supply or from an artesian well. On the accompanying map are indicated a number of existing artesian wells, which are situated to the south and west of Scinde Island, a hill consisting principally of limestone and marl, on which the residential part of Napier is built. These wells are apparently supplied from the Tutaekuri River. This river rises in the clay-slate ranges westward of Napier, and thence carries shingle down to, or within a short distance of, Taradale. From the point where shingle ceases the course of the river is a meandering one across an alluvial plain or delta formed of mud, which provides the necessary watertight covering over some shingle or sand strata to produce artesian water. The same conditions exist in the case of the Waimakariri River and its relation to Banks Peninsula, where artesian water is obtainable below the tidal estuary of the Heathcote River, and within a short distance of the sea. The resemblance of the two places is so close that it seemed reasonable to expect that artesian water could be obtained at Westshore It was therefore decided to sink a trial bore before undertaking the more expensive alternative of laying a main to convey water from the Spit. The result of the bore was, however, a disappointment, no water being obtained down to a depth of 380 ft. Unfortunately the casing became bound at a depth of 344 ft., otherwise the sinking would have been continued to at least 500 ft.
The bore is being preserved by casting a block of concrete over the top, so as to be available if it should be decided at some future time to continue an exploratory bore by inserting a smaller interior casing. A careful record was kept of the strata passed through, which are indicated on the accompanying section. The strata marked “Compressed silt” are very tight, and of a pug-like consistency; otherwise the country below the first bed of shingle was wholly sand.
Nearly all the sand beds from 110 ft below high-water mark carried an inflammable gas. This gas when lighted burned quietly at the mouth of the pipe, with a flame about 12 in. high, which was easily put out by dropping the hammer over it. At levels 110 ft, 140 ft., and 156 ft., however, the gas was under considerable pressure—sufficient to throw mud and water 10 ft out of the pipe.
The quicksand at 317 ft. caused considerable trouble; three lengths of the cleaning-out rods were caught as it forced its way up the casing, and were lost for a time, but were eventually picked up again.
As information of a negative character is quite as valuable as, and often more so than, that of a positive description, the results obtained by this bore should be placed on record in a manner easily accessible, for the information at some future time of those who may be again considering the question of artesian water at Westshore.
The Transactions of the New Zealand Institute affords a well-known and accessible means of placing on record valuable information, such as has been obtained by this bore; hence my decision to place this paper before the Geological Section of the Wellington Philosophical Society, in the hope that it may be accepted for publication.
The exact location of the bore is 140 links to the right of 1 mile 7860 links on the original railway-survey mileage.