ARTESIAN WATER LAYER

Dominion, Volume 38, Issue 7, 3 October 1944, Page 4

ARTESIAN WATER LAYER

Harbour Works Problem Further evidence on the possible effect on the artesian water supply m the Hutt Valiev of works involved in establishing a harbour at the Hutt River mouth or at Point Howard was given before the Hutt Harbour Commission yesterday by the director of geological survey, Dr. J. Henderson, who said he had visited the area when the dredging from the riverbed for the Hutt reclamation had cut into the impervious overlay of the artoThe chairman, Sir Francis Frazer: Supposing that piles about 88 feet i°ng had to be driven and an area dredged to 30 to 35 feet, what would be'the effect 3 Dr. Henderson: The piles would certainly break the seal and there would be a loss of water. . , With outlets near the river-mouth the pressure in the valley would fall. 3 —Yes. There would be a tendency for water from the sea, or from the land above to get into the artesian layer. 3 —There would be chances ofpollution. The acting-chief engineer of the M ellington Harbour Board, Mr. K. L Jenssen: If a long run of sheet piling were found necessary in the construction of harbour works it would result in a long scar cut in the seal? Dr. Henderson: Yes. The town clerk of Wellington, Mr. E. P. Norman: If positive pressure in tbe artesian layer were lost and negative pressure substituted you would expect •surface water to run into the artesian layer?—Yes. , . And also from the harbour if dredging took awav most of the.upper seal. 3 —Yes. Dr Henderson said he had seen it stated that there was a considerable diminution in the amount of water flowing in the Hutt River after it passed the Taita Gorge.. He considered it probable that the water entered the artesian layer there. There might, in addition, be some “edges” along the sides of rue valley where small amounts of water entered. .. . . , Mr E. R. MeKillop, assistant engineer, Wellington City Council, said an approximate estimate of the hydraulic grade of the Hutt Valley artesian water had been made. In a dry season tne river flow daily above the laita Gorge was 51,000,000 gallons. At Belmont it was 26,000,000 gallons daily. The outerop of the sealing layer could be plainly seen in the riverbed. The speed of flow of the water in the artesian layer had been estimated by charging with salt a row of wells across the valley at a selected point and waiting for galvanometers in wells lower down to register the arrival of the saline. It worked out to about three feet an hour. Tests had shown the water at the city’s wells on Gear Island to be sterile, but he did not know the result of similar tests higher up the valley. In a test drill put down to 485 feet before the city's wells were sunk water had been found nt various layers down Io 280 feet. The bulk of the water was between GO and 150 feet down. Tests had proved (hat the water in Hie difleient depths was connected. The charges on anv alternative major water undertaking would he higher than artesian pumping elm rges.