Monitoring water supply

Press, 6 August 1987, Page 17

Monitoring water supply

My guess is that most Christchurch residents take water for granted and little thought is given to its origin and permanence of supply, except when there are restrictions, or when a local main bursts. Today most households and

many industries are supplied with water that originates from wells penetrating the water-bear-ing gravels (or aquifers) below the city and is distributed by council reticulation networks. From the 1860 s until the early 1900 s most Christchurch residents obtained water from their own artesian wells. During this period several thousand artesian wells were sunk without restriction and a large quantity of water was allowed to run to waste from them.

Between 1880 and 1902, various public water-supply schemes using surface-underground water sources were proposed by engineers and their merits were debated by the ratepayers and councils without success.

Meanwhile, several observers reported on a continuing decline of artesian water levels and suggested that greater knowledge concerning the sources, extent and permanence of the artesian water supply was required. Some shallow wells had ceased to flow. Research by members of the Philosophical Institute of Canterbury provided most of this early information.

On July 3, 1895, a paper entitled “On the Behaviour of iTwo Artesian Wells at the Canterbury Museum” was read by Captain F. W. Hutton before the institute. This paper contained the first published detailed results and analysis of regularly recorded water fluctuations on two Christchurch artesian wells. Both wells are located in the Pacific Hall and the water levels

are still being monitored today, providing us with the longest records in Christchurch. The oldest museum well is 50 millimetres in diameter and 30.4 metres deep, and is believed to have been sunk in the early 1870 s to provide the museum’s first water supply. In 1894, because of insufficient supply, a new deeper well was sunk into the second aquifer to a depth of 55.4 metres by Job Osborne, a well sinker from Doyleston. Water from the well was raised by a hydraulic ram into an 1820-litre tank, and the well remained the museum’s only water source until it was replaced by the City Council supply when construction of the Pacific Hall began in 1958. Fortunately, permanent access to the wells was retained to allow the continuation of water-level recordings to the present day. ,The water-level recorder used for that purpose is in the cafeteria, and is connected to the well by an underground pipe. Why measure ground-water levels over such a long time? Most importantly, long-term records may indicate whether permanent depletion of water from the aquifers is occurring. So far this is not evident but there is no room for complacency. Also, by identifying the causes of waterlevel fluctuations, a greater understanding of how the artesian system operates is achieved. What causes groundwater

By

JOHN WEEBER

levels to fluctuate? They can fluctuate in response to both natural and man-induced influences which may operate simultaneously and require careful interpretation. Some of these were identified by Hutton in 1895. Natural causes include rainfall, river seepage, ocean tides, atmospheric pressure and earthquakes. For example, the Bay of Plenty earthquake on March 2 caused the water levels to move up and down through a range of 60 to 150 millimetres. Large quantities of water pumped from wells produce man’s greatest effect on the water levels. Daily and weekly cycles related to water demand may be recognised, for example, in the accompanying hydrograph recorded on the museum well.

The present total annual abstraction from all the aquifers below Christchurch is estimated to be about 110 million cubic metres. Put another way, this would be enough water to fill an imaginary container measuring 1000 by 1000 metres and 110 metres high. The monitoring of water levels in the museum wells and a number of other wells 25 to 150 metres deep scattered around the city, is the responsibility of the North Canterbury Catchment Board. It is essential that monitoring is continued so that use of the resource is managed effectively and safeguarded for future generations.