Hydrogeology of the southern Cook Islands, South Pacific

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Waterhouse, B.C.; Petty, D.R. 1986 Hydrogeology of the southern Cook Islands, South Pacific. Wellington: SIPC. New Zealand Geological Survey bulletin 98 93 p.

Abstract: On the Southern Cook Islands, water supplies are derived from several sources: roof catchment, streams, springs, galleries, dug wells, and drillholes. Roof catchment facilities are erected on all of the islands and development of this resource is encouraged by way of government assistance. The use of other supply sources depends on local demand as determined by population concentration and irrigation water for agriculture and horticulture. Although the climate of the Southern Cook Islands is sub-tropical with the annual rainfall averaging about 2100 mm and temperature 24 deg.C, unpredictable dry periods occur when the existing water supply sources are unable to meet the demand. A programme of investigation was started in 1974 aimed at identifying and locating additional water supply sources. A major part of this programme was the drilling of test bores to investigate the subsurface geology and the occurrence of aquifers for the main islands of the group. This work and previous geological surveys suggests that volcanic activity in the Southern Cook Islands commenced possibly as early as the Eocene on Mangaia. The most recent eruptions were probably those on Rarotonga (1.3 million years) and on Aitutaki (0.66-0.77 million years). Judged by interbedded coral and lavas penetrated by a drillhole on Mangaia, intermittent and spasmodic volcanic eruptions occurred over a span of some 40 million years. Drill core petrology (descriptive and chemical), radiometric dating, and paleontologic determinations support this geological history hypothesis. The results of drilling, pump tests, and water analyses suggests that in the Southern Cook Islands a reliable source of water quantitively and qualitively is likely to be available from drillholes which individually may yield up to 3500 l/hr from perched and water-table aquifers in volcanic and sedimentary rocks. The present investigation drillholes ranged from 13 to 100 m deep (drilling rig limit). Where water-table levels are close to sea level the Ghyben- Herzberg principle applies. The ''fresh'' water in such cases is likely to turn brackish if disturbed or pumped from below mean sea level. Target depths for drillholes should be to sea level or slightly below