Ground water of Hamilton lowland

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Schofield, J.C. 1972 Ground water of Hamilton lowland. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey bulletin 89 71 p.

Abstract: The Hamilton Lowland has Hamilton City near its centre with other major towns, Cambridge, Te Awamutu, and Ngaruawahia, near its eastern, southern and northern margins respectively. The extensively farmed lowland derives most of its water from ample ground-water resources close to the surface, but marginal areas and the neighbouring highlands (Hakarimata, Kapamahunga, Pateroa and Pakaroa Ranges, Hangawera Hills and Pirongia) depend mainly on surface supplies. The best and most widespread aquifers beneath the low hills and plains of the lowland are pumiceous sands and gravels of the Tauranga Group. This group includes many formations, of which the Hinuera Formation that underlies the plains and extensive surface peats is the most important, and wells within it have specific capacities as high as 1,500 gallons per hour per foot of drawdown. The next most widespread rocks are the argillites and greywackes of the Hokonui System which underlies the uplands surrounding the lowland. Although water of good quality is available from Hokonui rocks, they are poor aquifers with supplies usually ranging from nil to 200 gallons per hour and very rarely from 500 to 1,700 gallons per hour. Limestones, sandstones, and siltstones of the Te Kuiti and Waitemata Groups are confined to the western uplands, moderate supplies of ground water being available from the Mercer Sandstone, Waikawau Sandstone, Elgood Limestone, and Pukemiro Sandstone. The ground water is broadly divisible into three categories according to quality; low-salinity waters excellent for all purposes; those of low salinity containing troublesome amounts of iron salts; and minor amounts of warm, highly saline waters. The high iron content in roughly half the ground-water supplies from the Tauranga Group is related to acidic waters derived from widespread surface and buried peats. Such waters are often separated from better-quality waters, above and below, by impermeable strata, although, within the Hinuera Formation, waters high in iron invariably underlie an uppermost zone of good-quality water without an intervening impermeable layer. Where the Hinuera Formation is close to and below the surface peats, however, the uppermost zone of good-quality water is absent. Ground waters from peat and sediments close by are also often corrosive because of the presence of carbonic acid. Fluoride contents most often range from 0.1 to 0.3 ppm. Higher amounts of fluoride are recorded from water from the Rukuhia peat, 3.7 ppm, and from an underlying ground water, 1.3 ppm. Most of the ground water hardness is ''temporary'' and except in the highly saline, warm ground waters, is low or moderately low. The effects of the constituent chemicals on pastures when local ground waters are used for irrigation have not been investigated, but tentative use of certain criteria (salinity, sodium, boron, and bicarbonate-ion hazards) show that all but the warm ground waters from sediments of the Tauranga Group and Hokonui System appear to be excellent and good for irrigation. More analyses are required, especially for ground waters from the Te Kuiti and Waitemata Groups. Local climatic data have been combined in the form of graphs which compare favourably with local ground-water fluctuations - more favourably than a cumulative departure curve for rainfall alone. This comparison is made over a period of 5 years, but when extended back for 30 years, isolated recordings of the water table continue to fall close to the climatic graph, which suggests that the ground water is not being depleted. If all of the Hamilton Lowland were to be irrigated with ground water, it is considered that ground-water discharge would not quite equal recharge. Almost certainly considerably less than the total area is capable of being economically irrigated, and thus the amount of water likely to be used by spray irrigation would not be a serious threat to ground-water reserves. The average geothermal gradient in the Hamilton Lowland is 1oF per 50 feet, but in the Frankton and Orini areas it is 1oF per 20 feet which equals the common gradient of the neighbouring Hauraki Lowland. Clays tend to retard or even reverse the normal rise of temperature with depth, and a cap-rock at 370-380 feet below sea level in the Frankton-Tuikaramea areas retards the upward flow of heat so that there is a sudden increase in temperature of 5oF immediately below it. If the gradient of 1oF per 20 feet continues with depth, water of 390o (200oC), the temperature commonly used in the geothermal station at Wairakei, would be encountered between 6,000 and 7,000 feet. The high geothermal gradients in the Haurakei Lowland and in parts of the Hamilton Lowland, together with the saline content of the warm waters, is evidence for a magmatic body or bodies at depth.