Ground Water in New Zealand

New Zealand Journal of Agriculture, Volume 84, Issue 3, 15 March 1952, Page 164

Ground Water in New Zealand

By

D. L. JOHNS,

Assistant Instructor in Agriculture, Department

of Agriculture, Christchurch

AS a whole, New Zealand is, by virtue of its climate and topography, well supplied with lakes, rivers, streams, and springs. Hence in most parts of the country an adequate water supply is not a major problem. There are, however, many areas where farmers have to rely on the direct collection of rain-water from roofs or have to bring surface water long distances by pipeline or water-race. The former method depends on the vagaries of the weather and the latter is expensive to install and maintain. In those parts of New Zealand where an adequate water supply from other sources is a problem the use of ground water may solve the problem or provide an alternative means. This article discusses ground water and some of the factors involved in its occurrence and movements. GROUND water has very material advantages for farmers as well as for townspeople; some of the principal advantages are:— 7'l. Ground water is generally far more dependable in long periods of drought than surface water. 2. Ground water is relatively pure and free from suspended matter and does not require chemical treatment or filtration. 3. Ground water maintains an even temperature throughout the year, having a lower temperature than surface water in summer and a higher one in winter. This is an advantage of some importance to the dairy farmer and the housewife. 4. Provided wells are properly situated and' protected the danger of pollution or contamination is much less than with surface water. These factors as well as the fact that wells can often be situated at the point where the water is most needed without an expensive programme of pipelaying give ground-water supply many advantages over other sources of water supply. Occurrence and Movement Writings from ancient times show that the origin and movement of ground water puzzled learned men of their day. Many of the beliefs and superstitions which arose then still find support among some people today. Despite the great increase fin recent times in the geologist’s knowledge of the earth’s structure many people still rely on the “water diviner” to find the correct location for wells. Although scientists have been unable to refute the “magical influences” in the diviner’s work, practical tests, carried out all over the world on the diviner’s ability, have in no way been favourable to the diviner. Summarising results of tests carried out on 75 New Zealand diviners, P. A. Ongley (“New, Zealand Journal of Science and Technology”, volume 30 B No. 1, page 54) says: “Of the 75 diviners tested, representative of all occupations and from all parts of New Zealand, not one showed the slightest accuracy in any branch of divination. That 90 per cent, of the diviners are sincere does not lessen the harm they do.” It 1 is now well known that a zone of rock saturated with water exists below the earth’s surface at a depth which varies greatly in different places and at different times. This body of water is termed “ground water” and its upper limit the “water-table”. The water-table in a very general way follows the configuration of the land surface, being close to the surface near the sea, lakes, rivers, and swamps, and further from the surface under high ground and hills (see the diagram below). Other factors which influence the depth of the water-table are

the nature of the rock material in any area, seasonal fluctuations due to rainfall, flooding, over-pumping of wells, and artificial drainage. Supply Affected by Nature of Rock Of these the nature of the rock material in any area is of foremost importance in ground-water supply, as in conjunction with an adequate rainfall it controls the availability of ground water. The outer crust of the earth is composed of a very heterogenous mixture of rock materials which differ widely in their permeability, or ability to allow the passage of water between their constituent particles. Rocks near the surface of the earth contain pore spaces or interstices and the percentage of pore space to the total volume of rock is known as porosity. The main factors which control porosity are the size and shape of the individual grains, the amount of grading and sorting of the grains, and the amount and distribution of cementing material (see the upper left diagram on the opposite page). A moderately coarse-grained rock composed of rounded grains, well sorted and without much cementing material, carries most water. Thus, for instance, sand and gravel are highly porous. This type of porosity is known as granular porosity. There is also fracture or fissure porosity, in which the rocks themselves are impermeable to water due to compactness or abundance of cementing material, but during and since their, formation stresses and strains in the earth’s crust have caused fractures or fissures to form in the rocks and water is able to percolate through these cracks. For example, in a cracked concrete yard the mass of the concrete itself is impermeable, but water can percolate through the cracks. Many rocks in New Zealand such as the greywacke rock of the main mountain ranges and some of the volcanic rocks and limestones have this type of porosity. It is, however, the permeability of a rock formation that is most important in the supply of water. Permeability is a measure of the rate at which water can flow through the rock and is determined by the size and degree of connection between pore spaces and other interstices. A

rock may be highly porous and yet relatively impermeable, if the pores are very small, for example, as in a clay or silt. These types of deposit may have a porosity of 50 per cent., that is half their total volume is pore space, but they are so impermeable that even when saturated with water they will not yield it to wells. They are, therefore, very poor water-bearing beds. A sandstone or gravel, on the other hand, may have a porosity of only 30 to 40 per cent., but because of the larger size of its pores, its permeability will be much greater and it may be a valuable aquifer (conveyer of water). Below a certain depth the pressure or weight of the overlying rock materials closes up fractures in underlying rock, and with ' granular porous rocks the weaker mineral grains are crushed to fill up what would be pore space near the earth’s surface. It is for this reason that water is confined to a certain zone of the earth’s crust (see the upper right diagram above). In very deep mines, for instance, water may have to be pumped out of the upper shafts while conditions may be very dry and dusty in the lower levels.

t The natural force responsible for the occurrence of ground water is what is known as the hydrologic cycle (see the diagram below). Water is evaporated from the surface of the sea and carried inland by winds, where it is precipitated as rain, hail, and snow. Some of the precipitation is evaporated immediately from the surface of the soil and vegetation, some may run off the surface into streams and rivers, some is used by the growing vegetation, and some percolates into the soil and subsoil. Once the soil zone (the greatest depth reached by the roots of growing plants) reaches a certain moisture content (known as its field capacity) any surplus water percolates downward to the zone of saturation or ground water. Ground water, like rivers and streams, is part of the natural system of drainage and moves under the influence of gravity to lower levels and toward the sea. Because of the resistance by rocks to the passage of ground water, its rate of movement is very much slower than that of surface water. In well-sorted gravel or coarse sand ground water may move as much as several hundred feet per day down the slope of the water-table, but in poorly sorted silt or fine sand and

Ground water zones. gravel with low permeability the movement may be as little as a fraction of an inch per day.. Springs In its movement to lower levels ground water often emerges at the surface again as springs or seepages. Springs are of five main types (see diagrams on page 166): Depression, contact, tubular, fracture, and artesian springs. Depression springs, as the name implies, occur in hollows where the water-table intersects the ground surface. For example, springs are often found at the junctions of valley bottoms and the surrounding hills. The flow is generally in the form of a seepage which extends over a considerable area or in a line. Contact springs occur where impermeable rocks underlie permeable ones. The ground water percolates down through the permeable rock until it reaches the impermeable rock and, as it cannot move further downward, it emerges at the ground surface where the upper edge of the impermeable rock crops out. Tubular springs occur •in rocks such as limestone in which the percolating water has dissolved some of the rock material, forming tubular channels, which in some places are very large. Fracture springs emerge through cracks in otherwise impermeable rock material. Springs of this type are often distributed along straight lines for some distance, their position being, determined by lines of jointing or fracture. Artesian springs occur only under rather special conditions. The permeable rock containing the ground water must be confined between two layers of impermeable rock. The ground water must also be under hydrostatic pressure; that is, the water level in the permeable rock must be higher than the point of exit of the spring. If the spring flow is confined in a pipe, the water may rise to a considerable height above the spring mouth. No Survey of Water Resources Unfortunately no detailed or systematic survey of water-bearing formations has been carried out yet over large areas of New Zealand. However, with a knowledge of the geological structure and after a study of existing wells

in any area a geologist is able to forecast, with considerably more reliability than a “diviner”, the availability of ground water.

The advice of officers of the New Zealand Geological Survey (a branch of the Department of Scientific and Industrial Research) is available free to fanners to help them solve their water-supply problems. Water-bearing Rocks The water-bearing rocks of New Zealand can be classified most simply according to their age and origin. Broadly the four main groups of rocks are: Group 1: Relatively recent deposits of gravels, sands, and silts of various origins and means of deposition, which are widespread throughout New Zealand. They occur in valley floors, structural depressions, broad plains, and sand dunes. As a source of ground water this group is on the whole the most. productive, although the quality of the water varies from place to place. Added importance is gained from the fact that a large proportion of the intensive farming in New Zealand is carried on in these areas. In the North Island the main areas of these deposits are along the western side of the Auckland peninsula, at the base of the Auckland peninsula in the Mercer and Hamilton basins and in the Hauraki area, the Heretaunga Plains in Hawkes Bay, a coastal strip along the South Taranaki Bight which extends some distance inland in the vicinity of Palmerston North, and the two parallel depressions of the Hutt and Wairarapa valleys. In the South Island the main areas are the Canterbury Plains, the Southland Plains, the structural depressions in which Nelson and Blenheim are situated, . and the young deposits of the gravel terraces and moraines of Westland.. In all these areas abundant water. supplies are obtained from numerous wells of various types and depths.

Group 2: Sedimentary rocks younger than those of Group 1. These rocks, consisting mainly of limestones, sandstones, and mudstones (papa), occur extensively in North Auckland, Taranaki, north-west Wellington and along the east coast from Wellington to East Cape. In the South Island they are confined to small areas in Nelson, Marlborough, Otago, Southland, and Westland and to small scattered areas mainly along the eastern margin of the mountains in Canterbury. In the North Island these rocks are in places up to several miles thick. Most of the rocks of this type are unproductive of ground water, but' in some parts, especially Taranaki and North Auckland, wells yield good supplies of water. Group 3: The ancient sedimentary rocks originally consisting of sands and gravels which subsequently were greatly compacted and cemented and in some cases altered chemically by pressures and heat in the earth’s , crust. This type of formation, known generally as greywacke, is distributed very widely in the South Island, and in the North Island runs in a strip of varying width from Wellington to East Cape. These rocks are relatively impermeable and in most areas do not yield sufficient water to warrant the sinking of wells. Group 4: Volcanic rocks. There

are four types of volcanic rock yielding ground water:— 1. Andesitic rocks found in and around Coromandel Peninsula, in small areas in North Auckland, and in Banks and Otago Peninsulas. The ground water is found in fissures, shatter belts, and joints in the lava and in the fragmental rocks which

have been cemented and consolidated, In the Coromandel area huge quantities of ground water have to be pumped out of mine workings, but as there is a good supply of surface water in the area nracticallv ho use has been made a^ e , a l ’ Practically no use has been made of the ground water for farm factory, of roc^isObtained a from r^ll t ? I tS amea 11 om wells in tne uuneain area. 2. Rhyolitic rocks cover a very extensive area of the northern central part of the North Island. In this area the rocks are mostly very porous and well jointed, so that above the main drainage level surface water disappears underground rapidly to reappear as springs, which are numerous and help maintain a regular flow in the streams and rivers. Although the area is sparsely populated, the ground water is used. 3. Basalt lava flows that are fissured and cavernous, and scoria cones that are highly porous cover some .500 square miles of the Auckland peninsula. Percolating water finds its way readily through these rocks to emerge as. substantial springs. Water of this origin is identical in composition with surface water and is used by some local authorities and by farmers. 4. Two areas of successive flows of andesitic lavas and showers of tuffs and breccias. One covers about 450 square miles about Mt. Ruapehu and the other about 800 square miles about ' Mt. Egmont. Many springs occur in the higher parts of the valleys of the radiating streams. In the lower country there is much water at the contact of the lava and the underlying beds.

Besides these major areas there are many alluvial deposits in smaller valleys throughout the country which yield water.

Artesian Water

The conditions necessary for artesian wells occur in several widely scattered areas in New Zealand.' Although the typical basin structure as shown in the diagram on the opposite page, is not present as far as is known in New Zealand, similar conditions are produced by the arrangement of alternating permeable and impermeable beds sloping in one direction only. Where the outlets to the water-bearing beds are partially blocked by a de- . crease in permeability or by overlying mud on the sea floor the water may be under sufficient pressure to rise above the surface in wells drilled into them through the impermeable covering beds. Such artesian slopes, as they are called, are present in several parts of New Zealand, but the most important area is in and about Christchurch, where water from the deeper wells may rise to about 30ft. above the ground. This area is about 50 miles long and up to 10 miles wide and roughly concentric on the centre of Banks Peninsula. Thousands of wells provide water for domestic, municipal, and factory supplies. Other important artesian areas occur on the plains between Palmerston North and Paraparaumu, the Heretaunga Plains, a small area in the Hutt Valley, and the northern end of the Hamilton depression and the northern half of the adjoining Hauraki depression. Wells If a farmer has decided to sink a well, probably the most important factor is the type of well to be used. Of the various types in use each possesses advantages, under certain conditions encountered in sinking the well, and at the same time some disadvantages which may preclude its' use under a different set of conditions. The main points to be considered in selecting the type of well are the nature of the rock material to be encountered, the depth to which the well must be sunk, the relative costs, the quantity of water required, and safety, from pollution of the supply to be obtained. These conditions will vary from farm to farm and no hard-and-fast rules can be laid down. To avoid pollution of the supply the farmer should make every effort to have the well some distance from any source of contamination such as stock yards and sewerage

systems. Where possible the well should be higher up the water-table slope than these sources of contamination. Similarly the top of the well should be protected from any inflow of surface water. If a farmer has any doubts on these points, he would, be well advised to consult an officer of the Geological Survey of the Department of Scientific and Industrial Research or a local well, sinker. Types of Wells Of the various types of wells, the most commonly used in New Zealand are open or dug wells, drilled wells, and driven wells. Dug wells are suitable where water can be obtained at reasonably shallow depths and where the rock material is fairly soft. The storage capacity of a dug well is an advantage in relatively impermeable materials, as a larger quantity of water can be pumped from the well in a short time than from a narrow-diameter, drilled or driven well. The latter would produce much the same quantity over a longer pumping period. Where the deposits are more permeable sufficient water can be obtained usually from a drilled or driven well. The open or dug well is a type with which special care must be taken to protect it from pollution, because contaminated water from the surface may easily seep into the well through the permeable sides.. Drilled wells are suitable in the harder rocks and where the water is located at considerable depths. Drilled wells in alluvial deposits generally have to be cased with iron pipe to prevent collapse. Driven wells are suitable in soft or fine materials where the water occurs at shallow to moderate depths. With modern advances in well-sink-ing equipment and techniques it appears likely that ground water may yet be utilised successfully in localities where it was not previously thought possible. Acknowledgments Most of the information and the map in the section on occurrence of ground water in New Zealand have been adapted from an article, “Underground Water in New Zealand’’, by J. Henderson, in the “New Zealand Journal of Science and Technology’’, volume 23 B, pages 97-112 (1942). The diagram on page 165 showing different types of porosity in rocks is based on one first published by the United States Department of the Interior in “Geological Survey Water-supply Paper 489”, by O. E. Meinzer (1923).

The assistance given by B. W. Collins of the Christchurch office of the New Zealand Geological Survey is acknowledged.