When a hole is dug or drilled in the ground, water saturated rocks will usually be found at depth. This water is groundwater and is the natural store for water found in the pore-spaces and cracks of rocks. Although groundwater can be found almost anywhere, it may not be available for extraction because the pore-spaces or cracks are too small to allow the water to flow in sufficient quantities.
Saturated rocks from which groundwater can be taken are called aquifers of which there are two main types. Firstly in porous aquifers groundwater is contained in pore-spaces in largely unconsolidated rocks, such as alluvial sands and gravels. The water in the second type of aquifer is contained in cracks and fractures in consolidated volcanic and sedimentary rocks which are otherwise impermeable.
GROUNDWATER RECHARGE AND MOVEMENT
On reaching the ground, rainwater either runs into rivers and streams (surface flow) evaporates or is used by plants (evapo-transpiration) or percolates into the ground to the water-table to rise and when it is at a higher elevation than surface water, groundwater flows through the rocks towards rivers, streams and the sea causing the water-table to gradually fall.
During long dry periods groundwater is the only source of flow in streams. When this flow stops and the streams dry, the water table has fallen below the stream-bed. However, groundwater can still occur at depth.
Groundwater discharges from the ground as springs and seepages at high and low levels. Springs high in mountains are discharge points for water perched on impermeable layers of rock whereas low-level springs discharge groundwater from the aquifer into the lower reaches of rivers, near the coast or into the sea below sea-level.
Where fresh groundwater flow meets saline groundwater at the coast there is a zone of mixing, the fresh groundwater passing over the denser saline water to the sea.
EXPLOITATION OF GROUNDWATER
Groundwater is used by the large number of people in Fiji who obtain their water from springs, hand-dug wells and bore-holes. In low lying areas wells are dug by hand and lined with concrete blocks or rings or oil-drums. As the water-level falls during dry periods, wells are deepened to find the water-table. Hand-dug wells are usually sufficient for domestic requirements and water is extracted using a bucket, a hand pump or, where the depth to water-table is less than 7 metres, by a small surface pump.
Where the groundwater lies at greater depths, or larger quantities of water are required, then bore-holes are drilled, either by MRD or private drilling companies. Bore-holes are usually between 20-50m deep, 100-200mm in diameter and are lined with plastic or steel casing and screen. Groundwater is pumped from bore-holes using a variety of pumps and sources of power to drive them. Groundwater can flow naturally from bore-holes in a few geological situations and no pump is required. In Fiji, bore-holes usually yield between 0.5 and 15 litres per second but yields up to 50 l/s have been pumped.
|AQUIFERS IN FIJI
Hydrogeological maps of Vanua Levu and Viti Levu have been produced by MRD. These maps show the location of aquifers and bore-holes and other water-related information.
The highest yielding aquifers are found in the alluvial plains along the courses of the major rivers. These aquifers comprise unconsolidated sand and gravel and have been investigated along the Rewa, Waidina, Navua, Sigatoka, Nadi, and Labasa rivers. Sigatoka's main source of supply is from this type of aquifer.
The Meigunyah Aquifer, underlying the Nadi Basin, is another high-yielding aquifer which has been studied in detail. Groundwater from this aquifer is currently being used at Legalega for irrigation of mangoes and pawpaw (papaya) grown for export.
Over 200 private bore-holes have been drilled in the Western Division for domestic and village water supplies, in addition to bore-holes used for supplying Ba and Korotogo. These bore-holes exploit the fissured aquifers of the Ba Volcanic group and to a lesser extent, the Wainimala group of rocks. Investigations show that high-yielding bore-holes can be sited using modern geological and geophysical methods to locate water-bearing fracture zones.
Fissured aquifers have also been discovered on Vanua Levu and Taveuni where groundwater is used for rural and urban supplies at, for example, Vunivau (Bua), Naselesele (Taveuni) and several locations in the Labasa Basin, most notably the prolific source at Nabekavu.
Of the smaller islands, Rotuma is supplied with groundwater from bore-holes drilled in 1974 and recently successful bore-holes have been drilled on Lakeba and Vanua Balavu. Investigations carried out by GRADU indicate the presence of fresh-water aquifers on islands in the Lau, Yasawa and Kadavu groups.
WATER QUALITY AND POLLUTION OF AQUIFERS
Groundwater is a valuable resource stored in a natural reservoir, the aquifer. In general, it is a very pure water source free from contamination. Surface water, such as streams and rivers can easily be polluted by human and industrial waste and can carry large quantities of suspended silt and mud during times of flood. The ground acts as a natural filter to these contaminants so groundwater is relatively pure but care must still be taken not to pollute aquifers.
Hand-dug wells are constructed in areas where groundwater is at a shallow depth so it can be contaminated easily. It is recommended that the wells should be covered when not in use and concrete aprons and drains round the well tops should be constructed to prevent spilled water and animal waste from seeping directly into the wells.
To minimise potential risk of contamination, pit latrines and animal drinking troughs should be located ate least 30metres away from wells and if possible, down hill to make the underground path to the water-table and well as long as possible. Particular care is required in the disposal of small amounts of chemical waste or fuel/oil products. These should never be allowed to soak into the ground because they can pollute groundwater for many years.
Another source of contamination is from saline water at sites near the coast. Wells drilled, or dug too near the coast may strike saline water, particularly on small islands. A bore-hole sited near the coast may initially be fresh but turn saline due to over-pumping causing up-coning of the saline groundwater.
|AGENCIES INVOLVED WITH GROUNDWATER IN FIJI
Hydrogeological studies have been carried out in Fiji since 1967 by the MRD. In response to the rapidly increasing demand from the public to use groundwater, the MRD established the groundwater Resources Assessment and Development Unit (GRADU) in 1985, in cooperation with the British Government.
MRD is responsible for collecting and storing all information relating to groundwater in Fiji. Investigations carried out by GRADU are designed to improve knowledge of groundwater and are directed to areas of particular need so any groundwater discovered can be utilised as quickly as possible. A wide range of equipment and techniques are used to locate and assess groundwater resources, including the drilling of test bore-holes by MRD Drilling Section, which use several drilling rigs to carry out diverse drilling operations in all geological conditions. Detailed groundwater investigations are funded by MRD and PWD for urban and rural water supplies, and the Dept. of Agriculture for irrigation schemes.
The Public Works Department is responsible for urban and rural water supplies and inquiries should be directed via the local District Administration. Many bore-holes have been drilled for private use under the government's Bore-hole Subsidy Scheme, which is managed by the PWD. A subsidy of up to $1 000 towards the ost of drilling a bore-hole can be obtained, and the drilling is carried out by private companies.
Private companies that drill water bore-holes currently operating in Fiji are :
Drilling Fiji Co. Ltd., Ba
MRD Information Notes 2