Difference between revisions of "Groundwater development techniques"
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Groundwater can be developed for use in different ways. Many resources are available to support the choice of development technology. This page is not a comprehensive list of such resources, but it provides some basic information, and links to further information.
Groundwater is most often accessed through springs, hand-dug wells, or drilled boreholes.
- Springs are natural flows of groundwater from the underlying rock or unconsolidated sediment. Springs are dependent on the characteristics of the rocks, and their nature and yields are hugely variable. They often occur in specific hydrogeological environments. Because they are open at their source, springs are vulnerable to contamination. No equipment is needed to make a spring, but springs can be improved and made less vulnerable to contamination and drought by various developments, such as constructing a collection tank to store spring water, and installing a protective cover over the spring head.
- Hand-dug wells have been dug to access groundwater for thousands of years. They can only be dug in soft material, such as unconsolidated sediment like sand and gravel, weathered basement, or limestone. They are only appropriate where the groundwater level (water table) is shallow. They are usually less than 20m deep and 1-2m in diameter, but can be wider and much deeper. Little or no specialised equipment is needed to construct a well - just something to dig with, and a way of removing the spoil. Wells often need to be lined to keep them open, using materials like brick, stones, concrete rings or even lorry tyres. Open wells are vulnerable to contamination from the surface, and can be improved by installing a concrete apron around the top. Wells have large storage, which helps make them less vulnerable to drought, but because they typically tap only shallow groundwater, they can dry up in dry seasons or longer droughts.
- Boreholes are narrow diameter tubes drilled into the ground, usually vertically. Boreholes are also called tube wells or simply wells. They can be drilled more quickly and go deeper than hand-dug wells, and so can tap deeper, often more sustainable groundwater; they can be drilled though hard rocks; and they can be more easily protected from contamination. There are many different techniques for drilling boreholes, some of which are more suited to certain hydrogeological environments. Usually, a motorised drilling rig is used, operated by specialist drillers. There are also manual drilling techniques.
Other, less common ways of accessing groundwater are by:
- collector wells, which are vertical boreholes or wells modified by drilling horizontally out radially below the water table to increase the collection area for groundwater into the central well, from where water is abstracted. They are often constructed in alluvium, next to ephemerally dry ('sand') rivers, with the horizontal radials drilled into the river bed deposits; or in weathered basement.
- infiltration gallery, which is a horizontal trench or drain dug below the water table to abstract shallow groundwater, usually from unconsolidated alluvium, including sand rivers, or windblown deposits. The trench drains into a sump from where water is abstracted. The gallery may have to be lined to keep it open.
- qanats, which are an ancient water of tapping and transporting groundwater in many parts of North African and the Middle East. A qanat comprises a mother well, often in alluvial deposits at the edge of a mountain range, and a gently inclined covered, underground channel which allows groundwater to flow downhill to a village.
One approach to reduce drilling costs and increase cost-effectiveness of groundwater development programmes is manual drilling. Manual drilling methods are being used to provide water for drinking and other domestic needs in at least 36 countries around the world, and in some places are already well established.
UNICEF has worked with a range of partners to develop a toolkit for African countries wishing to embark on the professionalization of manual drilling. This toolkit includes Technical Notes and Technical Manuals, Advocacy Materials, Case Studies, and Implementation and Training Manuals for manual drilling. There is also a series of mapsshowing areas suitable for manual drilling in 12 countries in West Africa, and a report on the mapping methodologies used.
The Rural Water Supply Network (RWSN) has produced a Manual Drilling Compendium, which provides a useful overview of the impacts and challenges of manual drilling, and support for improving practices on the ground.
Groundwater can be abstracted from boreholes and hand-dug wells by traditional methods (buckets, etc), by handpumps, or by mechanical (eg diesel) or electrical submersible pump. Mechanical or electrical pumps are most appropriate for higher yielding wells or boreholes. Most rural water supply boreholes and wells in Africa are installed with handpumps. There are many different types of handpump, and the choice of which to use will depend on national standards, ease of maintenance and local expertise, availability of spare parts, the depth of water lift required, the groundwater chemistry (mild steel can corrode), and cost. RWSN provide many resources on handpumps, including technical manuals and a number of discussion documents on practice and policy.
DANERT K. 2015. Manual Drilling Compendium 2015. RWSN Publication 2015-2, Skat, St Gallen, Switzerland
DANERT K. 2015. Chad’s Growing Manual Drilling Industry. , Skat Foundation, St Gallen, Switzerland