Difference between revisions of "Groundwater development techniques"

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====Geophysical surveying====
 
====Geophysical surveying====
  
Geophysical surveying is often needed because reconaissance (maps, reports and other data) and field observations don't provide enough information to allow a confident assessment of where groundwater can be found. Geophysical surveying is a huge topic and only a very brief introduction is given here. There are many different geophysical techniques and countless types of equipment. Two of the main geophysical techniques used in borehole siting are electrical resistivity; and ground conductivity using FEM (frequency domain electromagnetic). A comprehensive list of techniques useful for borehole siting is given in [http://nora.nerc.ac.uk/501046/ MacDonald et al. (2001)], which is available online.
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Geophysical surveying is often needed because reconaissance (maps, reports and other data) and field observations don't provide enough information to allow a confident assessment of where groundwater can be found. Geophysical surveying is a huge topic and only a very brief introduction is given here. There are many different geophysical techniques and countless types of equipment. Two of the main geophysical techniques used in borehole siting are electrical resistivity; and ground conductivity using FEM (frequency domain electromagnetic). A comprehensive list of geophysical techniques that are useful for borehole siting is given in [http://nora.nerc.ac.uk/501046/ MacDonald et al. (2001)], which is available online.
  
 
It is important that geophysical techniques are carried out and interpreted carefully by well trained personnel. If surveying is done wrongly or survey results interpreted wrongly, at best the information given will be of no help, and at worse it can lead to expensive mistakes in borehole siting.
 
It is important that geophysical techniques are carried out and interpreted carefully by well trained personnel. If surveying is done wrongly or survey results interpreted wrongly, at best the information given will be of no help, and at worse it can lead to expensive mistakes in borehole siting.

Revision as of 11:31, 4 April 2016

Africa Groundwater Atlas >> Additional resources >> Groundwater Development Techniques

What does Groundwater Development involve?

Developing groundwater resources sustainably is easier in some hydrogeological environments than others. If good supplies of groundwater can't be found everywhere - for example, if the local aquifers are low permeability and/or complex, or there are groundwater quality issues - then new boreholes and wells must be sited and designed carefully if they are to be successful. To do this, it's important to understand how groundwater exists and behaves in the area.

The main stages of a groundwater exploration programme which will allow you to assess groundwater resources are summarised in the table below, with an indication of costs. Some more detail is given in the sections below, but this page is not a comprehensive guide to groundwater development. Many resources are available which do provide detailed guidance and support, including a detailed book by MacDonald et al. (2005); a report with less detail by MacDonald et al. (2001), but which is available online; and other references listed at the bottom of this page.

Stages of groundwater exploration and an indication of the costs of each
Stage Notes on costs
Reconnaissance: gathering maps and information (eg from existing reports, academic papers etc) on geological and hydrogeological conditions. This is an essential first step for understanding groundwater resources. A one-off cost: several weeks time of a project staff member or consultant. If new data have to be bought or generated (eg from satellite images or field mapping), costs will increase, but not prohibitively so.
Siting boreholes & wells: Hydrogeological fieldwork: assessing the groundwater potential of an area using an experienced eye (eg by someone who has developed boreholes in the area, or similar areas, before); examining local rocks and geomorphology; discussions with local communities on traditional water sources. This helps to 'ground-truth' the information gathered from the reconnaissance stage. Requires a well-trained engineer or hydrogeologist to visit the area.
Siting boreholes & wells: Geophysical surveying: eg resistivity or electromagnetic (EM) techniques. Must be combined with reconnaissance data and hydrogeological fieldwork. It is important to analyse geophysical data correctly so that it gives good information. Investment in training staff is often beneficial. Geophysical equipment varies in price, but for a single technique (eg resistivity) is generally less than $US20,000. A well trained geophysics team will need to spend at least 1 day in each area targetted for a new borehole.
Collecting information during borehole drilling: gathering information on geology (eg from logging drilled rock chips and measuring penetration rates) and hydrogeology (eg from water strikes). Borehole drilling is a unique opportunity to collect useful geological and hydrogeological data from deep underground - data that are not otherwise available. A well trained hydrogeologist or engineer should be onsite during drilling to supervise the drilling and collect good data.
Assessing source yield: assessing the sustainable yield of a borehole/well by carrying out a pumping test. It is important to measure how much the source will yield sustainably in order to know how many people it can serve. A well trained hydrogeologist or engineer is needed to carry out a pumping test, and normally they need at least 1 day per borehole. For higher yielding boreholes, an electric pump and generator are likely to be needed.
Assessing water quality: measuring the most important chemical and biological parameters that can impact human health. Some parameters can be measured quickly in the field using relatively simple equipment, but most need to be collected and sent to a laboratory. A well trained field technician may be needed to carry out sampling.

Types of Groundwater Sources

Groundwater can be abstracted from the ground in different ways. Many resources are available to support the choice of which abstraction method to use in different environments - there are some references at the bottom of this page.

Groundwater is most often accessed through springs, hand-dug wells, or drilled boreholes. (Drilled boreholes are also sometimes called wells!).

  • 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 20 m deep and 1-2 m 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 method 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.

Reconnaissance

Siting boreholes and wells

Siting boreholes and wells successfully requires a good understanding of where groundwater occurs and how it behaves in the local environment. Developing this understanding starts with reconnaissance, and is refined by hydrogeological fieldwork and geophysical surveying. Only a very brief introduction is given here. More information can be found in a book by MacDonald et al. (2005) or a report by MacDonald et al. (2001), which is available online.

Hydrogeological fieldwork

This involves making field observations of the local geology, hydrogeology and existing water sources, and gathering information from sources such as discussions with the local community. Local dry and wet season water sources should be visited, and discussions held with the community to find out more details: eg how much they yield; do yields fall or dry up in the dry season; are there water quality problems? Rock exposures(eg in river cuttings or cliffs) can give more information on local geology, as can chippings/cuttings from any local hand-dug wells or previously drilled boreholes (failed or working). It is also important to observe any local sources of pollution, such as pit latrines, burial grounds, cattle pens or market areas.

Geophysical surveying

Geophysical surveying is often needed because reconaissance (maps, reports and other data) and field observations don't provide enough information to allow a confident assessment of where groundwater can be found. Geophysical surveying is a huge topic and only a very brief introduction is given here. There are many different geophysical techniques and countless types of equipment. Two of the main geophysical techniques used in borehole siting are electrical resistivity; and ground conductivity using FEM (frequency domain electromagnetic). A comprehensive list of geophysical techniques that are useful for borehole siting is given in MacDonald et al. (2001), which is available online.

It is important that geophysical techniques are carried out and interpreted carefully by well trained personnel. If surveying is done wrongly or survey results interpreted wrongly, at best the information given will be of no help, and at worse it can lead to expensive mistakes in borehole siting.

Drilling boreholes and wells

Drilling with a rig

Most drilling methods use a motorised drilling rig (a different method is manual drilling - see next section). There are different types of drilling rig and methods of drilling, and these should be chosen to suit the local hydrogeology. The main types are cable tool percussion (also known as shell and auger), and rotary drilling. Rotary drilling can be air flush, sometimes with down-the-hole hammer; mud flush; or reverse circulation. Some more detail is available in MacDonald et al. (2001).

Manual drilling

Manual drilling is an approach that is appropriate in some hydrogeological environments, particularly in shallow unconsolidated aquifers with shallow water tables. It can reduce drilling costs and increase cost-effectiveness of groundwater development programmes. 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 professionalisation 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.

Collecting data during drilling

Whichever drilling technique is used, a very important part of successful groundwater development is collecting data during drilling. Drilling is usually the only opportunity to look below the ground and find out what the geology and hydrogeology is at depth, where it is usually hidden. One of the main aims is to identify groundwater-production zones at depth in the geological sequence - at what depths is groundwater found? Data on the local geology is also invaluable for developing understanding.

The following list is a summary of activities that should be carried out for good practice in drilling data collection:

  • a logbook should be kept with notes of drilling activities and what data are collected.
  • the borehole should be flushed clear of cuttings at the end of every sampling interval (eg every 1 m or every drill rod) and an accurate sample of drill (rock) cuttings/chippings collected for observation.
  • rock chip samples should be washed and logged (described)consistently: eg rock type, colour, texture,
  • information should be recorded in the logbook on: the penetration rate of drilling (how long it takes to drill a given interval, eg every 1 m or every drill rod); breaks or irregularities in drilling observed or reported by the driller; water strikes and/or flows; and dust production.
  • if possible, water conductivity (SEC) should be measured at regular intervals/depths during drilling, to observe any changes.

Pump type

Groundwater can be abstracted from boreholes and hand-dug wells by traditional methods (buckets, etc), by hand pumps, or by mechanical (e.g. 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 hand pumps. There are many different types of hand pump, 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 provides many resources on hand pumps, including technical manuals and a number of discussion documents on practice and policy.

Test pumping

References and links to more information

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.

MacDonald, A.M., Davies, J. & Ó Dochartaigh, B.É.. 2001. Simple methods for assessing groundwater resources in low permeability areas of Africa. British Geological Survey Commissioned Report CR/01/168N.

MacDonald, A.M., Davies, J., Calow R. & Chilton, J. 2005. Developing groundwater: a guide for rural water supply. ITDG Publishing, NERC 2005.

MacDonald, A.M. & Calow, R.C.. 2009. Developing groundwater for secure rural water supplies in Africa. Desalination, 248 (1-3), 546-556. doi: 10.1016/j.desal.2008.05.100

Africa Groundwater Atlas >> Additional resources >> Groundwater Development Techniques