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Section 2

Deciding Where to Drill

When drilling equipment is available, it is very tempting to get right to work drilling wells. However, wells should be carefully sited (Appendix D) so that drilling only occurs where there is a high probability of successfully penetrating into water-bearing formations and the wells can be effectively used, maintained (Section 17), and protected from contamination. While every borehole will not result in a good well, advanced planning with the community (Appendix R) will maximize the number of successful wells and minimize drilling costs.

In order to successfully site water wells, those involved must know something about the places where underground water occurs and how it got there (see Appendix C).

Aerial photographs, geologic reports, well logs and topographic maps are useful in studying these factors (see Section 2.9). Where available, specialists should be enlisted to use geophysical techniques to define subsurface conditions(1). This is most important in areas where air photo coverage and hydrogeological information are inadequate, where local rainfall is less than 700 mm/yr (White, ) and where adequate water supplies are only available in rock (Dijon, ). Usually, however, the best source of well siting information is talking to people who have dug local water wells and personal inspection of water wells in the area. Although this can be time consuming, it is very important and contributes to an understanding of local subsurface conditions and selection of the best place for successful water wells.

2.1 - Groundwater Depth, Quantity & Quality

Where dug wells exist, it is possible to determine the depth to water, geology and expected water quantity and quality. The history of old wells will indicate how far down the water table drops during dry seasons and will indicate how deep new wells must be. In general, the LS-100 mud rotary drill rig should only be used in areas where people are getting their water from hand dug water wells (less than 40 meters deep). Only after numerous water wells are drilled this way should drilling be attempted in areas where little information is available or where subsurface conditions (impermeable clay, hard rock, etc.) have prevented the construction of hand dug water wells.

If existing dug wells will be disinfected (Section 15) and continue to be used, the new well should be drilled as far away as possible to ensure that both wells will produce sufficient amounts of drinking water without interfering with one another (drawing water from the same part of the aquifer).

2.2 - Subsurface Soil Types

The amount of water supplied by an aquifer (water bearing formation) is as important as its quality. The only way to know exactly how much water is available is by pumping wells (Section 10.3). However, a rough estimate of yield can be made by identifying the soil and rock which comprise the aquifer.

Most sand and gravel deposits contain significant amounts of drinking water. However, the amount of water which can actually be pumped depends on how thick these deposits are and their permeability (how easy it is for water to flow through it). In general, the larger the grain size and the thicker the deposit, the higher the yield of the aquifer.

Unfortunately, the LS-100 cannot effectively drill past boulders (loose rocks greater than 10 cm in diameter) or through loose gravel greater than 1-2 cm diameter (it is very difficult to keep the borehole from collapsing and it is hard to carry the gravel up and out of the borehole). It is, therefore, important to inspect existing wells and valley sides, cliff faces, quarries, etc. to determine if there are boulders or coarse gravel present.

Try to avoid siting and developing wells in shallow sand and gravel deposits if the water table is less than 3 meters below surface. Under these conditions, waste water can easily infiltrate back down to the water table near the well and contaminate the drinking water supply.

Wells constructed in silt or clay soils will have very low yields regardless of how they are constructed. To compensate for this, large diameter water wells should be carefully dug so that large volumes of water can slowly accumulate in the well casing over time and provide sufficient quantities when required.

Finally, limestone, sandstone or quartzitic rock may also yield adequate quantities of water. Best yields are found where there is a thick zone of weathered rock with many cracks (fractures) (Dijon, ). In general, fine grained rocks, such as shale, do not serve as productive aquifers. Also, the LS-100 cannot effectively drill through cemented stone layers or hard rock like granite or gneiss. Therefore, if there are hard rock layers greater than 1 to 2 meters thick in the vicinity of the proposed drilling location, a different drill rig is needed.

2.3 - Vegetation

During the dry season, survey for indications of groundwater by looking at the alignment of ant mounds and green vegetation in the midst of an arid landscape. Annual plants, such as grasses and ferns, are not good indicators because they come and go with the seasons. However, year-round reeds and broad leaf trees and shrubs like cedar and willow tend to grow where water is close to the surface. Some water indicator trees in West Africa are Daniella (Daniella olivieri), Kapok (Ceiba pentandra) and Baobao (Adansonia digitata).

2.4 - Topography

The water table commonly follows the land surface (see Figure 1). While the lowest areas (valley bottoms or depressions where water accumulates after rains) are generally the best places to drill (Dijon, ), ensure that the site has good access, is not subject to flooding and is not close to where contaminated surface water ponds. The presence of water bearing fracture zones may be detected by surface features such as shallow linear depressions and abrupt changes in valley alignment. Often these features are difficult to see in the field but become apparent when viewed from the air.

2.5 - Surface Water

Successful wells are often drilled near rivers; groundwater may be available even if the river is temporarily dry (Figure 2). Reliable wells have even been located near broad sandy riverbeds which are active once every 5-10 years (Dijon, ). Water taken from wells located at least 15 m from a river is usually cleaner and cooler than water taken from the river. If the well water remains turbid after construction, the soils may be providing inadequate filtration and contaminated river water may be entering the well.

Look for springs since they indicate the presence of a water bearing formation (aquifer). A well can often be successfully drilled just uphill of the spring. Animal trails often lead to seeps and springs.

Finally, surface drainage patterns can be used to determine rock type (Figure 3):

• Trellis and rectangular drainage develops where dipping, fractured sedimentary rocks are present; these are the most favourable areas for high yield aquifers (Selby, );
• Contorted drainage develops over folded rocks. Water bearing tension fractures and gaps between layers of differing hardness sometimes develop near the top of folds;
• Annular drainage typically develops over volcanic or intrusive (granitic) domes, with streams flowing along water bearing fracture zones;
• Dendritic or branching patterns with a large number of tributaries are typical of drainage in areas of impermeable crystalline rock such as gneiss. Parallel drainage patterns may develop in areas with linear water bearing structures such as faults and dikes.

2.6 - Sources of Contamination

Well water should be tested (Section 16) to ensure that it is free from disease-causing organisms. Also, if it is not clear and good tasting, people may revert to traditional unsafe drinking water supplies. Therefore, avoid drilling in areas where unsuitable quality water is known to occur and keep wells as far away as possible from potential sources of pollution (see Figure 4 and Table 1):

Table 1: Minimum Separation Distances from Contaminant Sources Distance(m) Possible Source of Contamination 100 Garbage dumps/refuse piles, car repair or fuel (petrol) sales outlets, industrial operations/storage facilities etc. 50 Seepage pit or cesspool 30 Pit toilets, animal pens, barns, fields fertilized with dung 15 Septic tank, surface water body 7 Drain, ditch, house

Locate wells upgradient (uphill) of nearby potential sources of pollution (i.e., the land should NOT slope from pollution sources towards water wells). If this can not be avoided, try to locate wells as far to the side of the slope as possible (i.e., not directly downslope of possible contaminant sources).

2.7 - Accessibility

Issues of accessibility to well should be clearly addressed in the Community Water Supply Agreement (see Appendix R). Wells should be as close as possible to houses because people use a lot less water if wells are located far from their home. Usage drops from 40 litres per day (lpd) per person when water is supplied in the yard down to 15 lpd for sources 200 m away; this rate holds fairly constant for distances up to 1,000 meters (Cairncross, ). Only when water wells are located more than 1 km from home does the water consumption rate drop again, often declining to less than 7 lpd(2). This means that the most significant benefits (arising from increased water consumption) occur when water wells replace old water sources which were further than one kilometer away (Cairncross, )(3)!

Another factor in preparing Well Development Plans is to determine how many wells are needed to serve the population. When more than 300 people use one handpump, there will be significant waiting lines to get water.

Ensure that the site is accessible year-round and that the access route to the water well is not susceptible to flooding. Finally, ensure that the site has legal access which is acceptable to users from a societal standpoint. Land ownership law is usually different than what we are used to and requires careful consideration. Having a water well on someone's property enhances its value and therefore a formal arrangement for access needs to be clearly made before the well is drilled.

2.8 Preparation of a Site Map

A map of the village and surrounding area should be prepared. Add to the map all relevant features such as houses, animal pens, pit toilets, rivers, swampy areas, garbage disposal areas and indicate the direction in which the land slopes (see Figure 4). Draw all possible well sites on the map and select the best site (try a siting exercise - Appendix D).

There is rarely an ideal location and the relative advantages and disadvantages of each site must be weighed. The people that will be using the well and the drillers must together decide which site is best for the community. Since selecting the best site is a matter of judgement and experience, it always helps to seek assistance from hydrogeologists - while their investigation may be time consuming and add some cost to the drilling project, it will help ensure that a site is selected which will provide a safe, abundant supply of drinking water.

2.9 Sources of Groundwater Information

Information which can help effectively site wells includes aerial photographs, geologic reports, well logs, topographic maps, geophysical maps etc. Sources of this information are listed below. It should again be emphasized, however, that the best sources of well siting information are talking to people who have dug wells nearby and visiting these wells yourself. Keep in mind that:

1. Information is often available in-country from government agencies (such as Ministries of Development, Rural Affairs, Geological Survey);
2. In-country information is also often available from libraries and international development/aid agencies;
3. Often excellent information can be obtained by talking to consultants, hydrogeologists and well drillers who have worked in the area of interest;
4. Libraries in most large urban centres in the United States and Canada often contain good information (make use of inter-library loan facilities!). To determine which library is the closest depository for United Nations material, call the UN office at (212) 963-;
5. Many high quality maps are available from the U.S. Geological Survey (USGS) in Reston Virginia (703) 648- (Mapping Section) or (703) 648- (Geologic Inquiries) or (703) 648- (International Geology Program). They can search for material by key word. Material takes 2-3 weeks to deliver through their Denver office (303-236-). For a slightly higher fee, excellent overnight service is available from "Express Maps" (800-627- or 303-989-) or "Powers Elevation Co. Inc" (800-824- or 303- 321-);
6. USGS documents which pre-date can be obtained from the National Technical Information Service (703-487-). They require the NTIS number or the complete document title.
7. The United Nations Reference Library (Map Section) in New York is also a good source of information (212-963-). Delivery only available through regular mail.
8. The Library of Congress (Mapping Section) in Washington DC has many hydrogeological reports and maps for developing countries (202-707-). Requests must be made in person or in writing (response time to written search requests is very slow: 2-3 weeks); additional time is required for delivery.
9. Hydrogeological reports for many countries of the world are available from the National Groundwater Association (NGWA) Information Centre at Riverside Dr, Dublin OH, (800-332-). There is an on-line search facility available for a fee; credit card payment and overnight delivery service are available. There is a minimum $12 base fee for retrieving up to 20 pages (extra fee/page for additional pages).

1 Well siting success improved from 50-60 % (based on site reconnaissance and air photo interpretation) to over 90 percent when geophysical technology is used (White, ). Geophysical surveys employ instruments that quickly and cheaply measure the physical properties of soil and rock (density, magnetism, electrical conductivity, radioactivity etc.). Geophysical surveys can be very useful in locating water-bearing fault zones, in finding an adequate thickness of overburden or weathered bedrock, and in assessing the depth to the water table (see Hazell et al., and Reynolds, ). Success depends on the application of appropriate techniques, having enough time to do the investigation, and having the equipment operated and the results interpreted by trained geologists (Driscoll, ). In some countries, the government provides geophysical surveys as a service.

2 For design purposes, 5 lpd is the minimum consumption level and 25 lpd is an acceptable goal in places where piped connections to individual houses are not feasible (Brush, 197?). It should be noted that the amount of water needed may be much higher if livestock require water and if well water is to be used for irrigate gardens.

3 Education is crucial. Unless people understand the benefits clean water can bring, they will not make effective use of a new well and it may have little benefit (Brush, 197?)! Education is also important for on-going well maintenance; if people see clean water as being vital to them, they will be willing to occasionally spend a little time and money to keep their water supply safe and functioning reliably (Brush, 197?).

Cairncross, S. () "The Benefits of Water Supply", Developing World Water, Hong Kong: Grosvenor Press Int'l, pp. 30-34.

Dijon, R. () "Groundwater Exploration in Crystalline Rocks in Africa", Proceedings of the American Society of Civil Engineers, May 11-15, .

Selby, M.J. (). Earth's Changing Surface. Clarendon Press, Oxford, 607pp.

White, C. () "Bore Hole Siting Using Geophysics", Developing World Water, Hong Kong: Grosvenor Press Int'l, pp. 107-113.

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Well Site Location Should First Be Based on Hydrogeology, But Many Other Factors Come Into Play

Well site location should first be based on hydrogeology, but many other factors come into play.

NOTE: Much of the contents on this page has been adapted from publications of Lifewater International, written by Fred Proby.

Property owners and the people who will be using a water well will often base their choice of well location on the convenience of the site.

Understandably, a well located right outside their door would be desirable. Practically speaking, this may not be possible, and well site selection almost always involves some kind of compromise.

A couple of other considerations concerning well location become very important in our efforts to produce a dependable source of clean, safe water.

First of all, the drill rig being used must be able to penetrate the formations in the area and reach groundwater within the depth limit of the rig.

Secondly, wells must be located far enough away from latrines and other places where human and animal waste is concentrated to prevent the introduction of disease-causing pathogens from entering the well water.

If a well is located far enough away from a latrine, the soil is able to filter out harmful organisms before they reach the well. To determine the minimum safe separation distance between latrine and well we need to know three things:

1. Direction the groundwater is moving.

2. Distance between the bottom of the latrine and the water table.

3. Type of soil between the bottom of the latrine and the water table.

Following these guidelines (or any well site placement guidelines) will not guarantee that a well will be free from bacterial contamination. The well must also be properly constructed and the well water tested to assure the best possible water quality.

Groundwater Flow Direction

If groundwater flow direction is known, it is best to place the well up-gradient from a latrine, so that contamination moves away from the well.

It may be difficult to know the direction of flow. But, groundwater in an unconfined aquifer tends to flow in the direction that the ground slopes, and it flows from a recharge area to a discharge point. Knowing this, it will usually be better to locate a well uphill from a latrine than downhill from one.

The distance between the bottom of the latrine and the water table and the type of soil that lies between the bottom of the latrine and the water table will determine the minimum safe separation distance between the latrine and the well.

In The Case of a Deep Water Table

If the water table is at least 5 meters below the bottom of the latrine, most contamination will be removed before it reaches the groundwater if:

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1. There is clay, silt or fine sand between the bottom of the latrine and the water table.

2. The well is located at least 15 meters from the latrine

If the water table is at least 10 meters below the bottom of the latrine, most contamination will be removed before it reaches the groundwater if:

1. There is medium sand or gravel between the bottom of the latrine and the water table.

2. The well is located at least 15 meters from the latrine. 

In the Case of a Shallow Water Table

If the bottom of the latrine is at the water table, or close to it, then disease-causing organisms can get into the ground water. Finer grains soils, like clay, silt, or fine sand can more easily filter out pathogens than coarse sand or gravel.

So, the minimum safe separation distance is shorter for the finer soils than it is for the coarse soils. In either case, if flow direction is known, it is best to put the well up-gradient from the latrine. 

Environmental Factors

We have already discussed some environmental factors involved in locating a water well, so some of the items in the list below will look familiar. A few other factors must be noted. These environmental factors are natural phenomena that are not the result of any human activity.

1. The direction that groundwater flows, as noted above, is a very important environmental factor to know. A well should be placed so that contamination from any source moves away from the well and not toward it.

2. The type of soil near the surface is also important. As mentioned, clay, silt, and fine sand can keep contaminants from reaching the groundwater.

3. Surface waters, like streams, rivers, and ponds, may contain biological, agricultural, or industrial contamination so wells should be located at least 15 meters away from them.

4. Avoid areas that get flooded, since people cannot get to the well to the well during times of high water and the well may be contaminated by floodwaters overflowing and seeping into the well.

5. The well site should be elevated enough to direct surface runoff away from it.

6. Saltwater intrusion can affect shallow groundwater near the coast, so wells should be placed away from shore or drilled into a deep aquifer.

7. Naturally-occurring chemicals, like arsenic, boron, and selenium can affect groundwater quality, so water should be tested in areas where this may be a problem. 

Cultural Factors

Cultural factors can be thought of as anything related to human activity that should be considered when locating a well site. Examples of some cultural factors might include:

1. Proximity to where people live. Convenience is a very important factor to consider when locating a well. Studies have shown that when a water point is located less than 200 meters from a home, people tend to use more water than when the source is farther away. They drink more water and wash more often, which con tributes to better health.

2. If there are more people who want to use the well than the well can support, people will not have enough water for good health. One well for every 300 people is an appropriate goal. A large village may need several wells in order to experience all the benefits of clean water.

3. Are there areas in or around the community that are considered sacred? It is wise to respect the &#;spiritual landscape&#; of the area as viewed by those who live there. You may not share the community&#;s beliefs, but it is a demonstration of love to be sensitive to theirs. Asking about these things with gentleness can help build good relationships.

4. A cemetery is an especially sensitive cultural issue. In some places, people have refused to drink water that comes from the ground because people were buried in the ground. There is no real risk of contamination if a well is located a safe distance up-gradient from a cemetery. However, the perception that people have is a more important consideration when choosing a well site.

5. Even a small village may have a well thought-out plan for growth. So, consider what future development may take place in the village and locate the well in a place that will not conflict with that growth.

6. Some communities have cesspools or large seepage pits that receive much more human waste than a family latrine. A well should be placed at least 50 meters away from such a concentrated contamination source.

7. Animal pens concentrate waste in a small space and are similar to a latrine in their potential to contaminate groundwater. Use the &#;safe separation distance&#; criteria as you would for a latrine.

8. Industrial facilities or garbage dumps may discharge harmful chemicals into the soil and groundwater, so wells should be located at least 100 meters up-gradient.

9. A well should be no closer than 7 meters from overhead power lines, because the lines could be touched when installing or repairing the hand pump.

10. Before drilling, identify any underground pipes that might run near the well site.

11. If a new well is placed too close to another well that is still being used, they may interfere with each other. This can reduce the amount of water that each well is able to produce. The appropriate separation distance depends on the pumping rate of the wells and the characteristics of the aquifer.

12. An abandoned well may be filled with trash. Since it probably reaches to the water table, this can be a serious source of groundwater contamination. Use the &#;safe separation distance&#; criteria for a latrine that contacts the groundwater. 

Property Ownership

Property ownership is a cultural factor that deserves special consideration. Wherever there are living and working there will be some sense of property.

Ownership might be individual, collective, a mix of these, or something entirely different. In any case, it is not likely that property ownership will be evident to an outsider, even an outsider from within that culture.

If you are involved in surveying and walking around trying to find the best location for a well, it is always important to ask permission before walking around and looking at things.

It is best to be accompanied by a local leader so no suspicion will arise among those who may not know the purpose of your visit.

Who owns the well?

An essential part of the process of selecting a well site is determining property ownership of the well and access to the well. There are three arrangements that tend to be successful, as described below. But, these are not the only ones that will work.

The key is for the community to discuss the issue and agree on a solution before the well is drilled.

1. A publically-owned well on public property for public purposes

In this arrangement the involvement of the community leaders is essential. Typically the Village Water Committee is elected by the community to administer all aspects of the well.

The things that the Water Committee takes responsibility for include; collecting fees, establishing operating times (especially important if the well has limited yield), keeping the pump pad clean, and maintaining and repairing the pump.

The organization or individuals who construct the well may need to advise the Water Committee, but the policies are determined by the community.

2. Service contract variation of arrangement #1

In a variation of the above approach, the local constructors of the well and the community may agree on a service contract where the constructors provide maintenance and repair of the pump for a monthly fee.

The Water Committee collects this fee from the users of the well. This arrangement guarantees the community a fixed cost for their water supply and gives the local constructor a regular income.

The constructor has a strong incentive to perform routine maintenance on the pump, since that will reduce the need for expensive repairs.

3. A privately-owned well on private property for public purposes

In this case, the community contracts the responsibility for well maintenance to the property owner, who charges for that service. He keeps the pump working and the people pay a reasonable fee for the water. If he doesn&#;t maintain the pump, or if he cuts off access, he doesn&#;t earn any money. He is running a small-scale water utility.

Any contractual agreements are between the property owner and the community. In some communities it may be difficult for the leaders to impose any kind of fees, so this arrangement gets around that problem.

The people of the community are given a choice; they can either pay for safe water or get contaminated water for free. If the fee is based on water usage, then people will tend to use the safe water for drinking and cooking, and use other sources for washing and activities where water quality is not critical.

Community decisions concerning well ownership and community use of the well play an important role in deciding the best location for a well site. 

Make a Site Map

There may be a large number of latrines in a community, plus many other environmental and cultural factors to consider. Selecting a well site can be very difficult. It often involves making a trade-off between conflicting goals.

In these cases, making a simple map helps identify the most important factors to consider.

Start from the preferred well site location; this is usually suggested by community leaders or property owner. Walk around the well site looking for possible sources of contamination within 10, 20, and 30 meters of the well. If the soil is gravel and the water table high, you may need to search for contamination sources as far away as 500 meters away.

Make a simple sketch of what you find. The map does not have to be exact. Measure distances from the well site for each source of contamination you find. Take topography into account because it influences the direction of groundwater flow.

If you find a source of contamination too close to the proposed well site, then discuss the problem with the community leaders or property owner. Help them consider a better well site.

Work with them to find the best place for the new well, given the restrictions of the environmental and cultural factors. Remember that finding the best well site often requires tact and compromise.

Return to "Groundwater" from "Well Site" 

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