Section 2: Deciding Where to Drill

Section 2: Selecting the Right Drilling Site

Once the drilling machinery is ready, the eagerness to commence well drilling can be overwhelming. However, careful planning is crucial to ensure that wells are strategically located (Appendix D). Drilling should target areas with a high likelihood of accessing water-bearing formations, facilitating effective usage, maintenance (Section 17), and preventing contamination. While not all boreholes will yield a productive well, thorough planning in collaboration with the community (Appendix R) can significantly enhance the success rate while minimizing costs.

To adeptly identify suitable sites for water wells, it is essential for stakeholders to understand the occurrence of groundwater and its origins (refer to Appendix C).

Utilizing tools such as aerial imagery, geological surveys, well documentation, and topographical representations is invaluable for analyzing these aspects (consult Section 2.9). In areas where these resources are lacking, enlisting the expertise of specialists to employ geophysical methodologies can elucidate subsurface conditions(1). This is particularly critical in regions with insufficient aerial photography or hydrogeological data, those receiving under 700 mm of yearly rainfall (White, ) and where adequate water supplies exist primarily in rocky formations (Dijon, ). Often, however, the most reliable information comes from local experiences — engaging with individuals who have previously drilled water wells and conducting thorough onsite evaluations can provide invaluable insights into the subsurface environment and the optimal drilling locale.

2.1 - Water Characteristics: Depth, Quantity & Quality

In the presence of excavated wells, one can ascertain the water depth, geological composition, expected yield, and quality. Historical data from older wells reveals the extent of water level fluctuations during dry periods, informing the necessary depth for new wells. As a rule of thumb, the LS-100 mud rotary drill rig should be utilized where water is sourced from hand-dug wells (typically not exceeding 40 meters in depth). Drilling in less understood regions with sparse information or where subsurface challenges (such as impermeable soil or solid rock) exist should come after several successful wells have been established.

If existing hand-dug wells undergo disinfection (Section 15) and are to remain operational, it is prudent to drill the new well at a considerable distance to prevent interference between the two sources, ensuring both can effectively draw water from their respective portions of the aquifer.

2.2 - Understanding Subsurface Soil Types

The available water volume from an aquifer is as crucial as its purity. To accurately determine the groundwater yield, pump tests are ideal (Section 10.3). Nevertheless, estimating the yield can begin with a careful analysis of the soil and rock formations that constitute the aquifer.

Sand and gravel deposits often harbor significant quantities of potable water. However, the actual extractable volume is contingent on the thickness and permeability of these deposits; generally, larger grain sizes and thicker layers yield higher outputs.

Unfortunately, the LS-100 struggles with boulders (rocks larger than 10 cm) and loose gravel exceeding 1-2 cm in diameter, as this complicates borehole stability and extraction. Therefore, assessing existing wells, valley walls, and quarry landscapes is vital for identifying coarse materials before proceeding.

Avoid establishing wells in shallow sand and gravel if the water table lies beneath three meters from the surface. In such scenarios, contaminants could easily seep into the water source.

Wells situated in silt or clay will invariably yield limited water, irrespective of construction methods. To remedy this, larger diameter wells should be carefully designed to allow gradual accumulation of water over time.

Limestone, sandstone, or quartzitic rocks can also provide adequate yields, especially in regions where weathered rock zones are prevalent. Fine-grained rock such as shale typically does not make for effective aquifers, and the LS-100 is unable to penetrate hard layers like granite or gneiss. If hard rock layers thicker than 1-2 meters are encountered near the proposed drilling site, an alternative drilling rig will be necessary.

2.3 - Assessing Vegetation

In arid conditions, vegetation can signal groundwater presence. Observing the arrangement of ant mounds and patches of greenery can be revealing. Temporary plants like grasses do not serve as reliable indicators, but perennial species, such as reeds and specific trees, may thrive where water is near the surface. Notable water-tolerant species in West Africa include Daniella (Daniella olivieri), Kapok (Ceiba pentandra), and Baobao (Adansonia digitata).

2.4 - Evaluating Topography

The water table often reflects the surface geography (see Figure 1). Zones at lower elevations, such as valley floors where water collects, are typically prime drilling locations (Dijon, ). Ensure that the selected site is accessible, not prone to flooding, and distant from contaminated surface water. The presence of water-bearing fracture zones may be indicated by physical surface features, although these can be challenging to discern in person and are more apparent from aerial perspectives.

2.5 - Proximity to Surface Water

Successful wells are frequently established near rivers; groundwater can still be accessible even when a river is dry (Figure 2). Wells positioned at least 15 meters from a river often yield cleaner and cooler water than that obtained from the river directly. Should turbidity persist post-drilling, inadequate filtration could indicate the potential for contaminated river water intrusion.

Identifying springs is a good practice, as they highlight potential aquifer presence, and digging a well slightly uphill may prove effective. Animal trails can often lead to these valuable water sources.

Surface drainage patterns provide clues about rock type as well (Figure 3):

• Trellis and rectangular drainage occurs where folded, fractured sedimentary rocks are found; these areas are typically conducive to high-yield aquifers (Selby, );
• Contorted drainage generally arises above folded rocks, where water-bearing fractures can develop;
• Annular drainage is indicative of volcanic or intrusive (granitic) formations, where streams often follow fracture zones;
• Dendritic patterns with numerous tributaries are common in regions with impenetrable crystalline rock like gneiss, while parallel patterns signify linear water-bearing structures.

2.6 - Identifying Contamination Risks

Testing well water (Section 16) is essential to ensure safety from pathogenic organisms. Poorly tasting or unclear water may lead individuals back to unsafe sources. Therefore, avoid drilling near known sources of water quality issues and maintain adequate distances from potential contamination sources (see Figure 4 and Table 1):

Locate wells uphill from a potential contamination source. If impossible, positioning wells as far to the side of the slope as feasible is advisable, minimizing direct risk.

2.7 - Ensuring Accessibility

Access issues should be explicitly addressed in the Community Water Supply Agreement (refer to Appendix R). Well locations should prioritize proximity to residences, as water usage drops significantly with increasing distances. For instance, daily consumption is about 40 liters per person with wells nearby, compared to 15 liters for sources positioned 200 meters away, with minimal adjustments in usage rates for distances up to a kilometer (Cairncross, ). More than 300 users sharing a single handpump typically leads to long wait times for access.

Verify that the selected site is accessible year-round and that the access routes are flood-resistant. Additionally, consider legal access arrangements that are socially acceptable. Water well establishment can increase property value, necessitating formal agreements before drilling.

2.8 - Crafting a Site Map

Prepare a detailed map covering the village and surrounding terrain, indicating houses, animal areas, latrines, rivers, swamps, and other necessary features. Map potential drilling sites and evaluate for optimal placement (consider a siting exercise - Appendix D).

Ideal conditions are rare; thus, weighing the pros and cons of each site is vital. It is important for community members, who will rely on the well, to collaborate with drillers in determining the best site. Adopting the advice of hydrogeologists may enhance decision-making accuracy, although it may involve additional time and costs. Ultimately, careful site selection is key to ensuring a reliable and safe water supply.

2.9 - Resources for Groundwater Information

Gathering relevant information to aid in well placement includes reviewing aerial images, geological reports, well logs, topographic maps, geophysical studies, among other sources. Despite these resources, the firsthand accounts of individuals who have drilled wells nearby and personal visits hold considerable value.

1. Government agencies within the country (for example, Ministries dealing with Development, Rural Affairs, Geological Surveys) often have valuable information.
2. Libraries and international aid organizations frequently have pertinent resources.
3. Consulting local experts, hydrogeologists, and drillers offers valuable insights from those familiar with the area.
4. In the United States and Canada, urban libraries are often good information repositories (consider using inter-library loan services!). To find which library holds United Nations materials, contact the UN office at (212) 963-;
5. The U.S. Geological Survey (USGS) offers geographic and geological data from their offices, often requiring 2-3 weeks for material retrieval. Expedited services may incur additional fees.
6. The United Nations Reference Library in New York and the Library of Congress also house extensive collections of hydrogeological reports and resources.
7. Reports for various countries can be accessed through the National Groundwater Association (NGWA) Information Centre, which offers an online retrieval service for a fee.

1 Successful well siting improved from 50-60% with reconnaissance and air photo analysis to above 90% using geophysical methods (White, ). Geophysical evaluations utilize instruments to efficiently measure soil and rock properties which significantly inform the likelihood of identifying water-bearing zones and assessing the water table depth (refer to Hazell et al., and Reynolds, ).

2 Design considerations suggest a minimum consumption threshold of 5 lpd, while 25 lpd is a suitable benchmark where piped connections aren't feasible (Brush, 197?). The required water volume will increase if livestock or irrigation needs are factored in.

3 Education plays a crucial role, as community members must recognize the significance of clean water to maximize a new well's benefits (Brush, 197?)! Continual education is essential for effective well upkeep; when individuals acknowledge the value of clean water, they are more inclined to invest time and resources in maintaining their water supply (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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