Siting

Sub-principles

• Borehole siting should be undertaken by competent personnel.
• Prior to preparing any well construction contract, a hydrogeological desk study and field reconnaissance need to be carried out and the method of siting the wells agreed upon, based on expert opinion.
• The risk of drilling an unsuccessful borehole should be categorised. In proven areas where the geology is well understood and borehole success is high (say over 70%), it may not be necessary to site wells using geophysical survey techniques.
• Geophysical surveys should only be undertaken where the costs of drilling an unsuccessful well may justify the expense.
• The site selection needs to take into account community preferences with respect to convenience.

Discussion

Determining the best site for a borehole requires consideration of technical, environmental, social, financial and institutional issues. The siting process should show which groundwater conditions dominate the project area and enable the borehole(s) design to be specified. Professional siting involves desk and field reconnaissance, and makes full use of existing data. In order to determine the best place for a borehole ten factors are of particular importance:

• Sufficient yield for the intended purpose: The groundwater aquifer should have a sufficient yield for a rural water supply handpump (around 0.1-0.3 l/sec), for a small town water supply (2-10 l/sec), or for a larger scale need such as a significant irrigated area3. This information is sometimes available from existing documents or can be derived by performing a pump test (see forms in Annex E).
• Sufficient renewable water resources for the intended purpose. Although a well may be capable of delivering a certain yield in the short to medium term, if the groundwater is not regularly replenished by infiltration from rainfall or river flow, then that yield will not be sustained over the long term. It is therefore important to evaluate the likely recharge to the aquifer, and how this might vary with time. This estimate can be based on a water balance of an area calculated in a conceptual water model.
• Appropriate water quality for the intended purpose.  Different water uses impose different water quality requirements. Domestic water must be free of disease pathogens (which are carried in human excreta) and low in toxic chemical species such as arsenic or fluoride. When using groundwater for irrigation the level of salinity has to be checked. Well siting must therefore take account of knowledge of the occurrence of such undesirable substances. Water from the completed and developed well should be tested with the results tested against national standards. Where these are not available, the WHO guidelines may be used (WHO 2008).
• Avoidance of potential sources of contamination. It is essential to avoid point contamination sources such as pit latrines, septic tanks, livestock pens and solid waste dumps. There may be national guidelines on separation distances or groundwater protection zones. If these do not exist, they need to be developed.
• Engagement with the community to agree on the well location is essential and requires some negotiation to explain technical constraints whilst taking community preferences into account. Full consideration of the needs of women, who tend to be responsible for water collection, is essential. Land ownership issues also need to be considered.
• Proximity to the point of use. Within the constraints of geology, groundwater resources and groundwater quality, wells should ideally be sited as close as possible to the point of use. This means that walking distances to collect water from rural point sources (e.g. handpump wells) and energy costs for electric or fuel driven pumps and piped supplies should be minimised. Walkover surveys should be undertaken to prepare a map of the community. Interviews with householders will help to understand the community’s preference for well location. In general the community would be expected to indicate three preferred well sites in their locality, in order of priority.
• Access by construction and maintenance teams. In the case of wells constructed by heavy machinery, access by drilling rigs, compressors and support vehicles is crucial.  Even when lighter equipment is used, vehicle access for construction and for maintenance is important. Site selection must therefore take account of these needs.
• Avoidance of interference with other groundwater sources and uses.  In areas where some groundwater development has already taken place, the construction of a new well can lead to increased drawdown in existing sources. This in turn can lead to greater pumping (energy) costs in both the existing well and the new well, reduced yields, changes in groundwater quality and potential conflict between users. In an early phase of the siting process possible interference² and risks of derogation² have to be described and discussed. This means that the radius of influence of existing wells should be calculated and new wells located outside this zone. In high-risk situations possible alternative siting areas should be evaluated.
• Avoidance of interference with natural groundwater discharges. In a similar way, the construction of a well too near to natural springs, watercourses or wetlands can lead to a reduction of water levels, potentially drying up these important water sources and ecosystems and affecting uses and users dependent upon them. The intrusion of salt-water due to too high abstraction of groundwater near the coast could lead to irreversible decline of water quality.

What do we know?

MacDonald and Davies (2000) provide an overview of the four main hydrogeological environments in SSA (crystalline basement – 40% of land area; volcanic rocks – 6%; consolidated sedimentary rocks – 32% unconsolidated sediments – 22%) and the different methods for finding and abstracting groundwater from each.  Different hydrogeology requires different levels of technical capacity for development, and much is still not known about groundwater in Africa (MacDonald and Davies, 2000).  Drilling success rates influence boreholes costs.  Unfortunately in-depth knowledge of national hydrogeology is lacking in many countries (eg hydrogeological mapping is underway in Ethiopia and Uganda). 

Doyen (2003) reports on a Kenyan drilling programme where blind drilling and use of geophysical techniques achieved 51% and 89% success respectively.  In the challenging hydrogeological conditions of Mauritania, there are between two and three reconnaissance wells drilled per successful well (Antea, 2007). Extremely high success rates however are not economic if the costs are more than the savings.  In Tanzania, when siting, consultants are required to undertake a geophysical survey using at least two methods, including a VES resistively survey, which is not always necessary (Baumann, 2005).   

Hydrogeological data is extremely important and insufficient attention to the storage, analysis and utilisation of drilling data is a lost opportunity.  Although drilling records are often kept in Government ministries, the collation and analysis of information is rarely undertaken.

Improvements in knowledge of hydrogeology and enhanced experience in site survey can increase drilling success rates, and reduce the disparity between anticipated and actual drilling depths.

References

• Baumann, E. 2007. Groundwater theory. A paper prepared for the CAS Course on Integrated Water Resource Management at the Bern University of Applied Sciences