Key Factors to consider in choosing the right site for borehole drilling.

In order to determine the best location for a well, ten factors are of particular importance:

1. Sufficient yield for the intended purpose

The groundwater aquifer should have a sufficient yield for a rural water supply (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 area. This information is sometimes available from existing documents or maps or can be derived by performing a pumping test on an existing borehole

2. 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 calculated water balance of an area.

3. 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 should be checked. Well siting must therefore take account of knowledge of the occurrence of such undesirable substances. The quality of the water from the completed and developed well should be compared to national standards. Where these are not available, the WHO guidelines for drinking water (WHO 2008) may be used.

4. Avoidance of potential sources of contamination
It is essential to avoid point contamination sources such as pit latrines, septic tanks, livestock pens, burial grounds and solid waste dumps. There are international guidelines on separation distances or groundwater protection zones.

5. Client or Community preferences, user needs and land ownership
Engagement with the community to agree on the well location is essential. It 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 to avoid subsequent disputes between the land owner and water users. Formal agreements regarding land ownership and access to the supply may be required.

6. 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. hand pump wells), energy costs and piped supplies should be minimized. 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.

7. 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.

8. 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 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 interference and risks of derogation should 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.

9. Avoidance of interference with natural groundwater discharges.
In a similar way, 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 saltwater due to too high abstraction of groundwater near the coast could lead to irreversible decline of water quality.

10. Risk
As part of water-well siting, the risk of drilling a dry borehole should be categorized (e.g. high, medium and low risk). In the case of wells which are to be fitted with hand-pumps in areas with known hydrogeology, geophysical techniques (e.g. resistivity, conductivity) are rarely required so long as a desk study has been undertaken of the general hydrogeology of the area. Drilling small diameter exploratory wells (e.g. with a small hand auger) can also be a suitable siting method for shallow wells. However, this hole should be properly sealed afterwards to avoid aquifer contamination.