Choosing a Borehole Site: Factors to Consider for Safe and Reliable Groundwater Supply

Asikabew – ER

Choosing a borehole site is a critical part of the process of providing a safe and reliable supply of groundwater. The best sites are those in which catchment (natural water input) may be maximized. Such locations are not necessarily those that receive the highest rainfall (which may occur in upland watersheds). ‘Bottomlands’ – such as river valleys and lake basins – tend to be areas of maximum catchment as both surface water and groundwater migrate towards them under gravity. Fracture zones, although not always directly related to bottomland, can also be good reservoirs for groundwater, and may be located by ground observation or satellite images/aerial photographs, and by geophysical methods.

Another aspect of borehole siting that demands careful consideration in populated areas is the potential for contamination by livestock and pit latrines or other waste disposal facilities. Because near-surface groundwater migrates downslope, a shallow dug well or a borehole tapping shallow groundwater should be sited as far away as possible (while bearing in mind the human need for proximity to a source of water) and upslope of potential sources of pollution (latrines or sewage pipes, for instance). Deeper aquifers confined by impermeable layers are at less risk of contamination from surface pollutants.

One final consideration is the nature of the shallow aquifer. If the host formation is made of fine or medium-grain-sized sand, it will act as a natural filter for particulate pollutants, whereas fissured limestone, with a high rate of water transmission (transmissivity) will carry away pollutants faster and to greater distances from the source. It is estimated as a rule of thumb that most microorganisms do not survive more than 10 days of transportation by underground water.