WASH Technologies - Boreholes

A borehole is a small diameter drilled hole using a drilling rig. Borehole diameters range from 101 millimetres to 318 millimetres (4 inches to 12,5 inches). The standard drilling method is rotary percussion drilling in hard rock. In soft sediments the standard method is mud rotary drilling.


The type of construction method used will depend on the expected ground conditions and the availability of equipment, power sources, and skills. The water is brought to the surface by a pump or a small diameter bucket or bailer.

Borehole requirements
The depth of the borehole should accommodate seasonal or annual fluctuations in water levels so as to avoid the borehole going dry when the water table is low. A hydrogeological survey to determine the appropriate siting of the borehole is recommended. The optimum diameter of the borehole must be determined taking into account the anticipated yield, the demand, and the extraction method used.

Water quality
Water quality depends on the inherent properties of the groundwater when extracted, threats of contamination during the extraction process, and threats of contamination from the surrounding environment. However, it is important to remember that groundwater is still generally better quality than untreated surface water. Further, there is less of a chance of contamination from boreholes than from hand dug wells. A sanitary seal should prevent contaminants from entering down the side of the borehole casing.

Insitutional support

Consultation with qualified and experienced professionals is required to determine the position of the site and for drilling, testing, and pumping yield recommendations.
Capital requirements

• Costs are associated with:
• Drilling / auguring equipment;
• Casings;
• Screens; and
• A sanitary seal.

Operation and maintenance
If the borehole yield decreases over time, boreholes may need to be remediated. Clogging may be improved by cleaning out the borehole using a weak acid solution under high pressure. Abstraction from the borehole and water levels must be regularly measured to avoid over abstraction and the borehole running dry.
Advantages of boreholes
They include:

• Boreholes are the only means of extracting water in hard rock environments;
• They are more efficient per metre than hand dug wells, as less material needs to be removed;
• They can be constructed within short timeframes ; and
• The water source is generally reliable, as they can extend deeply, to reach reliable aquifers.

Limitations of boreholes include:

• The borehole offers limited storage capacity for low yielding aquifers;
• Problems with clogging of screens and scouring of sand from behind the screen may be encountered
• If the raw water has a high iron or manganese content the abstraction screens will clog, greatly reducing the yield
• Uncased boreholes are liable to collapse in highly weathered formation;
• Expert advice must be sought to establish when casing is necessary or not;
• Down-hole equipment must be used for down hole maintenance; and
• Borehole drilling costs are higher than hand excavation.