Boreholes can be an excellent water source in a range of sizes, from single boreholes equipped with a handpump, up to wellfields of multiple large diameter boreholes supplying whole towns.
The proper design and supervision of a borehole is crucial to its long term sustainability.
- All boreholes should have sanitary seals, concrete aprons with proper drainage to soak pits, and low-maintenance sustainable pumping devices that consider the highest flood level when selecting the height to raise the handpump.
- Where gravel packs are needed the borehole diameter should usually be 200mm greater than the OD of the screen/casing.
- Screen slot size should be 0.5mm or 1mm with an open area of 10%. Slot size should normally depend on the aquifer formation and the size of silt.
- The gravel pack should be well rounded and not angular, preferably 2-4mm diameter, and of silicate rich (>70%) or quartzitic nature.
- The gravel pack should completely enclose the screened portion to at least 5m above the highest screen level. A bentonitic clay seal of 0.5m should be placed above the gravel pack before backfilling.
- Pump testing should be carried out for 24 hours after the development of the borehole to determine its safe yield