Water underground: the three pictures to hold in your head
Porous rock and sediment (sedimentary aquifers). Sand, gravel and sandstone store water in the spaces between grains, like a sponge. These aquifers can be generous and predictable — much of the world's high-yield wells tap them.
Fractured hard rock (basement aquifers). Granite and gneiss hold almost no water in the rock itself; water lives in the weathered layer on top and in cracks (fractures) below. Wells here are a targeting exercise: hit a good fracture zone and the well produces; miss it by a few metres and the hole is dry. Large parts of sub-Saharan Africa drill in exactly this terrain.
Confined and artesian systems. Where a permeable layer is sandwiched under a clay or rock seal, water can be under pressure. These wells can flow strongly, but they are also the ones most often regulated, because they connect to regional systems.
Every siting decision starts with one question: which of these three am I in? Your country's geological survey maps, and the records of wells already drilled nearby, usually answer it before you spend anything.
The free evidence first
- Neighbouring wells. Depth, yield and water level of wells within a few kilometres are the best predictor you will ever get. Ask owners; many countries also keep borehole databases at the water authority.
- Geological maps. National geological surveys publish aquifer maps, often free. They tell you the formation, expected depth range and typical yields.
- Ground signs — useful, not magic. Lines of greener vegetation in the dry season, termite mounds and certain tree species can hint at shallow moisture. Treat them as hints to verify, never as a siting method on their own. Dowsing (water witching) has never passed a controlled scientific test — do not charge a client for a site chosen by dowsing alone.
The professional step: a resistivity survey
In fractured-rock terrain, professionals run an electrical resistivity survey (often a Vertical Electrical Sounding, VES). Electrodes in the ground pass a small current through the earth; water-bearing weathered and fractured zones conduct differently from solid dry rock. The survey produces a depth profile that shows where the weathered layer thickens and where fractures are likely — the two places water hides in basement terrain.
A survey costs a small fraction of a failed borehole. A dry hole costs your client the full drilling price and costs you your reputation in that community. This is why the order of operations is: records → maps → survey → drill. Clients who resist paying for a survey are telling you they may also resist paying for the well; price the risk honestly or walk away.
An honest limit: no method guarantees water. A good survey raises the odds; it does not remove them. Professional drillers say this to clients up front — and put in writing who carries the dry-hole risk before the rig moves.
Ready to estimate a well at a specific location? Our free groundwater well estimator combines location data with the questions in this lesson to size a well and a rig class.
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