A test hole is a narrow exploratory hole, 10-15 cm across, dug or augered to find the depth of the water table, the soil layers above it and the quality of the water before you commit weeks of labour to a well. Dig at least 3 in different places, wait for the water level in each to settle, record its depth, and the three levels also show which way the groundwater is flowing.
Part of Deep Wells
Test holes are small-diameter exploratory borings made before a well. They are far cheaper in effort than a full well in the wrong place, and give the depth to water, a log of the layers and a first look at water quality.
A Hole That Answers In A Day Saves Weeks In The Wrong Place
A hand-dug well is a large-diameter hole of 1.2-1.5 m that takes a team much longer than a narrow hole. A test hole of 10-15 cm removes only a small fraction of the soil. Two quantities decide whether a well site works, and a test hole measures both: how deep the water is, and what the ground is made of between the surface and the water. Sand and gravel yield water freely; clay holds it and gives little.
What You Need
- A hand auger with extension rods — see Augering, Auger Types and Extension Rods; a digging bar and narrow spade work in soft shallow ground
- A weighted measuring line — a stone or bolt on a knotted cord marked in 10 cm steps
- Sample bags or a row of small piles — one per layer, to keep the soil in order
- A level or a levelled string — to compare ground heights between holes (Water Tube Level)
- Clay and a rammer — to seal each hole afterwards
Method: Bore, Log, Settle, Measure, Seal
Step 1: Mark the holes. Choose three or more spots on the sites that landform and plants favour, spaced widely enough to show a difference. The example below spaces them 30 m apart; wider spacing shows more but costs more walking.
Step 2: Bore and log. Turn the auger down and lay out each load in order as it comes up. After every 0.5 m of depth, write down the depth, the soil (clay, silt, sand, gravel) and whether it is dry, damp or wet. The first wet load marks where water enters.
Step 3: Stop sensibly. Stop when water fills the hole, when the hole collapses, or when you reach the limit of your rods. A hole in loose sand will close; push a short length of pipe in as a lining or move over and start again.
Step 4: Let the level settle. Water often seeps in slowly. Come back after an hour, then the next morning, and measure again. The depth at which the level stops rising is the figure that counts. In clay it can take much longer than in sand.
Step 5: Measure the depth. Lower the weighted line until it goes slack, mark the cord at ground level and read the depth on the cord. Record it to the nearest 5 cm. Taste the water and look at it in a clear container, and note whether it is clear or muddy.
Step 6: Seal the hole. Pack clay in the top metre and ram it, then mound soil over it. An open test hole carries surface water straight to the aquifer.
Worked Example: Which Way The Water Moves
Three holes are levelled so that their ground heights are known relative to hole A. Hole B is 30 m east of A and hole C is 30 m north of A.
| Hole | Ground height above A | Settled depth to water | Water level above A’s ground |
|---|---|---|---|
| A | 0 | 4.2 m | 0 − 4.2 = −4.2 m |
| B | −1.0 m | 3.5 m | −1.0 − 3.5 = −4.5 m |
| C | +1.2 m | 5.6 m | 1.2 − 5.6 = −4.4 m |
The water level is 0.3 m lower at B than at A, over 30 m, so it falls 0.3 ÷ 30 = 0.010 (1%) toward the east. It is 0.2 m lower at C than at A, so it falls 0.2 ÷ 30 = 0.0067 (about 0.7%) toward the north. Combining the two, the water table slopes down by √(0.010² + 0.0067²) = 0.012 (1.2%) toward a point about 34° north of east (arctan(0.0067 ÷ 0.010) = 34°).
Groundwater moves down that slope. A latrine, pen or waste pit that lies to the west or south of the planned well is up-slope of it and would drain toward it. Put the well up-slope of every source of contamination. Note also that ground height and water depth do not move together: C is the highest ground and has the deepest water, but not the highest water level.
Common Mistakes
- Measuring the first wet load. In clay, water seeps in slowly and the settled level can be much higher than where it first appeared.
- Skipping the levelling. Depths below the ground are not water levels; direction needs heights.
- Reading the dry season in the wet. Repeat at the end of the dry season, or ask local people how far wells fall.
- Leaving the hole open. Seal it.
Safety
A pit is not a test hole
If a trial pit is dug instead of a bored hole, any excavation deeper than 1.2 m needs sloped or shored walls. Keep dug soil at least 0.6 m back from the edge, do not work alone, and do not enter an open bored hole of any depth. Narrow holes also trap animals, children and tools: cap or seal them the same day.
What’s Next
- Learn next: Hand Digging — turning a promising hole into a well
- Combine with: Augering — the tools that make test holes fast