Drilling
“How deep?” is the first question about any new well, and it hides three different numbers: how far the bore is drilled, where the water stands in it, and where the pump hangs. This guide separates them, explains how the drilled depth is actually decided at the rig, and why a deeper bore is not automatically a better one.
June 15, 2026 · 9 min read
When a neighbour says their well is a hundred metres deep, it sounds like a description of where their water is. Usually it is not. It is the length of the hole. Their water may stand thirty metres below the surface, the pump may hang at sixty, and most of what it delivers may come in through a single fracture at forty-five. Those are three or four different depths, and confusing them is the root of most surprises about wells: the quote that seemed too deep, the pump that seemed too shallow, the deep well that still runs short in April.
The drilled or total depth is how far down the bore goes. The water level is where water stands inside it, measured down from the ground: the static level when the pump has been off for a while, and the lower pumping level while it runs, because the well draws down as water is taken out. The pump depth is where the pump is hung, chosen to stay below the lowest pumping level with a margin, and above the bottom where sediment collects. The diagram below puts the three side by side.
Cross-section
The brackets on the right are measured from ground level. Only the third one is the depth of the hole; the other two are what the owner actually lives with.
Every depth is measured down from here. The wellhead is sealed and sits slightly above the ground.
Lines the bore through soil and weathered rock that would otherwise collapse. How far it goes is decided by what the drilling finds.
Where water stands with the pump off. It moves with the seasons, not with how deep the hole is.
Lower than the static level while the pump runs. The gap between the two is drawdown, and it is the number that sizes the pump.
Fractures or permeable layers where water actually enters the bore. A well may have one, several, or a deep one that adds little.
Set below the lowest expected pumping level with a margin, not at the bottom of the bore.
Where drilling stopped. Below the pump, it collects fine sediment away from the intake.
Ground to static level: what the neighbour's well can roughly tell you about.
Ground to pump: what decides cable, riser and how hard the pump works.
Ground to bottom: what the drilling is priced on, and the number people usually quote.
Most Samui bores pass through soil, then weathered granite, then harder rock. Groundwater moves through the weathered zone and through fractures in the hard rock; solid granite between fractures holds almost none. So a well does not fill evenly along its length. Water enters at particular depths, where the bore crosses an open fracture or a permeable band, and those zones decide how much the well can give. A bore drilled to a hundred metres with one good fracture at forty-five behaves much like a bore drilled to fifty with the same fracture, as long as the pump sits safely above the bottom of each. The extra fifty metres did not add water; they added cost and storage volume in the hole.
Nobody drills to a number chosen in an office. The driller reads the cuttings coming up to see where weathered rock gives way to hard rock, notes where water is struck and how strongly it flows, and watches whether the hole stays open. The aim is to get through the ground that would collapse and needs casing, then to cross enough water-bearing rock that the well holds up when it is pumped, with room below the pumping level for the pump. When the flow coming up suggests the well already meets the property's needs, drilling deeper is a decision taken with the owner, on what it is likely to add. When the flow is weak, continuing a little further can find another fracture, or can add nothing; experience on similar ground helps judge which. The real test comes afterwards, when the well is pumped for hours and the level is measured, as described in our guide to how much water a well produces.
A nearby well is useful evidence. Its static water level is a reasonable hint of where water stands in the area, its depth gives a sense of how thick the weathered ground is, and its yield shows whether the rock here is generous or stingy. What it cannot do is predict the fractures under your plot, which can be absent fifty metres away. Two wells on neighbouring plots routinely differ in depth and yield, and a neighbour's well pumped hard every day can lower the water level around it. Treat their numbers as a starting range, not a forecast.
Near the beach, fresh groundwater sits on top of salty water, and the boundary moves when wells pump hard. The coastal question is less “how deep can we go” than “how deep should we stop”, with a pump rate the well can sustain and salinity checked during testing. Our guide to saltwater intrusion explains the mechanism. On a hillside the challenge is the opposite: the water level may sit far below the wellhead, so the pump hangs deep and lifts a long way, and the cost moves from the hole into the pump, cable and head. The drilling pad, the casing through thick weathered ground and the pump specification all need planning for that.
On Samui the water level in a well is typically highest after the rains and lowest towards the end of the dry season, when demand is also at its peak. A well tested in November can look much stronger than it will in April. That does not change how deep the hole is; it changes how much room there is between the pumping level and the pump. Setting the pump with a dry-season margin is part of answering the depth question, and our guide to seasonal groundwater shows how to monitor it.
Once the yield test shows how far the level falls, the pump depth is chosen: deep enough that the lowest pumping level stays above the pump with a margin, shallow enough to keep clear of sediment at the bottom and of a water-bearing zone the pump would disturb. Hanging the pump at the very bottom does not buy more water and can pull in sand. Our guides to what is inside a deep well and to choosing a submersible pump cover the construction and the sizing that follow from it.
Planning a new well? Our drilling team can review the site and explain how the depth will be decided as the bore develops, and what the test will need to show before the pump is chosen.
Drilling
Once the rig leaves, almost all of your well is out of sight: tens of metres of pipe, gravel and seal that decide whether it gives clean water for decades or sand within a few years. This is what should be down there, and why.
October 3, 2026 · 9 min read
Drilling
The same well can give plenty in December and struggle in April without anything being wrong with it. A well isn't a tank of fixed size. Its useful supply rises and falls with the water around it, and the dry season is when that matters most.
October 8, 2026 · 9 min read
Drilling
"The pump gave 40 litres a minute" sounds like an answer, but it describes the pump on one afternoon, not what the well can keep giving. Yield is read from the water level: how far it falls while pumping, whether it settles, and how it comes back afterwards.
July 10, 2026 · 9 min read