Protecting a deep well from surface contamination

Owners of deep wells often assume that eighty metres of rock between the house and the water is protection enough. For most of the ground, it is. The weak point is the wellhead: the place where a pipe connects the ground surface to the aquifer. If water can run down that pipe, inside it or alongside it, the depth no longer matters, because the well itself has become the shortcut. On an island where a single afternoon downpour moves a lot of water across a garden, protecting a deep well from surface contamination is mostly about what happens in the first metre.

How surface water gets into a deep well

• Down the outside of the casing. Where the gap between casing and borehole isn't properly sealed near the surface, water pooling at the casing can run straight down it. That seal is explained in what's inside a deep well.
• Through the top. A loose or missing cap, or gaps where the riser pipe and pump cable leave the wellhead, let in rainwater, insects, small animals and debris.
• Through damage near the surface. Casing cracked by a vehicle, by settlement or by an old repair opens a path below ground level, where nobody sees it.
• By flooding. When the wellhead is submerged, any weakness at the top is underwater, and floodwater carries whatever it has picked up on the way.

What a protected wellhead looks like

• The casing ends above the ground and above any level where water collects or floods, not flush with the lawn and not in a pit.
• A sealed sanitary cap closes the top, with a screened vent so air can move but insects can't.
• The riser pipe and cable leave through sealed glands or fittings, never through a gap stuffed with a rag.
• A concrete apron surrounds the casing and slopes away from it, bonded to the casing without cracks or gaps.
• Underneath, a grout seal fills the space around the casing near the surface, installed when the well was built.

Public-health guidance on private wells, such as the US EPA's, describes the same basic picture: casing above grade, ground sloping away, and a vermin-proof sealed cap. Underground wellhead pits and chambers deserve a specific warning. They look tidy and keep the casing out of sight, but they collect water and put the top of the well below ground level, which is exactly where it shouldn't be.

Drainage around the well

Tropical rain arrives fast and in volume, and on a sloping Samui plot it travels. Walk the wellhead area after a heavy shower and watch where the water goes. It should be moving away from the casing. Common problems are easy to fix once seen: a roof downpipe discharging onto the lawn next to the well, a path or terrace draining towards it, pool overflow or backwash water released nearby, or an irrigation zone soaking the ground around the pad every evening. Regrading the ground, adding a gravel strip or a shallow swale, and moving downpipes usually costs little.

Keep chemicals away too. Pool chemicals, fertiliser, pesticides and fuel shouldn't be stored, mixed or spilled near the wellhead, and planting beds that are heavily watered and fed are better placed somewhere else.

Septic tanks, soakaways and why distance still matters

A septic tank or soakaway releases wastewater into the ground by design. Even when the well draws from deep rock, the horizontal relationship matters: a soakaway upslope of a poorly sealed casing gives contamination both a source and a shortcut. In fractured granite or sandy coastal soil, water can travel further than expected.

Many countries set minimum separation distances between wells and septic systems in public-health or building rules. The figures vary with jurisdiction, soil and system type. We don't quote one universal number, and any local requirement on Samui should be checked with the relevant authority alongside the groundwater permit. In practice, the site-specific answer is consistent: further is better, uphill of the well is worse than downhill, and a well-sealed wellhead matters most of all when the distance is short.

When the well is open: maintenance and building work

Wells are most exposed while someone is working on them. A pump pulled for service leaves the casing open; cover it whenever work pauses, and never overnight. Equipment going down the well should be clean, and sensible contractors disinfect anything that has touched the ground. After work, the cap, glands and seals should go back exactly as they were, or better.

Construction near an existing well needs a plan of its own. Fence the wellhead off, keep vehicles and excavators away from the casing, stop concrete washout and spoil from going near it, and don't let new paving or landscaping bury it or raise the ground around it. A wellhead that ends up below a new terrace is both hard to service and badly placed for drainage. Our guide to preparing for drilling covers where a new well should go in the first place.

Signs the wellhead needs inspection

• Water that turns cloudy or coloured after heavy rain. See sand or cloudy water from a deep well.
• Water standing against the casing after a shower.
• A cracked, broken or missing apron, or a visible gap between concrete and casing.
• A loose, cracked or improvised cap, or rags and tape around the pipe and cable.
• Insects, roots or debris found inside the casing during service.
• A change in taste or smell, or a positive bacteria result in a routine test.

After a flood

1. Switch off power to the well pump before going near a flooded wellhead.
2. Don't use the water for drinking, cooking or brushing teeth until it has been tested and found safe.
3. Once the water has gone down, inspect the wellhead: cap, glands, apron, and any sign of debris inside.
4. Have the well disinfected (shock chlorination) and flushed by someone experienced in doing it safely. Public-health guidance, including the CDC's, describes this procedure for flooded wells.
5. Take the sample for a laboratory water test only after the chlorine has been flushed out of the system.
6. Fix whatever let the water reach the wellhead, or the next storm will repeat it.

A bore that is no longer used is the extreme case of an unprotected wellhead. What to do with one, from keeping it safely to sealing it, is in our guide to unused and abandoned wells.

If runoff or flooding has reached your wellhead, ask us to inspect the installation. We'll check the cap, seals, apron and drainage, advise whether the water should be tested, and put right whatever let the water in as part of a well inspection.