Vivid Ready

Water

Test and disinfect a well or borehole

A private supply is the best water asset a household can have and the easiest to quietly poison. How to work out the volume, dose it, circulate it and flush it, and what testing actually tells you.

If you have a well, borehole or spring, you have something most households would trade a great deal for: water that keeps arriving when the mains stops. Around a million people in the UK are on private supplies, mostly in upland and rural areas.

It comes with an obligation the mains never asked of you. Nobody is testing it, nobody is treating it, and nobody will tell you when something has changed.

What a private supply actually is

The three types behave very differently and it matters which you have.

A borehole is drilled deep, often tens of metres, into rock or a confined aquifer. It is the best of the three: deep water is old water, filtered through a great deal of ground, and largely protected from surface contamination. Boreholes are hard to contaminate and hard to recover once contaminated.

A dug or shallow well draws from the water table at a few metres. Far more exposed. Surface water, farm runoff and septic leakage can reach it, and it responds to rainfall within days. Most contamination incidents in Britain are shallow wells.

A spring is groundwater emerging at the surface. Quality depends entirely on what is uphill, and it is fully exposed at the point of emergence. A spring collection chamber that is not properly sealed is effectively an open surface water source.

Working out the water volume

You cannot dose what you cannot measure. This is the step people skip and it is why so many home disinfections fail: a guessed volume gives a guessed dose, and an under-dose achieves nothing at all while feeling like it did.

For a well or borehole:

  1. Measure the depth to the water, from the top of the casing, with a weighted string. Mark where it goes wet.
  2. Measure the total depth the same way, until the weight rests on the bottom.
  3. Subtract: total depth minus depth to water gives the standing water column.
  4. Measure or find the internal diameter of the casing.

A worked example. A 150 mm borehole with 20 m of standing water: 20 × 17.7 = 354 litres, plus a 200 litre storage tank and perhaps 50 litres of pipework and cylinder, giving roughly 600 litres to dose.

The dose

Use plain unscented sodium hypochlorite bleach with no thickener, fragrance or detergent. Bleach loses strength on the shelf, substantially within two years, so buy fresh for this and date the bottle. If yours is 10%, halve the quantities; if 3%, use about 1.7 times as much.

Doing the disinfection

The step that separates a working disinfection from a wasted afternoon is circulation. Poured in and left, bleach sinks and sits at the bottom of the column, leaving the upper water, the casing walls and the entire distribution system untreated.

  1. Turn off any UV lamp, and bypass or remove any carbon filter, softener or cartridge filter. Carbon strips chlorine out immediately, so leaving it in line means the far side of it never gets disinfected. Cartridges should be replaced afterwards, not reused; they will hold contamination.
  2. Dilute the bleach in a bucket of water first. Never pour neat bleach down a borehole: it is dense, it plunges straight to the bottom, and it can damage a pump seal on the way past.
  3. Pour it in around the casing wall, letting it run down the sides rather than straight down the middle, so it wets the whole casing.
  4. Circulate. Attach a hose to a nearby tap, run it back down into the well, and let it run for 15 to 30 minutes. The water is pumped up, dosed, and returned, mixing the whole column properly. This step is what actually disinfects the well rather than a portion of it.
  5. Scrub the casing walls with the hose stream if you can reach, especially above the water line where biofilm sits.
  6. Draw it through every outlet. Go round the house and open every tap, hot and cold, inside and out, one at a time, until you smell chlorine at each. Then close it. Do not forget the outside tap, the shower, the toilet cisterns, the washing machine and dishwasher feeds, and any drinking water tap.
  7. Leave everything standing 12 to 24 hours. No water use at all during this period, so fill jugs and buckets beforehand and plan for a day without the supply.

Flushing, and where the water goes

After the contact time, run the water off until the chlorine smell is gone completely. This usually takes longer than expected, several hours on a large system, and the last traces linger in the hot water cylinder.

Where that water must not go:

Run the hose to bare ground or hardstanding well away from any water, planting or soakaway, and let it disperse slowly. Letting the flow run thin over a long period is much better than dumping it all in one spot.

Afterwards, replace any filter cartridges you bypassed, restart the UV lamp, and give the system a further flush before drinking.

Testing, and what it does not tell you

Home test strips for a private supply tell you very little. They typically check chlorine, hardness, pH and nitrate. Those are useful, but they are not the things that make people ill.

What matters is bacteriological testing, specifically E. coli and total coliforms, and that needs a laboratory. Your local authority has a duty around private water supplies and is the right first call; they can test, and in many areas will do so for a modest fee. Get a baseline test done while everything is normal, so that a later result means something.

Signs something has changed, none of which are reliable on their own: a change in taste, smell or clarity; water that suddenly runs cloudy after rain; sediment where there was none; a drop in yield; anyone in the household with unexplained stomach upset.

The absence of all of those means nothing. Contamination that makes people ill is generally invisible, odourless and tasteless, which is exactly why the laboratory test exists.

Protecting it in the first place

Far cheaper than disinfecting it afterwards.