You cannot see bacteria and you cannot smell a virus, so nothing on this page tells you water is safe. What it does is tell you two useful things: whether to walk away, and which treatment method has any chance of working.
That second one matters more than people expect. Boiling, filtering, chlorinating and distilling each fail against different problems, so choosing wrongly can leave you confidently drinking something dangerous.
Look uphill first
Spend more time on this than on the water. Where water has been tells you more than what it looks like.
Walk upstream if you can, or think it through from the map:
- Farmland? Assume slurry, fertiliser and cryptosporidium. Boil rather than chlorinate.
- A road, car park or industrial estate? Assume oil, brake dust and road salt. None of that is removed by boiling, filtering or bleach. Walk away.
- Housing, especially older housing? Assume sewage misconnections, which are common in Britain.
- A landfill, tip, or old industrial ground? Walk away. Chemicals are the one category where all your normal tools fail.
- Open moor or upland above all of it? This is the best surface water Britain offers. Still boil it.
Moving water beats standing water, almost always. Flow means oxygen, dilution, and less chance of a stagnant bloom. A stream running over stones is a far better bet than a pond, even a clear-looking one.
What the water itself tells you
| What you see or smell | What it suggests | What to do |
|---|---|---|
| Clear, moving, over stone or gravel | Best case available | Still boil or filter |
| Cloudy or silty | Suspended solids, and bleach will be largely wasted on them | Settle or pre-filter first, then treat |
| Brown, peaty, tea-coloured | Usually organic staining from upland peat, often harmless | Filter and boil. Colour may remain, which is fine |
| Green tinge, surface scum, paint-like sheen | Possible algal bloom | Do not use. See UK water risks |
| Rainbow film on the surface | Oil or fuel | Do not use. Nothing you have will fix it |
| Smells of rotten eggs | Sulphur, often from anaerobic mud | Usually not dangerous, but suggests stagnation. Prefer another source |
| Smells musty or earthy over still water in summer | Possible cyanobacteria | Do not use |
| Smells of chemicals, solvent or fuel at all | Chemical contamination | Do not use |
| Foam or suds at the edge | Can be natural agitation, can be detergent | Investigate upstream before using |
| Dead fish, dead birds, or no life at all | Something is killing things in it | Do not use |
Look for life. Water with insects skating on it, small fish, larvae under stones, and green plants growing in it is water that is supporting life rather than ending it. A stretch of water with nothing living in or around it, in summer, is a warning worth taking seriously.
The hard stops
Matching the treatment to the problem
This is the part worth internalising, because it is where good instincts go wrong.
The practical reading of that table:
- Boiling is the best single method for almost every British biological risk, and the one to default to.
- Bleach has two real gaps: cryptosporidium and algal toxins. After heavy rain on farmland, boil instead.
- A filter plus boiling covers nearly everything biological, which is why that pairing is the standard advice.
- Only distillation touches salt and chemicals, and it yields almost nothing. See solar still and transpiration bag.
Always clarify before you disinfect. Let cloudy water stand until the silt drops, draw off the top, or pass it through cloth or a biosand filter first. Chlorine gets used up on suspended solids before it reaches the organisms, so clear water makes your bleach go several times further and makes boiling more reliable too.