Most survival water advice is written for North America and assumes the threats are giardia, cryptosporidium and beavers. Britain has its own list, and one item on it defeats the method almost everybody reaches for first.
Blue-green algae, and why boiling makes it worse
Blue-green algae is not algae. It is cyanobacteria, and in warm still water it blooms across British lakes, reservoirs, canals and farm ponds, most reliably from late spring through to autumn. A bloom looks like green or blue-green paint on the surface, or a scum along the downwind edge, and sometimes like pea soup through the whole body of water.
Some blooms produce toxins. Microcystin is the common one and it damages the liver.
What to do instead: do not use the water at all. Not for drinking, not for cooking, not for washing food, not for the dog. A bloom can persist after the visible scum has gone, and toxin can remain in the water after the cells that made it have died.
If you have no alternative whatsoever, chlorination and ultrafiltration are the methods that actually work on cyanotoxins, but neither is something to attempt casually with a bottle of household bleach and no way to measure the result. Treat a bloom as a hard stop and find another source.
Recognising it matters more than treating it:
- Green, blue-green or turquoise scum on the surface or blown against one bank.
- Water like pea soup, or with a paint-like sheen when disturbed.
- Dead fish, or dead birds, at the edge.
- A musty or earthy smell over still water in warm weather.
Dogs die of this in Britain most summers, usually after swimming and then licking their coats. If you have a dog, keep them out.
Farm runoff
Britain is a small, intensively farmed country and very little of its surface water is far from a field.
The risk spikes after heavy rain following a dry spell, which is exactly when a stream looks fullest and most inviting. That first flush carries slurry, fertiliser and whatever was on the surface straight into the watercourse. Cryptosporidium is the classic result and it is unpleasant, long-lasting, and not touched by chlorine at normal doses.
Practical rules:
- Prefer water upstream of farmland rather than downstream, and walk up to check what is above your collection point rather than assuming.
- Wait 24 to 48 hours after heavy rain if you can, and let the first flush pass.
- Treat any water downstream of grazing or arable land as contaminated regardless of how it looks.
- Boiling handles cryptosporidium. Chlorine largely does not, which is a genuine gap in the bleach method and worth knowing before you rely on it.
Road salt and road water
From roughly November to March, British roads are salted heavily. Snow banked at a roadside, meltwater running off tarmac, and drainage from a road are all salty enough to be worth avoiding, and the salt cannot be removed by boiling, filtering or chlorinating. Only distillation removes it.
Road runoff also carries oil, brake dust, rubber and whatever was spilled. The rule is simple: nothing that has run across a road.
The same applies to snow. Snow banked up by a plough or lying within a few metres of a road is not a water source, however clean and white it looks. Take snow from open ground well back from any road, as covered in melting snow and ice.
If somebody has already drunk it
The page has said dogs die of this most summers and has not said what to do about it, which is a gap worth closing.
Then find another source. See judging raw water before you treat it for the general assessment, and remember that a bloom can persist after the visible scum has cleared.
A quick risk order for British water
When you have a choice, take them in this order.
- Mains water, while it still runs, even at low pressure.
- Stored water you put by yourself.
- Rainwater caught from a clean roof, filtered and disinfected. See the rainwater system page.
- Upland running water above any farmland, boiled.
- Lowland running water, boiled, accepting the runoff risk.
- Standing water in a pond or lake, only after satisfying yourself there is no bloom, and only boiled.
- Never: anything with a visible algal bloom, anything that has crossed a road, and anything near an obvious industrial or landfill site.