Why Clear Water Can Still Make You Ill

A tumbling Highland burn or a chalk stream in a beech wood looks like the purest thing on earth. It almost never is. Any water in the British countryside has passed through soil grazed by livestock, roamed by deer, and colonised by rodents. That means it can carry Cryptosporidium, Giardia, E. coli, Campylobacter and, in some lowland areas, leptospira from rat and cattle urine. None of these announce themselves. A stream running gin-clear over gravel can still put you flat on your back for a fortnight.

The other common trap is assuming upland equals clean. A dead sheep lying in the headwaters, a herd of cattle crossing a mile upstream, or a slurry spreading operation the day before your visit will all end up in your cup. Treat every source as suspect, no matter how remote the setting feels, and you will never be caught out.

Choosing Your Source

Where you fill up matters as much as what you do with it afterwards. Give yourself the best possible starting water and you make the rest of the job easier.

• Favour flowing over still. Fast, aerated water from a riffle carries fewer pathogens than a stagnant pool, which can also harbour blue-green algae and heavy silt.
• Work uphill of your camp. Collect far upstream of where you are washing, toileting or doing the washing-up, and never fill from a spot other people may have used as a latrine.
• Look at what is around you. Field drains, farm gates, obvious stock crossings and recently spread fields are all reasons to walk on to another source.
• Avoid obvious blooms. Green, soupy or pea-coloured water in a still pool is a no. Boiling will not remove algal toxins.
• Springs are excellent if they emerge directly from rock and flow continuously. If the water seeps through open ground first, treat it exactly as you would a stream.

Gathering Water Cleanly

Collecting is where most people introduce contamination themselves. Carry a dedicated scoop or collapsible bucket and keep it for water only. Never dunk your drinking bottle into the source, and never let the threads of a bottle touch the stream — that is where muck clings and later drips into your mouth.

If a side stream is silty or the bank is churned to mud, dig a seep pit. Choose a spot roughly 20–30 cm back from the bank, scoop a hole down until water seeps in, then let it settle and refill two or three times before collecting from the top. The soil does a decent job of filtering grit and organic matter. It does not remove pathogens, so treatment is still essential, but your filter will thank you and your water will taste far better.

For a really murky source, a Millbank bag or a folded cotton cloth over your pot will strip out the worst of the sediment in a minute or two. Change or rinse the cloth often.

Filters, Pumps and Squeeze Systems

Hollow-fibre filters rated at 0.1 micron are the workhorse of UK woodland campcraft. They remove bacteria and protozoa reliably and turn a muddy puddle into something drinkable, and they are fast enough to keep a group supplied. Understand the limits, though: they do not remove viruses, which are small enough to pass straight through, and they do not remove dissolved chemicals or pesticides.

• Backflush regularly. A clogged filter slows to a trickle and pushes water past the seals.
• Never let it freeze. Ice expands inside the hollow fibres and splits them invisibly. Sleep with the filter in your bag on cold trips.
• Keep dirty and clean hands separate. The inlet side is contaminated; treat it that way.
• Combine methods. Filter first, then boil or chemically treat, and you cover viruses and bacteria in one routine.

Boiling and Chemical Treatment

Boiling is the gold standard for pathogens and it needs no consumables. Bring the water to a rolling boil and hold it there for a full minute — at UK altitudes a minute is ample, and longer is purely a waste of fuel. Boiling does nothing for chemical contamination, so if you suspect agricultural runoff, no amount of bubbling will help.

Chemical treatment is lighter and works while you walk. Chlorine dioxide is the best all-rounder, as it deals with Cryptosporidium, which ordinary chlorine tablets often fail against. Iodine is less effective and unsuitable for anyone pregnant or with thyroid problems. Follow the stated contact time to the letter and note that cold water takes far longer — at near-freezing temperatures some products need hours rather than the thirty minutes quoted for warm water. Ultraviolet pens are quick and effective, but only in clear water and only with fresh batteries.

Storage, Hygiene and Aftercare

Once treated, keep your water treated. Store it in clean, dedicated bottles and resist topping them up from a raw source later in the day. Wash your hands or use sanitiser before eating and after every trip into the bushes, and cover any cuts before you handle stream water — leptospirosis enters through broken skin.

Do your washing-up well away from the water, at least 30 metres back, straining food scraps and packing them out. Soap and food waste do not belong in a woodland burn.

Finally, if you develop diarrhoea, cramps or fever in the days or weeks after a trip, see your GP and mention that you drank wild water. Giardia in particular can take a fortnight or more to appear, and knowing the source helps enormously. Treat it as a lesson, not a disaster — and enjoy the woods.