Chemical contamination: what cannot be fixed in the field

Bacteria, viruses and protozoa are particles. Heat destroys their proteins, chlorine attacks their outer layer, UV light damages their genetic material, and a filter with small enough pores holds them back mechanically. All these methods rely on the problem being a living thing, or at least a particle.

Chemicals such as fuels, solvents, pesticides or heavy metals, by contrast, are usually dissolved. They are spread so finely through the water that they pass through common filter pores. Heat, chlorine and UV do not make them harmless. When boiling, non-volatile substances stay behind in the pot and become more concentrated; volatile substances can partly escape with the steam – and be breathed in while cooking.

Activated carbon can bind some substances and improve taste and smell. But in the field you do not know how much it has already absorbed or exactly which substances are in the water. Its capacity runs out unnoticed. That is why activated carbon is not reliable protection against suspected chemical contamination.

Because you cannot measure chemicals, the surroundings are your most important tool. Look at what lies upstream of the source and around it: industry, mining, spoil heaps, landfills, roads, railways, fuel stations, intensively farmed fields, burned areas or accident sites. The more of these are nearby, the more careful you should be.

There are warning signs at the water itself: an oily film or shimmering surface, a smell of fuel or chemicals, unusual colouring, foam, dead fish or animals, and banks without vegetation. Each of these signs is a reason to avoid the source.

Very important: the absence of these signs proves nothing. Many dangerous substances are invisible, odourless and tasteless. Clear water from a mining area can be heavily contaminated. Naturally occurring substances in groundwater, such as arsenic in some regions of the world, cannot be detected in the field either.

In disasters, water utilities and authorities announce whether tap water may still be used. A boil-water notice means: pathogens are suspected, and boiling protects against them. A do-not-use notice, by contrast, often means that chemical contamination is present or cannot be ruled out – and then boiling does not help.

Confusing these two notices is one of the most consequential mistakes. So read or listen to official announcements carefully and ask if in doubt. Follow the instructions even if the water looks unchanged.

The safe field decision when contamination is suspected is: do not treat – switch. Possible alternatives are your own supplies, bottled water, water supplied by the authorities, rainwater from clean surfaces or a source upstream of the possible contamination. These alternatives must of course still be treated against pathogens unless they are already safe.

Flood water is almost always mixed with sewage, fuels and chemicals. It is not a drinking-water source, not even after boiling or filtering.

If poisoning is suspected after drinking – for example nausea, dizziness or unusual symptoms after water that smelled of chemicals – call the local emergency number (112 in the EU) or a poison centre. This entry gives no medical instructions.

In its drinking-water guidelines, WHO draws a clear distinction between microbiological and chemical risks. Pathogens often act quickly and can make many people ill at once. Many chemicals, by contrast, only cause harm when taken in over a long time – but some are also immediately toxic. Most chemicals require special, often large-scale treatment plants that are not available in the field.

Distillation – evaporating water and catching the steam again – separates many dissolved substances such as salt. Volatile chemicals, such as components of fuels, can however evaporate along with the water and end up in the distillate. An improvised solar still is therefore no solution for chemically contaminated water either.