Understanding that there is no such thing as ‘pure water’ helps defuse the panic that some people display when they learn that there is anything dissolved in water at all. All natural waters contain at least some dissolved chemicals, and much of this is beneficial to our health, as it supplies some of the trace nutrients which healthy bodies depend on. Indeed, it is a very bad idea to drink the unnatural, distilled deionised water which is used in laboratories: it would play war with your digestion. Of course, some natural waters contain so much dissolved matter that they cease to be healthy to drink.
For the most part our taste buds are a good guide, save for the few harmful solutes which are occasionally present in harmful concentrations even at levels below the taste threshold. Such, tragically, has been the case, for instance, with arsenic in deep groundwaters in parts of Bangladesh (and elsewhere), where well-intentioned development projects unwittingly exposed hundreds of thousands of people to long-term arsenic poisoning. However, for the most part Mother Nature is relatively benign in her dealings with freshwater quality – if you really want to find dangerous water, you need to examine human influences.
Sullying the waters pollution
While the natural chemistry of water gives powerful clues about its history, the tell-tale signs of human interference are even more blatant. Most notable is the presence of pathogens – that is to say, harmful microbes of faecal provenance – as well as high concentrations of nutrients, particularly compounds of nitrogen, phosphorous and carbon. These indicators are not only associated with the bodily wastes of humans, but are common to other mammals, too, and hence are particularly elevated where large herds of farm animals are held in small areas. While pathogens are directly hazardous to other mammals, the nutrient pollutants mainly pose hazards to aquatic life.
So prevalent are these pollutants that it may be assumed that any stream or river downstream of an inhabited (and/ or live-stocked) area will be sufficiently contaminated that it cannot be used for drinking without prior water treatment. The situation is often somewhat better where groundwater is concerned, as many pathogens are strained out of the water as it infiltrates through the soil and deeper strata. Nevertheless, the simple pleasure of drinking from a natural spring or stream should be strictly reserved for remote rural areas where animals are scarce, such as high mountainsides.
While it is true that some waters are more polluted than others, even a small number of pathogens can render a water very dangerous to drink. The same may be said about most other waterborne pathogens: as most surface waters are eventually exposed to animal and/ or human faecal material, drinking untreated surface water can be confidently expected to lead to enteric diseases.