Drinking stagnant water may be unhealthy, whether the water comes from outdoors or through pipes. Some bacteria, algae, and parasites are better able to multiply in water that is stagnant.
The causes of stagnant water differ somewhat between the developed and the developing world, with the former usually suffering troubles with drinking stagnant water after watermain breaks.
The danger of drinking stagnant water is not to be conflated with stagnant water increasing vector-borne (e.g. mosquito) diseases like dengue and malaria, or a glass of chlorinated tap water sitting out for days.
Causes of stagnant water
In the developing world, stagnant water comes from improper drainage, and a population exceeding water capacity of outdated infrastructure. Irrigating and washing crops with contaminated water can also cause disease.
After a water main break, workers need to shut off water to fix the main. When water pressure then drops, back-flow caused by back-siphoning or back-pressure can occur if flowback prevention systems fail. This could allow contaminated water to enter the public system, such as brackish water from a fire extinguisher system, fertiliser from a nursery, or chemicals from an industrial plant.
Contaminants can also find their way into public drinking water directly through the break in the water main. Studies have shown that soil surrounding buried pipe can contain fecal pathogens. Additionally, runoff from streets and farmland can be tainted with microbial and chemical contaminants.
In the U.S., the CDC links 70% of waterborne diseases to two parasites: Crytosporidium parvum and Giardia lamblia. There are no effective drugs for Cryptosporidium; treatment is by rehydration.
But water main break contamination is not confined to these two pathogens. Others are Legionnaires' disease, which can cause pneumonia, and E. coli infection.
Why do bacteria like stagnant water?
Anaerobic bacteria thrive in stagnant and oxygen-deficient water, and are impaired to varying degrees when exposed to oxygen. Denitrifying bacteria, which metabolise nitrogenous compounds, are an example. Bacteria that die when exposed to oxygen, called obligate anaerobes, do not have an enzyme to convert lethal superoxide into water or O2. As fish aquarium owners know, surface agitation provides water oxygenation.
When camping, don't drink pond water without treating it. The presence of algae is a good indicator that water is too stagnant to drink. Purifying water while camping should be a multi-stage process. Boiling, filtration, and water purification tablets preferably should all be used to make water as safe to drink as possible.