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Three Categories of Water Pollutants

Water pollution makes headlines when it is cataclysmic, or when its affects are visible and dramatic. It is also common--we are accustomed to seeing trash in streams or on beaches. More potentially dangerous, however, are the forms of pollution that are not so easy to see. Water pollution can be categorized as three basic types: mineral, biological, or thermal.

Mineral Pollution
- Oil is one of the most visible of water pollutants, but other minerals can also be dangerous, even toxic, in water.
  
  These pollutants can include chemicals, heavy metals and other minerals that are, as the National Institute of Environmental Health Sciences, puts it, "detrimental to human, plant, or animal health." Heavy metals, such as lead and mercury, can build up in tissue over time, remaining in the food chain. They are potentially dangerous; mercury, for instance, is blamed for impeding nervous system development in infants. Other examples of mineral pollution includes such toxins as arsenic or carcinogenic PCBs. Some types of pollution, such as fertilizer runoff and petroleum, may contain more than one specific type of toxin.
Biological Pollution
- Bacteria and viruses can pollute water, making people and animals ill.
  
  Less familiar and less well-understood are biological pollutants, like bacteria and viruses. Some microbial life should be expected in natural water sources, but the U.S. Geological Survey explains that the presence of bacteria similar to those in human and animal digestive tracts "signal[s] that disease-causing pathogens may be present." Specific pathogens include giardia and cryptosporidium. The National Institute of Environmental Health Sciences warns against such diseases as shigellosis, giardiasis, amoebic dysentery, skin rashes, conjunctivitis and infection.
Thermal Pollution
- While power plants are most associated with air pollution, the water used to cool them may contaminate local water with heat.
  
  While the most problematic forms of pollution involve substances in water, thermal pollution involves the temperature of the water itself. Thermal pollution occurs when warm or hot water--typically used for cooling power plants--is released into the environment, killing plants or animals with the sudden change in temperature or subjecting them to a rise in temperature they cannot withstand. The U.S. Clean Water Act (Title 33 of the U.S. Code) calls this type of pollution "waste heat and thermal discharge."

Nature