Drinking water may reasonably be expected to contain at least small amounts of contaminants. Some drinking water contaminants may be harmful if consumed at certain levels in drinking water while others may be harmless.
The following are general categories of contaminants in drinking water:
Physical contaminants primarily impact the physical properties of water (e.g. sediment or organic material suspended in the water of lakes, rivers and streams from soil erosion)
Chemical contaminants are naturally occurring or man-made (e.g. nitrogen, bleach, salts, pesticides, metals, toxins produced by bacteria, human or animal drugs)
Biological contaminants are organisms in water (e.g. bacteria, viruses, protozoan, parasites)
Radiological contaminants are chemical elements with an unbalanced number of protons and neutrons resulting in unstable atoms that can emit ionizing radiation (e.g. cesium, plutonium, uranium)
-US Environmental Protection Agency