There are five basic groups of contaminants: pathogens, inorganic chemicals, synthetic organic chemicals, radioactive substances, and additives. There is also a long list of chemicals that are regulated by the Safe Drinking Water Act which including these catagories, here are some of the major contaminants:

Giardia:

Giardia, a common bacteria and pathogen which cause great intestinal pain, often appears in the Rocky Mountains, Pacific Northwest, and New England. Giardia causes over 50% of all waterborne outbreaks, and can slip past most water filtration systems. This can be prevented by a combination of disinfection and chlorination.

Oregon Department of Human Services

Arsenic:

Arsenic is an extremely toxic substance, and it belongs to the group of inorganic chemicals. The short-term effects can include skin irritation, gastrointestinal problems, central nervous system disorders, stress on the cardiovascular system, and contamination of the blood, liver and kidney. Some long-term effects can be: open soars in the skin, kidney or liver malfunction, bladder, liver, or lung cancer, and death. The act states that a maximum of 1.1 ppb (parts per billion) is allowed in public drinking water (EPA).

Lead:

Lead is yet another common heavy metal found in our water sources, and it is catagorized as an inorganic chemical. Some of the shot-term effects of ingesting lead are lead poisoning, gastrointestinal problems such as colic, and brain and kidney damage. The long-term effects include anemia (deficiency of red blood cells), neurological disorders, effects on blood pressure, altered kidney function, interference with Vitamin D metabolism, and delayed hearing and growth development in children. Lead also affects the reproductive system. It can cause a miscarriage, and is linked to low birth weights and delayed mental development. Drinking water accounts for 1.2% of lead exposure to humans. Lead can enter water systems through atmospheric particles from leaded gasoline combustion or smelting and burning fossil fuels. Three hundred thousand people drink water with over 50 parts per billion (ppb), which is the EPA maximum allowed.

Weinrich, Dan

Mercury:

Mercury is also an inorganic chemical. It is a very toxic chemical, and is most toxic in its organic form, methyl mercury. Many household appliances from paint to batteries use mercury. Many mercury products get thrown into our landfills, and the mercury leaks into our underground water systems. Once mercury enters the body it takes months to finally exit. The intake iof mercury affects your kidneys as well as your nervous system.

Volatile Organic Chemicals:

Volatile organic chemicals (VOC) belong to the group of synthetic organic chemicals. VOC's are toxic and pose a health threat to humans. They consist of pesticides and herbicides. VOC's get into the main source of our water through agricultural run-off and illegal toxic waste dumpings. Prior to filtration the water in twenty-eight out of twenty-nine cities contains volatile organic chemicals. VOC's cause cancer in the liver and kidney, as well as damage to the brain, nerves, reproductive system, and the immune system.

Swanhose

Radon:

Radon is a toxic radioactive gas that is released from most soils. It belongs ot the group of radioactive substances. Although ingestion of radon in water is not much of a threat, during water usage such as showering, radon in the water can become airborne.. Inhalation of radon is much more harmful than ingestion. The particles can enter the lungs and cause tissue damage, and eventually lung cancer. It can enter your home through the tinniest of cracks, and is considered the second leading cause of lung cancer.

Disinfection By-Products:

Disinfection By-Products are catagorized as additives. They are contaminants which are left behind by chlorine and other chemicals used during the filtration cycle. One of the disinfection by-products is formed when chlorine reacts with organic materials such as dead leaves. Didinfection by-products can cause bladder cancer.

For more information on chemicals restricted by the SDWA go to:

Pollutants: chlorinated solvents and hydrocarbons

Pollutants: DDT, PCB's and dioxins