Aesthetics 
Otherwise harmless contaminants like chlorine, sulfur, iron, and manganese that cause taste, color, and odor problems. 

Water Hardness 
Hard water contains excessive levels of the minerals calcium and magnesium. Hard water shortens the life of household plumbing and water-using appliances, makes cleaning and laundering tasks more difficult and gradually decreases the efficiency of water heaters. 

Lead
Used extensively in plumbing materials (pipes and lead-based solder) , lead can leach into water supplies. Low levels of lead have been linked to learning disabilities in young children, and high levels can cause hypertension in adults. 

Biological Pathogens
Waterborne organisms that can cause disease in humans. They include cysts like Cryptosporidium and Giardia; bacteria like typhus, fecal coliform and cholera; and viruses like influenza. These organisms typically cause unpleasant intestinal disorders and can pose a significant threat to the immune-impaired. 

Nitrates
Nitrogen compounds are sometimes found in ground and surface water in rural areas, often as a result of nitrogen-based fertilizer runoff. Excess nitrate levels can interfere with the oxygen-carrying capacity of blood, especially in babies, and have been linked to high incidences of miscarriages. 

Heavy Metals 
Metals like mercury, zinc, copper, and cadmium usually enter the water supply as industrial waste and, in excessive concentrations, can cause physiological damage to humans, including damage to the central nervous system. 

Radium/Radon
Naturally occurring radioactive elements linked to cancer in humans. Radon is found in gaseous form, and is absorbed through drinking, as well as through inhalation during washing or showering. 

VOC's
Volatile organic compounds, such as the petroleum distillate benzene and the industrial degreasing compound trichloroethylene. High concentrations of VOC's are linked to organ damage and cancer in humans. 

THM's
Trihalomethanes are by-products produced when chlorine reacts with organic compounds in water. THM's are primarily absorbed through inhalation, and have been linked to bladder and rectal cancer. 

Asbestos
A fibrous mineral that contaminates water naturally or through its past use in concrete water pipes. Asbestos has been linked to lung and other forms of cancer. 

Arsenic
Both a natural and manufacturing-induced ground water contaminant, arsenic is linked to various cancers and may damage the circulatory and central nervous systems. 

Sediments 
Solid particulates in water that can settle out over time. The presence of sediments in water is typically an aesthetic concern. 

Low/High pH
pH refers to "potential hydrogen," and is a measure of acidity or alkalinity on a 14-point scale (zero through six is acidic; seven is neutral; and eight through 14 are alkaline). Extreme measures of acidity in water can be corrosive, whereas high alkalinity can be the source of aesthetic problems.