The US

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 Safe Drinking Water Act

Safe Drinking Water Act

The Safe Drinking Water Act is the principle federal law in the United States intended to ensure safe drinking water for the public. Pursuant to the act, the Environmental Protection Agency is required to set standards for drinking water quality and oversee all states, localities, and water...

 and derivative legislation define public water system as an entity that provides "water for human consumption

Drinking water

Drinking water or potable water is water pure enough to be consumed or used with low risk of immediate or long term harm. In most developed countries, the water supplied to households, commerce and industry is all of drinking water standard, even though only a very small proportion is actually...

 through pipes or other constructed conveyances to at least 15 service connections or serves an average of at least 25 people for at least 60 days a year."

The Canadian provinces of Manitoba

Manitoba

Manitoba is a Canadian prairie province with an area of . The province has over 110,000 lakes and has a largely continental climate because of its flat topography. Agriculture, mostly concentrated in the fertile southern and western parts of the province, is vital to the province's economy; other...

 and Nova Scotia

Nova Scotia

Nova Scotia is one of Canada's three Maritime provinces and is the most populous province in Atlantic Canada. The name of the province is Latin for "New Scotland," but "Nova Scotia" is the recognized, English-language name of the province. The provincial capital is Halifax. Nova Scotia is the...

 also use this definition.

Some US states (e.g. New York

New York

New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...

) have varying definitions.
Of the approximately 155,693 public water systems in the United States, 52,110 (33.5%) are community systems and 103,583 (66.5%) are noncommunity systems, including 84,744 transient systems and 18,839 nontransient systems.

According to the EPA, over 286 million Americans get their tap water from a community water system.8% of U.S. community water systems provide water to 82% of the U.S. population through large municipal water systems.

Additional US Public Water System Categorization

The United States Environmental Protection Agency‎ (EPA) has defined three types of public water systems:

• Community Water System (CWS): A public water system that supplies water to the same population year-round.
• Non-Transient Non-Community Water System (NTNCWS): A public water system that regularly supplies water to at least 25 of the same people at least six months per year, but not year-round. Some examples are schools, factories, office buildings, and hospital
  Hospital
  A hospital is a health care institution providing patient treatment by specialized staff and equipment. Hospitals often, but not always, provide for inpatient care or longer-term patient stays....
  
  s which have their own water systems.
• Transient Non-Community Water System (TNCWS): A public water system that provides water in a place such as a gas station or campground where people do not remain for long periods of time.

The EPA also classifies water systems according to the number of people they serve:

• Very Small water systems serve 25-500 people
• Small water systems serve 501-3,300 people
• Medium water systems serve 3,301-10,000 people
• Large water systems serve 10,001-100,000 people
• Very Large water systems serve 100,001+ people

Water systems may be categorized by their source of water:

• Groundwater
  Groundwater
  Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
  
  , generally from wells
  Water well
  A water well is an excavation or structure created in the ground by digging, driving, boring or drilling to access groundwater in underground aquifers. The well water is drawn by an electric submersible pump, a trash pump, a vertical turbine pump, a handpump or a mechanical pump...
  
  
• Surface water
  Surface water
  Surface water is water collecting on the ground or in a stream, river, lake, wetland, or ocean; it is related to water collecting as groundwater or atmospheric water....
  
  and groundwater "under the influence" of surface water
• Purchase of water from another Public Water System

The term "public" in "public water system" refers to the people drinking the water, not to the ownership of the system.

Water-related Diseases and Contaminants in Public Water Systems

Sources of drinking water are subject to contamination and require appropriate treatment to remove disease-causing contaminants. Contamination of drinking water supplies can occur in the source water as well as in the distribution system after water treatment has already occurred. There are many sources of water contamination, including naturally occurring chemicals and minerals (for example, arsenic, radon, uranium), local land use practices (fertilizers, pesticides, concentrated feeding operations), manufacturing processes, and sewer overflows or wastewater releases.

The presence of contaminants in water can lead to adverse health effects, including gastrointestinal illness, reproductive problems, and neurological disorders. Infants, young children, pregnant women, the elderly, and people whose immune systems are compromised because of AIDS, chemotherapy, or transplant medications, may be especially susceptible to illness from some contaminants.

To see a list of The Top 10 Causes of Waterborne Outbreaks in Public Water Systems go to CDC's Drinking Water Site.