A drinking water supply comes from one of two sources. Urban areas usually get drinking water from surface water such as rivers, lakes, reservoirs or streams. In this case, it is necessary to consider the land area over which the water flows. For this reason, community water suppliers are required to provide consumers with a report each year describing the source of the drinking water, any contaminants found in the water, as well as how the water is treated. Surface water is usually pumped to storage reservoirs where it is treated to make it safe for drinking. The water is first filtered to isolate harmful bacteria and remove sludge and other residual matter and contaminants. Once the pH is adjusted as needed, chemicals are added to disinfect the water and kill bacteria and viruses. Many community water suppliers also chlorinate and fluoridate the water before distributing it to homes through pipelines. The U.S. Environmental Protection Agency (EPA) regulates the standards for the safety and quality of local drinking water supplies (see Resources).
People who live in rural areas typically get drinking water pumped to the surface from a well. While most public water systems in this country get drinking water from surface water sources, there are still many rural communities that rely on groundwater for a clean supply of drinking water. Just like for surface sources of drinking water, any above ground industries, agricultural activities, hazardous waste disposal or other environmental conditions have the potential to affect the quality of groundwater. Even though groundwater for drinking is not regulated by the EPA, the agency does make certain recommendations. Water pumped from underground wells generally contains fewer organic materials than surface water, and therefore, requires less treatment. Groundwater is filtered naturally as it passes through several layers of the earth on its course underground. However, people who get their drinking water from private wells should have the water chlorinated at least annually. This disinfects the water and kills coliform bacteria. The pH of well water should be routinely tested, as a low pH increases the risk of lead and other harmful metals dissolving in the water. Certain types of bacteria also thrive in water with a low pH. Well water may need to be treated if the pH level is too acidic.
Both groundwater and surface water initially come from precipitation in the form of rain or snow. It helps to understand that there is a limited amount of water on the planet, and that water is used over and over again. This hydrologic cycle represents the continuous movement of water evaporated from the oceans into the air. The sun heats the sources of surface water, causing the water to evaporate. Once the water vapour rises into the earth's atmosphere, it cools and condenses into liquid droplets. These droplets continue to combine with each other until they get too heavy. The water then falls back to the earth in the form of precipitation, seeping into rock and soil, replenishing the lakes, streams and rivers, which eventually flow back into the oceans where the process begins once more.