Public drinking water dataset details

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Interpreting the data

What these data tell us:
  • These data can be used to identify measures of arsenic in community drinking water systems.
  • These data represent two metrics for arsenic in drinking water: the mean (average) measurement of arsenic in drinking water for the year, and the maximum amount of arsenic measured in drinking water for the year.
​What these data do not tell us:
  • These data do not give us measures of actual exposure to arsenic for individuals. How much arsenic in water an individual is exposed to depends on many things, including how much tap water a person drinks each day, if a water filtration system is used in the home and consumption of water outside of the home.
  • The value measured for arsenic when the water system is tested may not be an accurate measure of the average value present in a person’s home tap water throughout the year. Water distribution systems are often large and complex, and changing conditions at the water source and throughout the water distribution system can change the level of contaminants in tap water.
  • Comparison of water quality data to health measures is done at an aggregate level, and just because events occur in the same geographic area does not mean one must cause the other for each individual person. Elevated rates of certain health effects in areas with potentially higher exposure to arsenic in drinking water do not necessarily mean the arsenic is causing that health outcome. There may be different factors contributing to the health of different individuals.
Limitations of the data
These data only give us information on community water systems. They do not give us any information about drinking water from private wells or public water systems that are not used the whole year. Levels of arsenic are likely to be higher in private drinking water wells.

For most community water systems, arsenic samples are taken once a year or less, This may not accurately capture variations in arsenic levels, but since health concerns are due to long-term exposure infrequent testing is considered safe.
 
Estimates of the number of people served by a community water system might be inaccurate. These estimates are not updated every year. Water testing schedules are partially determined by how many people a community water system serves. The sampling regulatory framework is tiered. This means that systems are only required to update their population data as the system’s population served, typically estimated by number of service connections or customer lists, changes enough to move into the next regulatory tier. So the estimate of the number of people exposed to any particular community water system may be low.

These data provide limited ability to compare between community water systems. This is because different community water systems test for arsenic more or less often based on how many people get water from that community water system, the source of the water for that system, and if testing has detected levels of arsenic above the EPA maximum contaminant level (MCL) in the past.
 
About these measurements
Measures included This indicator is comprised of two measures: the annual mean concentration of arsenic measured in community water systems (CWSs), and the annual maximum concentration of arsenic measured in CWSs. Both these measures are presented by number of systems and by number of people served. All measures of water quality are conducted by laboratories certified by the U.S. Environmental Protection Agency (EPA).

Frequency of measurement
Community water systems that use surface water test for arsenic once a year. Those that use ground water sources test once every three years. Water systems that have received a waiver from the Colorado Department of Public Health and Environment test for arsenic once every nine years. The most recent measurement available is reported for each consecutive year until the next water test is scheduled to occur.
 
What is considered to be a high measure of arsenic?
A concentration above the maximum contaminant level (MCL) set by the U.S. Environmental Protection Agency (EPA) for arsenic in drinking water is considered to be too high, although exposure to a concentration above the MCL does not necessarily mean a person will get sick. Health concerns due to arsenic in drinking water are the result of exposure to levels above the MCL for many years.
 
The EPA’s MCL for arsenic in drinking water was recently lowered to 10 µg/L, which is the same as 10 parts per billion (10ppb). CWSs that get their water from surface water started meeting this MCL standard in 2006, and CWSs that get their water from groundwater started meeting this standard in 2007. Before then, the MCL for arsenic in drinking water was 50 µg/L.
 
Calculation methods
When testing did not detect any arsenic, a value of half the detection limit was used as the value of the annual measure. It is assumed there may be some level of arsenic in the water sample even if the amount is so low it cannot be detected by the test.
 
Because of wide variability in arsenic sampling schedules and reduced sampling frequency in systems with low detection values, arsenic data was carried over from the last year that testing was done. Missing annual means and maximums were calculated from the latest day of sampling of the most proximate prior year that arsenic sampling data was available.
 
Dataset information
The indicator is comprised of two measures: annual mean concentration of arsenic measured in community water systems (CWSs) and the annual maximum concentration of arsenic measured in CWSs. These measures are displayed by number of water systems and by number of people served. The maximum contaminant level (MCL) for arsenic is 10 µg/L. Before 2006, the MCL for arsenic was 50 µg/L.
Data prepared by: Environmental Programs, Colorado Department of Public Health and Environment
Data source: Safe Drinking Water Information System (SDWIS-State) database. The SDWIS-State database is maintained by the Colorado Department of Public Health and Environment.
 
Metadata