Public drinking water dataset details


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 first metric is the mean (average) measurement of arsenic in drinking water for the year. The second metric is 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. Exposure depends on how much tap water a person drinks inside and outside of the home. In addition, exposure depends on if a water filtration system is used.

  • The arsenic value measured may not be an accurate measure of the average value present in someone's tap water. A change in the level of contaminates in tap water may be due to the following:

    • Water distribution systems are often large and complex.

    • Changing conditions at the water source and through the distribution system. 

  • Comparison of water quality data to health measures is done at an aggregate level. Areas with higher exposure to arsenic does not necessarily mean that there will be higher rates for certain health effects. Different factors, such as genetics, can contribute to the health of an individual.

Limitations of the data

These data only give us information on community water systems. They do not give us information on private well water or public water systems that aren't 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. However, because health effects may be 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. 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. The reasons are listed below:

  • The frequency of testing is often based on how many people get water from that community water system.
  • The source of the water for that system.
  • If testing has detected levels of arsenic above the U.S. Environmental Protection Agency's (EPA) maximum contaminant level (MCL) previously.
About these measurements

This indicator is comprised of two measures.

  1. The annual mean concentration of arsenic measured in community water systems (CWSs).
  2. 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 CDPHE 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 MCL set by the EPA for arsenic in drinking water is considered to be too high. However, exposure to a concentration above the MCL does not necessarily mean a person will get sick. Health concerns due to arsenic 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. This value is the same as 10 parts per billion (10ppb). CWSs that got their water from surface water started meeting this MCL standard in 2006. CWSs that got 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 annual measure. It's assumed there may be some level of arsenic in the water sample even if the amount cannot be detected.

Arsenic data was carried over from the last year that testing was done due to schedules and reduced sampling frequency. Missing annual means and maximums were calculated from the latest day of sampling of the most proximate prior sampling year.

Dataset information

The indicator is comprised of two measures.

  1. The annual mean concentration of arsenic measured in CWSs.
  2. 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 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 CDPHE.


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