Interpreting the data
What these data tell us:
- These data can be used to identify measures of arsenic, nitrate, uranium, and coliform bacteria in private drinking water wells.
- These data represent two metrics for each contaminant in drinking water from private wells: the maximum levels of each contaminant measured in private well water and the percent of test results for each contaminant over the Environmental Protection Agency’s Maximum Contaminant Level (MCL).
- These data show the prevalence of contaminant levels above the MCL.
- These data are aggregated from various data sets and include data from 2011 to 2020.
What these data do not tell us:
- These data do not give us measures of actual exposure to contaminants for individuals. How much of these contaminants an individual is exposed to through private well water can depend 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.
- These data do not tell us where in the home's water supply system the sample was taken from. It may have been taken directly from the well, a kitchen sink, or before or after a filtration system, for example.
- The value measured for these contaminants when the well is tested may not be an accurate measure of the average value present in a person’s home tap water throughout the year. Underground aquifers are often complex, and changing conditions, including the amount of water in the aquifer, can change the level of contaminants in tap water.
- Comparison of these water quality data to health measures can only be done at an aggregate level, so because events occur in the same geographic area does not mean one must cause the other at an individual level. Elevated rates of certain health effects in areas with potentially higher exposure to these contaminants in drinking water do not indicate that exposure to these contaminants is causing harmful health impacts. There may be different factors contributing to the health of different individuals.
Limitations of the data
These data do not come from a comprehensive survey, and may not be representative of the contaminant levels in other private drinking water wells. These results cannot be used as a substitute for individual well testing.
These data only give us information on private drinking water wells. They do not give us any information about drinking water from community or non-community public water systems.
These data provide limited ability to compare private wells. This is because different characteristics of the well are important in determining the well water quality including, depth, the level at which the well is screened, the amount of water in the aquifer, season, the amount of water produced by the well, the aquifer the well is in, and surrounding land uses.
About these measurements
This indicator is comprised of two measures for each contaminant: the county level maximum concentration of each contaminant measured in private well water and the percent of tests results with levels over the MCL. All measures of water quality should be conducted by laboratories certified by the U.S. Environmental Protection Agency (EPA).
What is considered to be a high measure?
There is no regulatory level established for these contaminants in private wells. A good guideline to consider is the maximum contaminant level (MCL) set by the U.S. Environmental Protection Agency (EPA) for public water systems for each contaminant. Exposure to a concentration above the MCL does not necessarily mean a person will get sick.
For the four contaminants displayed on this page, their MCLs are:
Arsenic: 10 µg/L
Uranium: 30 µg/L
Nitrate: 10 mg/L
Total Coliform Bacteria: Less than 5% of samples in a month with detectable levels of coliforms.
When testing did not detect any arsenic, uranium, or nitrate, 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 these contaminants in the water sample even if the amount is so low it cannot be detected by the test. If no detection limit was specified in the original raw data, a detection limit was selected from a list of EPA-approved methods and used throughout the entire data set.
Data Set Information: This indicator is comprised of two measures for each of arsenic, nitrate, uranium, and coliform bacteria: the county level maximum concentration of each contaminant measured in private well water and the percent of tests results with levels over the MCL.
Data prepared by: Environmental Programs, Colorado Department of Public Health and Environment
Data source: Private Well Initiative, and Environmental Public Health Tracking funds made possible the identification, collection, and analysis of numerous disparate well water quality data sets including those held at county public health departments, universities, and state laboratories.