Private well water dataset details

Interpreting the data

What these data tell us:
  • These data can be used to identify measures of arsenic in private drinking water wells.
  • These data represent three metrics for arsenic in drinking water from private wells: the mean (average) measurement of arsenic in private well water, the maximum amount of arsenic measured in private well water and the percent of test results for arsenic over the Environmental Protection Agency’s Maximum Contaminant Level (MCL). The MCL for arsenic is 10 µg/L.
  • These data show the prevalence of arsenic levels above the MCL.
  • These data are aggregated from various data sets and include data from 1990 – 2013.​
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 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 homes 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 arsenic 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 large, 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 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 private drinking water wells. They do not give us any information about drinking water from community or non-community public water systems. Levels of arsenic are likely to be higher in private drinking water wells.
Estimates of the number of people served by a private well might be inaccurate. These estimates are not recorded in the data compiled for this project and were not found in any of the data sets collected. While it is assumed that typically one well serves only one household, and may serve on average 3- 4 people. We do know that in some cases a single well may serve multiple homes or a small housing development where fewer than 25 people consume the water for no more than 60 days of the year. At this point, the Safe Drinking Water Act is triggered and the well would fall under regulatory requirements as a public water system.

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 three measures: the county level mean concentration of arsenic measured in private well water. the county level maximum concentration of arsenic measured in private well waterand the percent of private wells tested 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 of arsenic?
There is no regulatory level established for arsenic in private wells. A good guideline to consider, however, would be 10 µg/L, the maximum contaminant level (MCL) set by the U.S. Environmental Protection Agency (EPA). A test result equal to or higher than 10 µg/L in drinking water from a private well 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.
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. 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.
Dataset information
Data Set Information: The indicator is comprised of three measures: mean concentration of arsenic measured in private well water, the max concentration of arsenic measured in private well water, the percent of wells tested that had an arsenic level above the Environmental Protection Agency’s Maximum Contaminant Level (MCL). The current MCL for arsenic is 10 µg/L.

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.
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