About the drinking water quality data

The following web page provides general information about drinking water quality data and measures developed by the Minnesota Environmental Public Health Tracking Program. For more information about these data, contact Minnesota Public Health Data Access.

What do these data tell us?

Community Water Systems:

  • The annual mean (average) and maximum concentrations of arsenic, nitrate, radium, uranium and disinfection byproducts (DBPs) for community water systems in Minnesota.
  • If the concentrations of arsenic, nitrates, radium, uranium and DBPs are increasing or decreasing over time.

Private Wells:

  • The number of new private wells constructed and sampled for arsenic from August 2008 through February 2016.
  • The percent of new private wells with arsenic concentrations above 2 μg/L and 10 μg/L (micrograms per liter), by county, the median arsenic concentration, and the 95th percentile arsenic concentration (the value below which 95 percent of test results are found).
    • Cutpoints of 2 μg/L and 10 μg/L were chosen because 2 μg/L is a common limit of detection for Minnesota Department of Health (MDH)-certified laboratories, and 10 μg/L is the federal Maximum Contaminant Level for community water systems.
    • There is no enforceable health-based standard for arsenic in private wells. MDH recommends that people don't use water with more than 10 μg/L of arsenic for drinking or cooking. Levels below 10 μg/L may also increase health risks, depending on several factors, such as the type of arsenic, the concentration of arsenic, the amount and duration of arsenic exposure, and individual susceptibility. For questions or additional information about the health risks from arsenic in private well water, contact the MDH Well Management Program. 

How can we use these data?

  • The data provides summary information on arsenic, nitrate, radium, uranium and disinfection byproduct concentrations in drinking water from community water systems, and arsenic in new private wells, in Minnesota.
  • State and local partners can use the data for program planning and evaluation.

What can these data not tell us?

  • The origins of entry for contaminants in drinking water.
  • The levels of arsenic, nitrate, radium, uranium and DBPs, in individual community water systems.
  • Information about water quality in water sources other than community water systems and private wells.
  • The levels of arsenic in individual private wells or in private wells that have not been tested.

What is the source of the data?

Community Water Systems:

Private Wells:

  • Water samples are collected from all new drinking water wells by the well contractor. Results are submitted to the MDH Well Management Program of the Minnesota Department of Health, as required by Minnesota Rules. Information about geology, including the outline for the Des Moines lobe till, was adapted by MDH from the University of Minnesota.

Why are the data focused on arsenic, nitrate, radium, uranium and disinfection byproducts?

How were non-detects, the levels below lab testing limits, handled in the data set?

Community Water Systems:

  • Sample results with concentrations less than the detection limit have been replaced with a value equal to 1/2 the detection limit for the analytical method used.
  • No sensitivity analyses were conducted to characterize how the approaches to handling non-detects may impact the drinking water quality data; therefore, it is possible that the analysis may produce biased estimates of the mean - especially if a large proportion of samples are below the limit of detection for the analytical method..

Private Wells:

  • Almost all arsenic sample results had laboratory reporting limits of 2 μg/L or less. To assure data quality, thirty-two non-detects with laboratory reporting limits larger than 2 μg/L were not included in any maps or charts. There was no need to assign a numerical value to non-detects, since statistics such as average concentrations were not displayed. 

What are the limitations of the data?

Community Water Systems:

  • These data are limited to community water systems in Minnesota (i.e., systems that serve 15 or more locations or 25 or more people, year-round in Minnesota.
  • Concentrations in drinking water systems cannot be directly linked to exposures and health risks, because they do not account for water use and consumption practices. In addition, people may obtain drinking water from sources other than community water systems (e.g., private wells, bottled water, workplace, school). Concentrations may not reflect levels in finished drinking water or water at the consumer's tap.
  • The current form of the data and measures contain one record for each active community water system per year (i.e., active at the time of data analysis). Sampling results from inactive systems that were active prior to data analysis are not included in the data and measures. Early missing values may be because the water system was not yet active, or the system was not required to take a compliance sample during that particular time period. The data set does not distinguish between these two scenarios.
  • Trends can be difficult to interpret if water quality standards and treatment requirements change over time.
  • Presenting average concentrations by calendar year is not usually consistent with how maximum contaminant levels (MCLs) established by US Environmental Protection Agency (EPA) are calculated (i.e., typically based on running averages).
  • The accuracy of the population served information is unknown. Water systems derive population estimates using census estimates, number of water connections, or other means. Furthermore, system population is a current estimate, so population-based estimates for past years may be inaccurate (likely over-estimated). Historical information on population served is not available.
  • Systems may receive a sampling waiver from the state for reduced frequency of monitoring based on low analytical results. Reduced monitoring frequencies lead to a reduced number of samples. If a system was not required to sample within a calendar year, the most recent available sampling result was carried forward.

Private Wells:

  • These data are limited to new private wells constructed in or after August 2008. Therefore, these data are not representative of all private wells in the State.
  • County-wide test results and the geology overlay are useful tools for identifying areas of the state where arsenic values may be high. However, they cannot be used to accurately predict arsenic levels for individual wells. The only way to know the arsenic concentration in a well is to have the well water tested. 
  • Results were flagged as unstable for two counties, Lincoln and Kittson, which had less than 20 data points. Values based on such a small sample size are considered unstable.
  • Reported concentrations in private wells cannot be directly linked to exposures and health risks, because they do not account for water use and consumption practices. In addition, people may obtain drinking water from other sources (e.g., bottled water, workplace, school).
  • Each well is represented by only a single sample, so the data cannot be used to assess well-specific trends in water quality over time.

  • Samples are collected by licensed well contractors and analyzed by several state-certified private and public laboratories without direct oversight by the MDH. There is likely considerable variability in sampling methods and some variability in laboratory analytical methods, which could affect sample results.

Where can I find more technical information about the data?