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Private Wells

This page provides information about the private well data and measures developed by the Minnesota Environmental Public Health Tracking (MN EPHT) Program. These include:

  • Occurrence of homes on private wells
  • Homes on private wells at increased risk of nitrate contamination

For more information, contact us.

Data Sources

Occurrence of homes on private wells

The Minnesota Department of Revenue provided MDH with a dataset of residential properties from their Property Record Information System of Minnesota data system (PRISM). The dataset contained street address and property type for all residential properties in Minnesota through 2022. The following residential property types were included:

  • Residential Agricultural Homestead,
  • Residential Agricultural - NonHomestead,
  • Manufactured Home Park,
  • Residential - Homestead,
  • Residential - NonHomestead,
  • Seasonal Residential Recreational – NonCommercial,
  • Seasonal Residential Recreational – Commercial Homesteaded Resorts.

The addresses in the PRISM dataset were then geocoded; meaning, residential street addresses were converted into geographic coordinates. Next, estimated Community Water System service areas, provided by the MDH Drinking Water Protection Program as boundary polygons in shapefile format, were overlaid on the geolocated residential properties. Residential properties outside of the community water system service area boundaries were assumed to get drinking water from a private well.

Once residential properties on private wells were identified, the total number within each census tract and county were summed, as well as the total number by property type (primary residence versus seasonal property). The denominator in the percent of homes on private wells calculation is the total number of residential properties in the county or census tract identified through PRISM.

Number of homes on private wells at increased risk of nitrate contamination

Each residential property on a private well, identified through the process described above, was assigned a nitrate risk category based on conditions within the quarter section of land in which it is located. Quarter sections are ¼ mile by ¼ mile, or 160 acres and their boundaries are publicly available. Nitrate risk classifications were based on the percent of row crop agriculture and the percent of geologically vulnerable land in the residential property's quarter section:

  • Lower risk: Less than 30% of the quarter section is geologically vulnerable to groundwater contamination and less than 20% is in row crop production.

  • Increased risk:

    • At least 30% of the quarter section is geologically vulnerable to groundwater contamination and at least 20% is in row crop production, OR
    • The residential property's quarter section intersects a township where at least 10% of private wells were found to have nitrate concentrations above the federal drinking water standard of 10 mg/L in the Township Testing Program. MDA tested townships from 2013-2019.

Use the query tool or download the data to see the number of homes in these two increased risk categories by census tract and county.

Row crop production areas were identified from the U.S. Department of Agriculture’s 2022 crop-specific land cover data for Minnesota. The following row crops or crops that have a high likelihood of going into a corn/soybean rotation were included: alfalfa, corn (all types), dry beans, oats, potatoes, soybeans, sugarbeet, and wheat. The Minnesota Department of Agriculture (MDA) recommended the percent row crop production and geologic vulnerability thresholds used to define the risk categories, as they were found to be optimal at identifying areas with potentially elevated nitrate levels in private well water in MDA's Township Testing Program.

Geologic sensitivity was based on pollution sensitivity of near-surface materials, which is provided by the Minnesota Department of Natural Resources in a geospatial database. Pollution sensitivity of near surface materials is defined as the transmission time of water through three feet of soil and seven feet of surficial geology, to a depth of ten feet from the land surface. The following pollution sensitivity classifications were considered vulnerable: karst, peatlands, disturbed lands, and high (≤170 hours) or moderate (>170 to 430 hours) transmission time.

The denominator in the percent of homes on private wells at increased risk calculation is the total number of residential properties in the county or census tract on private wells.

Nitrate testing in Southeast Minnesota Wells

Nitrate testing data is from Southeastern Minnesota Water Analysis Laboratory from 2016-2023.

Data Questions

No comprehensive state database of homes served by private wells exists. There is a publicly available database maintained by the Minnesota Geological Survey and MDH called the County Well Index (CWI) . CWI contains basic information on Minnesota wells and borings (e.g., well depth, geologic materials encountered during drilling). While CWI is the most comprehensive database of water well information in the state, it only contains information on wells for which a well contractor report is available. Most of the records in CWI are for wells drilled after 1974, when the well construction code first required contractors to submit records for wells drilled in Minnesota to MDH. There were many existing wells constructed prior to the well code. Further, many of the early records did not have sufficient information to accurately identify the location of the well. Because CWI is incomplete, a different approach, described above, was taken to identify homes on private wells.

Occurrence of homes on private wells

The data provide information about where residential properties on private wells are located in Minnesota, specifically,

  • The estimated number of residential properties that rely on private wells for drinking water within each Minnesota census tract and county.

  • The estimated percent of residential properties that rely on private wells for drinking water within each Minnesota census tract and county.

  • The estimated number and percent of primary residence residential properties on private wells within each Minnesota census tract and county. People drink more well water for longer periods of time at their primary residence compared to seasonal or vacation homes.

Number of homes on private wells at increased risk of nitrate water contamination

For each census tract and county, the data provide the estimated number and percent of homes on private wells at increased risk of nitrate water contamination.

Nitrate testing in Southeast Minnesota Wells

The maps display data from testing done in SE Minesota by Southeastern Minnesota Water Analysis Laboratory. The map shows apparent differences in nitrate occurrence, however it is important to note that nitrate levels can vary significantly between wells due to a variety of factors including, but not limited to: well depth, age, and location. MDH recommends that all private well users test for nitrate yearly.

Occurrence of homes on private wells

  • State and local partners can use the data for program planning and evaluation, particularly programs focused on groundwater protection and those that conduct outreach, education, and other services for private well users.

  • The general public can use this information to better understand residential drinking water sources in Minnesota and in their own area.

Number of homes on private wells at increased risk of nitrate contamination

State and local partners can use the data to plan and target programs that:

  • monitor water quality in private wells

  • provide support to private well users, such as education and outreach programs or well testing clinics.

Other data users, such as policy makers and researchers can use these data to inform their planning and analyses. The general public can use this information to better understand the potential impact of nitrate contamination throughout the state and in their own area.

It is important to remember that all private wells should be tested for nitrate every year regardless of the number of homes on private wells identified to be at increased risk of nitrate contamination in an area.

Nitrate testing in Southeast Minnesota Wells

State and local partners can use the data to understand the nitrate occurrences within their own county and surrounding counties and use that information to encourage private well testing.

Occurrence of homes on private wells

  • The data cannot tell us if people living in residential properties served by private wells are drinking untreated well water. People may use water treatment devices or get water from another source, such as bottled water.

  • Estimates of the number and percent of residential properties on private wells do not directly translate to the number and percent of households or people served by private wells. For example, a fraction of residential properties may be vacant.

Number of homes on private wells at increased risk of nitrate contamination

  • Increased risk resulting from proximity to potential point sources of nitrate contamination such as animal feedlots or faulty septic systems were not considered. While elevated levels of nitrate can originate from multiple sources, MDA has identified agricultural fertilizer use as the major contributor to nitrate contamination of groundwater in Minnesota.

  • The data cannot tell us about the increased risk of nitrate contamination resulting from improperly sited, poorly constructed, or damaged wells. Wells constructed before the 1974 state well code may be more likely to be improperly located or constructed.

  • The data do not tell us if people living in homes on private wells are drinking untreated well water. People may use water treatment devices that remove nitrate or get water from another source, such as bottled water.

Nitrate testing in Southeast Minnesota Wells

  • The data do not tell us if people living in homes on private wells are drinking untreated well water. People may use water treatment devices that remove nitrate or get water from another source, such as bottled water.

Due to the limitations described below, the data should be considered estimates.

Occurrence of homes on private wells

Several issues may result in either an over- or under count of residential properties on private wells.

  • Community Water System service area boundaries may be inaccurate. Boundaries for Minnesota’s rural water systems are considered the least reliable. MDH’s Drinking Water Protection Program continues to refine the estimated boundaries.

  • Some addresses may not geocode correctly. Data quality checks found a ≤0.5% error rate between the county identified by geocoding residential properties on private wells and the county listed in the PRISM dataset, with the majority of mismatched properties falling along the county border. A small number of residential property addresses could not be geocoded, and a subset of these properties presumably get drinking water from a private well. Geocoding problems may occur for different reasons such as address errors in the source data or incomplete geocoding coverage in parts of Minnesota’s unorganized territory.

  • Seasonal properties were presumed to be served by a private well if located outside community water system service area boundaries. However, some seasonal properties may not have a source of drinking water. This would overestimate the total number of homes on wells. Properties not classified as seasonal were presumed to be primary residences, but this may not always be the case.

  • Since the method relies on state property tax records, any homes on private wells within tribal lands are not included since they are typically not subject to state property tax. An option to view tribal land boundaries is included in the maps.

  • There may be residential properties within community water system service areas that are not yet hooked up to public water and still get drinking water from a private well.

  • A 30-foot buffer was added around community water system service area polygons to capture residential properties at the very edge of the service area; i.e., residential properties on the other side of street. This may incorrectly classify properties on private wells as connected to a community water system.

Number of homes on private wells at increased risk of nitrate contamination

  • Land cover data to identify row crops reflect one point in time and do not account for subsequent changes in land use.

  • The vulnerability of the underlying geology is based on vertical migration of nitrate to groundwater. Nitrate contamination from the horizontal flow of groundwater within specific aquifers was not considered.

  • Private well homes may be located close to or along a quarter section’s border with an adjacent quarter section that has a different risk classification.

  • While nitrate in groundwater can originate from multiple sources, the "lower" and "higher" nitrate risk classifications are based on potential nitrogen fertilizer impacts from row crop agriculture. Proximity to point sources of nitrate was not considered.

  • The increased risk classification based on initial results from the MDA Township Testing Program represent the percent of tested wells that exceeded the federal drinking water standard for nitrate regardless of the potential source of nitrate or condition of the well. While the township testing program offered testing in 344 vulnerable townships from 50 counties across Minnesota, not all townships were tested, as this program was focused on areas most vulnerable to nitrate contamination.

For these and other reasons, all private wells should be tested for nitrate annually regardless of the risk estimate shown for a particular county or census tract.

Nitrate testing in Southeast Minnesota Wells

Limitations of the Nitrate testing in Southeast Minnesota data include:

  • Test results from a single lab: SEMWAL

  • Test results from 2016-2023

  • Map does not show individual test results.

  • Not every private well in the county or census tract has been tested for nitrate, nor is every well’s nitrate test result represented.

  • This data cannot definitively tell a well owner if their well water may have elevated nitrate.

For these and other reasons, all private wells should be tested for nitrate annually, regardless of rates shown for a particular county or census tract.