Private Well Water

Many Marion County residents rely on private wells for their drinking water. The two main contaminants of concern in private wells are coliform bacteria and arsenic. The dashboard below shows the percentage of positive coliform bacteria tests and the percentage of arsenic tests above 10 parts per billion (ppb) by ZIP Code.

Use the buttons at the bottom of the dashboard to switch between arsenic and bacteria. Hover over the map, click the bar chart, or select from the list to the left to see values for specific zip codes.

Green represents lower percentages while blue represents higher percentages. Gray zip codes are excluded due to not having enough data. 

Use the button at the bottom right corner of the dashboard to switch between arsenic and bacteria.

Bacteria

One contaminant of concern that is often found in well water samples is bacteria. One group of bacteria called coliform bacteria, also referred to as coliform, can enter well water from multiple sources such as animal feces, compromised septic systems, or runoff from feedlots, pastures, and woodlands. Many types of bacteria found in well water are harmless, but some, like E. coli, can cause illness or death. Bacteria in private well water is usually evaluated simply on the basis of whether it is present or not. If a well tests positive for coliform bacteria, it does not necessarily mean that harmful bacteria is present, but treatment is recommended to be on the safe side. A positive bacteria test means that there is a contamination pathway present that could lead to harmful bacteria in the water. Wells are more likely to test positive for bacteria during warmer and wetter times of the year. In Marion County, 41% of coliform tests conducted were positive. In comparison, it is estimated that 35% of private wells in the United States are contaminated with coliform at any given time. 

Source: Coliform Bacteria. (2025, September 08). Penn State Extension. Retrieved from https://extension.psu.edu/coliform-bacteria 

Arsenic

Another contaminant of concern that is often found in well water samples is arsenic. This naturally occurring contaminant is found in the soil and impacts groundwater more than surface water. Exposure is associated with cancer, preterm births, metabolic diseases, and other chronic health conditions. Arsenic is tasteless, colorless, and odorless. The only way to know if your well is contaminated is to have it tested. Arsenic in water is measured using micrograms per liter (μg/L) and parts per billion (ppb), which are the same unit of measurement. The maximum level of arsenic allowed in public drinking water is 10 ppb. Private well water is not regulated, but this standard is used to assess its safety. Of arsenic tests conducted between 2020 and 2024 by MCPHD, 42% were found to contain arsenic and 14% were over the 10 ppb limit.

Source: Arsenic. (2025, August 06). National Institute of Environmental Health Sciences. Retrieved from www.niehs.nih.gov/health/topics/agents/arsenic

Resources

About the Data

  • Although water quality is generally not enforced for private well owners, MCPHD uses water quality standards set by the Environmental Protection Agency (EPA) for safety guidance. Visit www.epa.gov/dwreginfo/drinking-water-regulations to learn more about drinking water regulations.

  • MCPHD regulates construction of private wells and well pumps through local and state ordinances. Environmental Health Specialists also conduct neighborhood surveys, educate well owners, and provide well water sampling at no cost to well owners. Environmental Health Specialists will provide a free water quality report and may offer guidance and recommendations. MCPHD does NOT condemn wells and does not require homeowners to install water treatment systems. Rental properties are required to meet water quality standards.

  • The total number of wells in Marion County is unknown, which can affect certain data values. For this reason, information is presented using the number of wells tested instead of the total number of wells.

  • The distribution of wells in Marion County is uneven. Some areas are made up almost entirely of homes supplied by well water while others contain few or no homes supplied by well water.

  • This data only includes testing conducted by MCPHD. Testing conducted by private contractors is not included.

  • ZIP Codes with fewer than five tests are excluded to maintain the accuracy of our estimates.

  • Data is displayed over a five-year period so that enough testing data is available to create an accurate assessment of contamination.