Summary of the Study
Find the study here: Arsenic and Uranium in Water from Private Wells Completed in Bedrock of East-Central Massachusetts—Concentrations, Correlations with Bedrock Units, and Estimated Probability Maps (PDF).
The investigation studied water samples collected from 478 private drinking water wells in 116 communities. These wells were drilled in bedrock. The investigation found correlations between the bedrock geology and concentrations of arsenic and uranium. However, there was also a strong correlation between arsenic concentrations and proximity to the Clinton-Newbury fault zone that was not dependent on the type of bedrock. The Clinton-Newbury fault zone extends from near the eastern end of the Massachusetts and Connecticut border in the south to the Merrimack River Valley in the northeast.
The USGS used the results of the sampling to predict the number of wells in the study area that might exceed the arsenic standard. This included the use of probability statistics for a zone to the west of the Clinton-Newbury fault zone and probability statistics for individual bedrock geology units for areas outside of this zone.
The predicted number of wells exceeding the uranium standard were calculated in a similar manner, except that there was no high-probability zone for uranium independent of bedrock geology, as there was for arsenic. Based on the results of this investigation, the USGS predicted the following within this study area:
- approximately 5,700 residential bedrock water supply wells out of 90,000 may have arsenic concentrations exceeding the public drinking water standard of 0.010 milligrams per liter (mg/L), and approximately 3,800 of these wells are being used without treatment for the removal of arsenic.
- approximately 3,300 residential bedrock water supply wells out of 90,000 may have uranium concentrations exceeding the public drinking water standard of 0.030 mg/L, and approximately 3,000 of these wells are being used without treatment for the removal of uranium.
Based on information obtained from public water supply records, the USGS and MassDEP believe that the study area includes most of the areas of high arsenic within Massachusetts. However, the distribution of those sampling points is not sufficient enough for us to know for certain whether or not there are some areas where the bedrock wells may have an increased probability of exceeding the arsenic standard.
Based on the results of this study and information obtained from public water supply records and other investigations, MassDEP believes that there are many areas located outside of the study area in which bedrock well water has an increased probability of exceeding the uranium standard and other radiological standards such as radon and radium. In general, areas in Massachusetts where the bedrock is mapped as granite or pegmatites have an increased probability of exceeding the drinking water standard for uranium and other radiological contaminants. There may be other types of bedrock or bedrock formations that may contain unacceptable concentrations of uranium and other radiological contaminants, but there is insufficient water quality data to assess the bedrock formations in areas outside the study area.
There are reliable methods to test and treat water to remove both arsenic and uranium. A drinking-water-supply well that has increased concentrations of arsenic and uranium may still be used as a safe and reliable source of drinking water if the necessary water treatment equipment is installed.
Additional Resources
Maps of Arsenic and Uranium Probability Zones
Statewide overview maps
Detailed town maps
All PDF maps of the arsenic or uranium probability zones by town have been archived. They are being replaced by an accessible GIS application which will be online in late 2026.
In the interim, if you would like to request a copy of a town PDF map, please contact program.director-dwp@mass.gov.
Additional Resources
Find a testing lab
Note to private well owners:
Labs provide different services. See the spreadsheet linked below for a list of labs currently certified to test for arsenic and/or uranium. Some may be able to arrange testing for both arsenic and uranium. Other labs may not accept samples from the public. Check with the lab before sending samples.
Important notes on arsenic:
Inorganic arsenic occurs naturally as arsenic 3 and arsenic 5. Arsenic 3 is much more difficult to remove from water than arsenic 5. If the total arsenic concentration in your well water exceeds the drinking water standard, you should consider having a sample tested to identify the type of arsenic in order to select a treatment option. This testing is called “arsenic speciation.”
For the laboratories that have indicated that they offer arsenic speciation testing services, some offer field filtration kits for the homeowner to use to isolate the arsenic 3 sample. Those that don't offer field filtration kits complete the filtration process in the laboratory.
Field filtration is preferable, especially if the water sample contains a significant concentration of iron. However, laboratory-filtered samples are also a useful method for determining arsenic speciation.
If you have the laboratory run the sample filtration, you should discuss with them the optimum day (or days) of the week that you should collect and deliver the sample bottle, in order to minimize the time that elapses between sample collection and sample filtration.
Lab Services: Many laboratories will offer analytical services for analytes other than those for which they are certified, by sub-contracting to other certified labs. Laboratories will typically provide sample bottles to their customers with instructions on how to collect samples and many also offer the option of sending a technician to your home to collect the samples. Check with the lab before sending samples.
Lab Certification Status: To check the current certification status of any of the laboratories listed in the spreadsheet below, use the Online Searchable Laboratory Certification Listing, enter the analyte you wish to test for (e.g., arsenic, gross alpha, radium, or uranium), select "Potable (Drinking Water)" and click Search.