Summary of the Study
Find the study here: https://pubs.usgs.gov/sir/2011/5013/
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.
Maps of Arsenic and Uranium Probability Zones
These maps include statewide overviews for arsenic and uranium, and a listing of all of the towns that were either completely or partially located within the study area.
Statewide overview maps
Probability of Exceeding the Arsenic Drinking Water Standard in Private Drinking Water Wells in Massachusetts - Map
Probability of Exceeding the Uranium Drinking Water Standard in Private Drinking Water Wells in Massachusetts - Map
Detailed town maps
The town listing includes towns that were outside the study area but include bedrock units that were in the study area and for which probability information is available. The listing also includes towns that are mapped as areas of granite or pegmatite bedrock units that were not included in the USGS study area, but may have an increased probability for containing radionuclide contaminants such as radon, radium and uranium.
Please note that not all of the towns included in the list have increased probabilities of having arsenic and radionuclide contamination in private wells. Those towns marked with an asterisk were not identified with bedrock units having an increased probability of containing water with naturally occurring arsenic or uranium levels exceeding public drinking water standards. Furthermore, there are many towns on this list that are either mostly or entirely served by public water supplies.
To review the town maps, open the PDF file below and click on the links next to your town to obtain a PDF map of the arsenic or uranium probability zones for the town. If your town is not included in the list, then there is no arsenic and uranium probability information available for your town. Please note that for many of these towns there are some areas for which arsenic or uranium probability information is not available.
Note: Information in these maps only applies to bedrock aquifers. If your well is located in a sand or sand and gravel aquifer, this information does not apply. Most, if not all, drinking water wells installed in the Plymouth-Carver Aquifer and on Cape Cod, Martha's Vineyard, and Nantucket are completed in sand or sand and gravel deposits rather than bedrock. Therefore, these maps do not apply to those areas.
Detailed Town Maps Showing the Probability of Exceeding the Arsenic and/or the Uranium Drinking Water Standards in Private Drinking Water Wells in Massachusetts
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.
- Standards & Guidelines for Contaminants in Massachusetts Drinking Water
- UMass Extension Service Well-water Fact Sheets
- Dartmouth Toxic Metals Research Program YouTube Video on Arsenic in Drinking Water
- US EPA: Private Drinking Water Well Information
- US EPA: Arsenic Rule
- US EPA: Radionuclides Rule
- US EPA: Laboratory Study on Arsenic Oxidation
- USDA Rural Development Program
- CDC Case Studies in Environmental Medicine