Summary of the Study Results
The United States Geological Survey (USGS), in cooperation with Massachusetts Department of Environmental Protection (MassDEP) and Massachusetts Department of Public Health (MDPH,) investigated the presence of arsenic and uranium in private bedrock water supply wells in a study area located in east central and northeastern Massachusetts. The study area was selected because existing public bedrock well records indicate that high arsenic concentrations occur in this area. Less is known about the distribution of uranium in bedrock well water. The study included uranium analysis in order to obtain a better understanding of its relationship with types of bedrock in Massachusetts.
The USGS investigation studied water samples that were collected from 478 private bedrock drinking water wells in 116 communities. These samples were distributed among the multiple bedrock geologic units that are mapped in the study area. The investigation found correlations between the bedrock geology and the concentrations of arsenic and uranium. However, there was also a strong correlation between arsenic concentrations and the well's proximity to the Clinton-Newbury fault zone that was not dependent upon the type of bedrock. Specifically, there was a higher probability arsenic zone that extended west of this fault zone. 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 are expected to exceed the arsenic standard by multiplying the probabilities obtained from the samples results by the estimated number of wells in the area. This included the use of the probability statistics for a high probability zone to the west of the Clinton-Newbury fault zone and the probability statistics for the individual bedrock geology units for areas outside of this high probability zone.
The predicted number of wells exceeding the uranium standard were calculated in a similar manner, except there was no high uranium probability zone that was independent of bedrock geology as there was for arsenic. Based upon the results of this investigation, the USGS predicts the following within this study area:
- approximately 5,700 residential bedrock water supply wells out of 90,000 may have arsenic concentrations that exceed 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 that exceed the public drinking water standard of 0.030 milligrams per liter (mg/L) and approximately 3,000 of these wells are being used without treatment for the removal of uranium.
Based upon information obtained from public water supply well 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 public water supply sampling points is not sufficient enough for us to know for certain whether or not there are some areas not yet identified where the bedrock wells may have an increased probability of exceeding the arsenic standard.
Based upon the results of this study and information obtained from public water supply well records and other investigations on the occurrence of uranium, 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 of the study area.
See Additional Resources USGS Scientific Investigations Report 2011-5013 for additional information regarding the investigation methodologies and results.
It should be noted that there are reliable methods to test and treat water to remove both arsenic and uranium contamination. 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 supplied with the necessary water treatment equipment.
See Additional Resources Town Maps of Arsenic and Uranium Probability Zones, below. Links to maps for all of the towns that were either completely or partially located within the USGS study area for the 2011 report. Includes towns that were outside the study area but for which probability data is available or which have types of bedrock that may have an increased probability for containing radionuclide contaminants such as uranium.