Technical Announcement
May 3, 2011 |
Rod Caldwell |
406-457-5933 |
USGS Samples for Radioactive Constituents in Groundwater of Southwestern Montana
The U.S. Geological Survey (USGS), in cooperation with Jefferson County, recently sampled groundwater in southwestern Montana for uranium and other radioactive constituents. Scientists with the USGS Montana Water Science Center collected 165 groundwater samples from 128 wells in Broadwater, Deer Lodge, Jefferson, Lewis and Clark, Madison, Powell, and Silver Bow Counties. Most of the wells included in the study provide water for human consumption. The objectives of the study were to evaluate the occurrence and concentration of naturally occurring radioactive constituents and the geologic settings and conditions in which elevated concentrations occur. The USGS released preliminary findings to Jefferson County officials on May 3, 2011. The USGS will publish a report describing methods and results later in 2012.
Radioactive elements, or radionuclides, are unstable trace elements that naturally occur in rocks, soil, and groundwater. These elements undergo radioactive decay through the emission of alpha particles, beta particles, or gamma rays to form more stable elements. For example, during decay of uranium, alpha and beta particles are emitted and radium and radon are formed as interim products; these, in turn, ultimately decay to a stable form of lead. Radioactive constituents in water generally are colorless, odorless, and tasteless and typically cannot be detected by our senses.
The presence of radioactive constituents, uranium in particular, in Jefferson County groundwater was previously documented in a 2007 USGS study. Although not regulated in domestic or livestock wells, concentrations of some constituents were greater than drinking-water standards or Maximum Contaminant Levels (MCLs) set by the U.S. Environmental Protection Agency (USEPA) for public drinking-water supplies. According to the USEPA, some people who drink water containing radionuclides in excess of the specified MCLs over a lifetime may have an increased risk of cancer or, in the case of uranium, toxic effects to the kidneys. For example, the USEPA has estimated that the risk of contracting cancer is about 1 in 10,000 for people who, during their lifetime, consume 2 liters of water per day that contains 30 micrograms per liter (µg/L) of uranium.
To examine this issue, USGS scientists collected samples from 128 wells, including 88 wells sampled from November 2009 through September 2010, and the 40 wells sampled as part of the 2007 USGS study. Samples were analyzed for uranium and radon as well as gross alpha and beta radioactivity. Wells with elevated concentrations or activities were sampled a second time to examine variability of water quality over time, and these samples were analyzed for an expanded list of radioactive constituents including polonium-210, three isotopes of radium, and three isotopes of uranium.
Preliminary results indicate that radioactive constituents were detected in groundwater from all wells sampled, regardless of geologic formation tapped by the well or location of the well. Although most values were low, 29 percent of the wells exceeded the drinking-water standard for at least one of the analyzed radioactive constituents (see figure 1). Uranium concentrations were higher than the USEPA MCL of 30 micrograms per liter (µg/L) in samples from 18 of 128 wells. In addition, radon concentrations exceeded the proposed MCL of 4,000 picocuries per liter (pCi/L) in 27 percent of the wells. Adjusted gross alpha radioactivity exceeded the MCL of 15 picocuries per liter (pCi/L) in 25 of the 104 of the samples analyzed. Analyses for radium isotopes (radium-226 and radium-228) were conducted on samples from 46 wells, with resulting combined concentrations exceeding the MCL of 5 pCi/L in 22 percent of the samples.
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Figure 1. Wells sampled by the U.S. Geological Survey as part of a study of radioactive constituents in groundwater of southwestern Montana, 2007 – 2010.
Radioactive constituents exceeded drinking-water standards (MCLs or proposed MCLs) in 38 of the 58 sampled wells that obtained water from rocks of the Boulder batholith. The Boulder batholith is a common geologic unit found in the study area, particularly in Jefferson and Silver Bow Counties. These igneous rocks are known to contain uranium-bearing minerals in some areas. The Boulder batholith was intensively prospected for uranium in the 1950s, and some uranium ore was mined locally. However, it is important to note that elevated values were also found in geologic units other than the Boulder batholith. This study detected values that exceeded at least one of the drinking-water standards for radioactive constituents in samples from 11 of 70 wells in geologic units other than the Boulder batholith.
Many of the 128 wells were sampled twice to examine variations in radioactivity and concentrations of radioactive elements over time. Concentrations and activities of radioactive constituents in most of the wells that were sampled more than once remained relatively constant. However, values in a few individual wells varied substantially over time.
Private well owners who participated in the USGS study received results of their water-quality analysis and information about USEPA drinking-water standards. Well owners also were supplied with information about possible options for treating well water to reduce concentrations of uranium and other radioactive constituents.
County and State health officials recommend that private well owners have the quality of their well water tested. To have private well-water samples tested, citizens may contact the Montana Department of Public Health and Human Services Environmental Laboratory at 406-444-2642 or a private laboratory. In Jefferson County, the County Sanitarian, 406-442-4126, is available to answer questions regarding drinking-water quality and has bottles available for collecting samples for water-quality analysis.
Information on EPA drinking-water standards and other aspects of water quality can be obtained from the EPA “Safe Drinking Water Hotline” at 1-800-426-4791 or at http://www.epa.gov/safewater/.
For more information about this study contact Rod Caldwell at the U.S. Geological Survey, 3162 Bozeman Ave, Helena, MT 59601, e-mail at caldwell@usgs.gov or call 406-457-5933.
USGS provides science for a changing world. For more information visit www.usgs.gov.
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