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Radium occurrence and geochemistry in groundwater in the United States

Radium Publications

Current Radium Report:

Related Publications on radium in drinking water:

More USGS Radionuclide Publications

Glossary of terms

Frequently Asked Questions




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Radium is a naturally occurring radioactive element and known carcinogen that usually is present at low levels in rocks, soils, and groundwater.  Its presence in groundwater is largely the result of minerals dissolving from weathered rocks and soils. Radium has several isotopes. These include radium-224 (Ra-224), radium-226 (Ra-226) and radium-228 (Ra-228).  The US Environmental Protection Agency (USEPA) has established maximum contaminant level (MCL) for combined Ra-226 plus Ra-228 of 5 picocuries per liter (pCi/L) for public water supplies. USEPA in coordination with USGS is evaluating the occurrence and potential guidance required for Ra-224 as noted in the provisions of the Radionuclide Rule of 2000.

To understand how radium is distributed in aquifers across the United States, and to develop a greater understanding of geochemical conditions that control its release from aquifer materials, the USGS NAWQA program, in cooperation with the USEPA, sampled the untreated source water from1270 domestic, public supply and monitoring wells between 1987 and 2005.

Study Results

About 3 percent of the sampled wells had combined radium concentrations greater than the EPA standard for drinking water. Almost all wells with concentrations greater than the standard were located in the eastern United States (figure 1). More importantly, one in five wells in the Mid-Continent and Ozark Plateau Cambro-Ordovician aquifer systems, and in the North Atlantic Coastal Plain aquifer system had combined radium concentrations greater than the EPA standard. It is important to note that for this study untreated water was sampled, and that concentrations in sampled wells do not necessarily reflect the quality of finished water from wells with treatment systems.

Several geochemical factors control the concentration of radium in groundwater — including the following:

  • Low dissolved oxygen (generally less than 1 milligram per liter) figure 2
  • Low pH (pH generally less than 6) figure 3
  • High dissolved solids

For example, in the Ozark Plateau Cambro-Ordovician aquifer system, low dissolved oxygen was associated with higher radium concentrations, whereas in the North Atlantic Coastal Plain aquifer system low pH was associated with higher concentrations. Similar geochemical conditions were associated with the release of radium to groundwater in other aquifers as well.

For more information on this study see the publication links on the upper left of this page.

Maps, Figures for Radium

National location map:
Location of wells sampled for the study and radium concentrations.

National location map

Radium vs Dissolved Oxygen graph:
Radium is at higher concentrations in untreated groundwater and more likely to exceed a drinking water standard under anoxic conditions.

Radium vs Dissolved Oxygen graph

Radium vs pH graph:
Radium is at higher concentrations in untreated groundwater and more likely to exceed a drinking water standard under low pH, or acidic, conditions.

Radium vs pH graph

State Guidance on Radium:

Many state environmental and health agencies have detailed guidance and information on radium and radionuclides. Information from several states is listed below.

Radium links

US Environmental Protection Agency

World Health Organization (PDF)

Center for Disease Control

Radium Treatment and Information Links

For Public Purveyors For Private Well Owners

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