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. 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)
- Low pH (pH generally less than 6)
- 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.
Further Information Available @
Radium is ordinarily present at low levels in all soil, water, and rocks, including groundwater. However, the study found that if the groundwater has low oxygen or low pH, radium is more likely to dissolve and become present in the groundwater. Low oxygen conditions were prevalent in the Mid-Continent and Ozark Plateau Cambro-Ordovician aquifer systems, and low pH conditions were prevalent in the North Atlantic Coastal Plain aquifer system. Groundwater Monitoring
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