The U.S. Geological Survey (USGS) has conducted a study of naturally occurring radium in drinking water. They found that elevated levels of radium occur most often in the central and eastern part of the country, see map.
Radium forms from natural radioactive decay of uranium and thorium in rocks and sediments derived from those rocks. “Most rocks and sediments contain some uranium and thorium and, thereby, contain radium as well, but usually in small quantities. Uranium and thorium are most common in granitic and metamorphic crystalline rocks and in associated weathered sedimentary deposits in the central United States and mountainous regions of the East and West.”
Major findings from USGS report:
Concentrations of radium in principal aquifers used for drinking water throughout the United States generally were below 5 picocuries per liter (pCi/L), the U.S. Environmental Protection Agency maximum contaminant level (MCL) for combined radium—radium-226 (Ra-226) plus radium-228 (Ra-228) —in public water supplies. About 3 percent of sampled wells had combined radium concentrations greater than the MCL.
The highest concentrations of combined radium were in the Mid-Continent and Ozark Plateau Cambro-Ordovician aquifer system and the Northern Atlantic Coastal Plain aquifer system. More than 20 percent of sampled wells in these aquifers had combined radium concentrations that were greater than or equal to the MCL.
Three common geochemical factors are associated with the highest radium concentrations in groundwater: (1) oxygen-poor water, (2) acidic conditions (low pH), and (3) high concentrations of dissolved solids.
The USGS notes: “Exposure to radium over long periods of time can increase the risk of cancer….Radium in the body behaves similarly to calcium and can replace calcium in tissues, particularly bone…Radiation exposure from radium received externally through washing, showering, or other uses of water is less of a concern since human skin tends to block exposure to alpha radiation and minimize penetration of beta radiation.”
Note that the USGS sampled only 1,266 wells nationwide and only 3% of those contained radium concentrations above the MCL.
Arizona State Geologist Lee Allison notes in his blog that “the water wells tested in Arizona are all below 1 picocurie per liter. However, only a handful of wells in Arizona were tested, all in alluvial aquifers in the south. No tests are reported from the Colorado Plateau, which has some of the highest concentrations and largest deposits of uranium in the nation.”
On an individual household level, radium is removed by ion-exchange (salt recharged) water softeners.
See a USGS fact sheet here.