Radium (Ra) and water

Radium (Ra) and water: occurrence, chemistry and treatment

Radium is a naturally occurring radioactive element that can dissolve into groundwater from rocks and sediments. Its occurrence depends on local aquifer chemistry and is not the same as uranium occurrence: elevated radium and uranium do not necessarily appear together. Testing is needed to determine whether treatment is required.

A groundwater sample and mineral core being analyzed in a water laboratory. — Illustration of groundwater sampling and mineral analysis; it does not represent a measured sample.

How does radium enter water?

Radium isotopes form naturally in radioactive decay chains in rocks and sediments. Radium-226 and radium-228 are the isotopes most commonly considered in drinking-water monitoring. They can enter groundwater as minerals weather and water moves through an aquifer. Concentrations depend on geology, mineral surfaces, redox conditions, salinity and the presence of competing ions.

The chemical conditions that mobilize radium differ from those that mobilize uranium. U.S. Geological Survey studies report that high concentrations of uranium and radium rarely coincide in the principal aquifers they assessed. A uranium result therefore cannot be used as a substitute for a radium analysis.

Radium species and water chemistry

In water, radium is generally present as the dissolved Ra²⁺ ion or associated with particles and mineral surfaces. As a divalent alkaline-earth metal, it can behave in some ways like calcium and barium. Sulfate, hardness, salinity and ion-exchange sites may influence its mobility and how it responds to treatment.

Radium is radioactive; it is not the same as radon. Radon-222 is a radioactive gas formed in the decay chain of radium-226, and radon in water has separate monitoring and treatment considerations.

Health and radiological considerations

Radium emits ionizing radiation. If ingested over time, it can accumulate in bones and increase lifetime cancer risk. The health assessment concerns radioactivity and is usually reported in activity units such as becquerels per litre (Bq/L) or picocuries per litre (pCi/L), not simply as a mass concentration.

As a U.S. drinking-water example, the EPA maximum contaminant level is 5 pCi/L for combined radium-226 and radium-228. This is a jurisdiction-specific regulatory limit, not a global standard. Requirements and monitoring protocols differ by country; use the current local rules and an appropriate laboratory method.

How can radium be removed from water?

Potential treatment methods include cation exchange, lime or lime-soda softening, and reverse osmosis. Selection depends on radium concentration, hardness, barium, sulfate, total dissolved solids, flow and residual-management requirements. Competing cations can affect ion-exchange performance, while precipitation or membrane processes create residual streams that require attention.

Before design, confirm radium-226 and radium-228 concentrations and characterize the water. Verify performance through testing and ongoing monitoring. Ion-exchange brine, softening sludge and reverse-osmosis concentrate can contain higher radionuclide levels than the feed water; assess their handling and disposal under applicable rules.

Read more about ion exchange and reverse osmosis.

Last reviewed: 31 October 2026 by Lenntech.