Uranium ore is often formed by the dissolution, transport, and precipitation of uranium-rich groundwater and for that reason the geochemical behavior of uranium has been extensively studied. Solubility is dependent on pH, the presence (or absence) of oxygen, as well as the presence of other elements, such as sulfur and iron. Dissolved uranium is present in natural waters in concentrations ranging from 0.1 ug/L (parts per billion) with concentrations greater than 1 mg/L (parts per million) typically associated with uranium ore chemically precipitated in "roll-fronts" in the aquifer. Uranium is also found in igneous rocks and hydrothermal rock veins, and some hot springs are known for their naturally occurring radioactivity due to uranium.
Natural uranium is composed of several isotopes, of which uranium-238 is the most prevalent (99.3 percent of all naturally occurring uranium) and is the starting point in a radioactive-decay series that ends with the stable lead isotope lead-206. The uranium-238 series, with a half-life of uranium-238 being 4.46 billion years, produces the greatest part of radioactivity in natural water. Although radioactive, the most convenient way to detect dissolved uranium in drinking water is by chemical means.
Uranium cannot be detected by taste, sight, or smell. The only way to know the concentration is through sampling and testing.
The health effects of uranium in drinking water are chronic (the delayed result of continuous consumption over a long period of time) rather than acute (the immediate result of consumption). Individual risk depends on the concentration, how much water was consumed and for how long, as well as the age and general health of the individual.
Studies suggest that ingesting high levels of uranium may be associated with an increased risk of kidney damage. Exposure to soluble uranium in drinking water has not been shown to increase the risk of developing cancer.
The quality of water supplied by public water systems is regulated by the U.S. Environmental Protection Agency (EPA) under the federal Safe Drinking Water Act. Under these regulations, public water supplies classified as community water systems (CWS) are required to test for uranium. This includes most cities and towns. While most Safe Drinking Water Act regulations apply to all public water supplies, the uranium regulation is an exception. It does not apply to public water supplies classified as non-community. A non-community water system (NCWS) is a public water system that is not a CWS. Examples include schools, factories, businesses, rest areas, convenience stores, and camping facilities with their own water supplies. The EPA issued the Federal Radionuclides Rule, establishing an enforceable Primary Standard for uranium in public CWS of 30 micrograms per liter (ug/L), which also can be expressed as 30 parts per billion (ppb). Uranium concentrations can vary over time. For this reason, compliance is based on a running annual average, which is the average of four consecutive quarters of monitoring results. The Rule established enforceable standards for additional radionuclides, which are not discussed in this article.
If users want to know the concentration of uranium in a private water supply or a NCWS, they must have the water tested by a state certified laboratory. Uranium mass analysis is the appropriate water quality test to determine the level of uranium concentration in a private water.
Public community water supplies must provide drinking water that is in compliance with the maximum contaminant level allowed by the Environmental Protection Agency.
Typically, the most feasible treatment alternative to remove uranium in private water supplies is the Point-of-Use system. A POU system is usually placed under or near one faucet and treats only the water coming out of that tap for drinking or cooking. While no POU system is certified to remove uranium at this time, documentary proof exists to show that reverse osmosis, distillation, special adsorbent media (such as titanium dioxide) and anion exchange remove uranium and a variety of other contaminants. Boiling water is not an effective means of removing uranium. Pour-through, faucet-mounted and POU activated carbon filters also are not an effective means of removing uranium.