Tritium (3H) is a radioactive isotope of hydrogen that decays with a 12.312 year half-life exclusively via beta emission to form stable helium-3. Low-energy beta particles emitted during the decay have a maximum energy of 18.591 keV and an average energy of 5.68 keV.1
Tritium exists fairly uniformly in the environment. It is produced naturally through interactions of high-energy cosmic rays with gases in the upper atmosphere, the most abundant of which are nitrogen and oxygen. Since the 1940s, various anthropogenic sources of tritium have significantly increased the quantity of tritium in the environment. Tritium in the environment is found almost exclusively as a very labile, exchangeable hydrogen atom in water molecules. Its occurrence in liquid form, coupled with the low energy of its beta emissions, limit the radiometric techniques that can be used for its detection.
Naturally-occurring tritium is most abundant in precipitation and surface waters with concentrations typically in the 10 to 30 pCi/L range.2 In contrast, the tritium in aged waters, such as ground water in aquifers isolated from surface water recharge, is often present well below the single pCi/L level.
Significant contributions of anthropogenic 3H were released to the environment as a result of nuclear weapons testing from the 1950s through the 1980s. Tritium is produced in light-water nuclear reactors by ternary fission, neutron capture in coolant additives, control rods and plates, and activation of deuterium. Releases associated with nuclear power represent a relatively smaller proportion of environmental 3H although nuclear reactors and fuel-processing plants may be localized sources of 3H because of discharges during normal operation. In general, only about 1% of the 3H in the primary coolant is released in gaseous form to the atmosphere while the remainder eventually is released in liquid waste discharges. Most 3H produced in reactors remains in the fuel but may be released when fuel is reprocessed. As 3H produced by nuclear weapons testing decays over time, contributions from nuclear power production are expected to become the major source of anthropogenic 3H in the environment. In industry, 3H has found use as an energy source for radioluminescent light sources (e.g., exit signs, luminous dials, night sights for guns). It is also used as a tracer in research laboratories.
Method 7500-3H B describes the use of distillation and liquid scintillation spectrometry for the measurement of water-exchangeable 3H in water. Some or all organically-bound 3H present in samples may be measured using Method 7500-3H B depending on the reduction/oxidation (redox) properties of the organically-bound matrix and the completeness of the permanganate oxidation. Method 7500-3H B readily meets detection capability requirements stipulated in 40 CFR 141.25 (c) for compliance determinations of 3H in water under the US EPA Safe Drinking Water Act (Required Detection Limit of 1000 pCi/L).
Method 7500-3H B, when run using an ultra-low-level liquid scintillation counter, the largest practicable sample size (10 mL of distilled sample), and count times in excess of 600 minutes, can theoretically achieve MDCs as low as 75-100 pCi/L and SDWA DLs as low as 40-60 pCi/L. It is not, however, sensitive enough to detect the significantly lower background concentrations of tritium typically found in surface and ground water. To achieve more sensitive measurements, such as are required for geochronometry, specialized methodologies, such as the electrolytic enrichment of the tritium content in the water, are needed prior to radiometric measurement.