Radionuclides in ground water

Radionuclides

Elements that undergo radioactive decay as a result of an unstable nuclear structure are referred to as radionuclides. Isotopes have nucleus with the same number of protons, but different numbers of neutrons. Radionuclides are typically written with the chemical symbol and a superscript to the left of the symbol that denotes the atomic mass (total number of protons and neutrons) of that isotope. For example, ¹²C and ¹⁴C are isotopes of the element carbon; each has 6 protons, but are associated with 6 and 8 neutrons, respectively.

As part of the decay process, radionuclides release ionizing radiation in the form of alpha particles, beta particles , and gamma radiation.

Radionuclides Present in Ground water

There exist a variety of sources of radionuclides which can be dissolved in groundwater. Sources are typically categorized as natural or anthropogenic. Natural sources of radionuclides include formation in the atmosphere caused by cosmic irradiation, and presence in mineral deposits of long-lived radioisotopes of heavy elements and their associated daughters. Anthropogenic sources consist of nuclear power plants, nuclear weapons explosions and testing, medical, research, industrial, and pharmaceutical radionuclide applications, and mining wastes. A number of specific radionuclides are known to represent potential problems in groundwater systems. Each is described below.

Radon (²²²Rn)

The only long-lived isotope of radon is ²²²Rn, which undergoes adecay with a half-life of 3.82 days. This radionuclide is produced by decay of ²²⁶Ra, and is often associated with deposits that have significant uranium concentrations (²²⁶Ra and ²²²Rn are part of the ²³⁸U decay chain). ²²²Rn represents a significant health concern in some homes, entering as a dissolved species in drinking water or directly from soil as a gas. Additional radiation is emitted from daughter products of ²²²Rn as it decays to a stable element. ²²²Rn is a naturally occurring radioisotope, and can be significant for uranium mining in terms of worker safety and waste disposal (Fetter, 1993).

Radium (²²⁶Ra and ²²⁸Ra)

²²⁶Ra (t_1/2 = 1600 years) is a daughter of ²³⁰Th, part of the ²³⁸U decay chain, and decays subsequently to ²²²Rn. Because ²²⁸Ra (t_1/2 = 5.8 years) is a daughter of ²³²Th, both radium radionuclides are associated with uranium- and thorium-containing deposits. Radium is more soluble than either thorium or uranium, and can therefore be transported in higher concentrations with an associated higher activity, particularly when considering the relatively short t_1/2 of ²²⁸Ra. Because radium is associated with uranium deposits, it is therefore a potentially important problem in mining wastes (Freeze and Cherry, 1979).

Uranium (²³⁸U and ²³⁵U)

Uranium in mineral deposits is composed of 99.3% ²³⁸U (t_1/2 = 4.5 X10⁹ years) and 0.7% ²³⁵U (t_1/2 = 7.1 X 10⁸ years). Because ²³⁵U is fissile (split by low-energy neutrons), it is used as fuel for nuclear power reactors worldwide. Both uranium isotopes are long lived and part of long decay chains. Each has numerous radioactive daughter nuclei that produce emissions before decaying to stable states. Uranium is typically mined in open pits or underground and concentrated to 65 to 70% uranium via flotation, dissolution, and solvent extraction processes. Further purification to U₃O₈ and enrichment to increase the fraction of the ²³⁵U isotope can achieve desired levels of ²³⁵U, with 3 to 4% being typical for nuclear power reactors (Choppin et al., 1995).

Neptunium (²³⁷Np)

²³⁷Np can be found in nuclear waste as the product of nuclear reactions (in a nuclear power reactor) converting ²³⁸U to ²³⁷U, which then beta decays to ²³⁷Np. It has a t_1/2 of 2.2 X 10⁶ years and undergoes a decay to ²³³Pa, with associated daughter decays from that element and others. ²³⁷Np is of great concern in high level and spent nuclear fuel disposal because it is only slightly sorbed (i.e., high mobility) in many groundwater environments and has a long half-life (USDOE, 1995).

Tritium (³H)

Tritium undergoes beta decay with a half-life of 12.43 years, producing ³He as a daughter element. Because tritium can combine with hydrogen and oxygen to form tritiated water, it poses a significant threat in terms of groundwater transport. Tritium is especially useful in delineating groundwater recharge occurring since the 1963 spike of tritium released by the U.S. and Soviet Union prior to the implementation of an above-ground testing treaty (Fritz et al., 1991). Tritium concentrations in natural waters have increased nearly tenfold since the advent of the nuclear age (Fetter, 1993). Tritium is formed naturally in the atmosphere when high-energy neutrons bombard ¹⁴N, producing ¹²C and ³H. It is also produced in large quantities at nuclear power plants, and in nuclear weapons testing and production.

Carbon (¹⁴C)

Like tritium, ¹⁴C can be used for dating purposes, in this case for organic material. It decays by beta emission to ¹⁴N, and has a t_1/2 of 5715 years. ¹⁴C labeling of organic compounds is often done to provide sensitive detection methods for experiments with organic compounds. Because it can be oxidized to soluble or gaseous ¹⁴CO₂, or form a variety of organic compounds, ¹⁴C can pose a significant transport threat as a radionuclide. It can be produced naturally in the atmosphere when low-energy neutrons bombard ¹⁴N, producing ¹⁴C and ¹H. Anthropogenic sources include nuclear power plants, nuclear weapons testing, and research laboratories.

Fission Fragments (⁹⁰Sr ⁹⁹Tc, ¹²⁹I, and ¹³⁷Cs)

Fission fragments are radioactive materials produced by fission of heavy isotopes such as ²³⁵U. The fission reaction typically splits the heavy atom into two (sometimes three) lighter isotopes, with associated neutron, neutrino, and gamma radiation emissions as well. The lighter isotopes are produced primarily in the atomic mass range of 80 to 155. Although a variety of fission fragments are produced, ⁹⁰Sr ⁹⁹Tc, ¹²⁹I, and ¹³⁷Cs are of primary concern due to the significant amounts that are produced and their relatively long half-lives.

⁹⁰Sr decays by beta emission to ⁹⁰Y with a t_1/2 of 29.1 years. ⁹⁰Y then decays to stable ⁹⁰Zr via beta emission.Strontium is a prominent fission fragment with no associated gamma emissions. In high-level nuclear waste and spent fuel, it is one of the highest activity radioisotopes for the first 200 years after fission ceases. It can be particularly dangerous when ingested by humans because it is known to accumulate in or near the bones, resulting in greater irradiation of sensitive bone marrow.

⁹⁹Tc undergoes beta decay to ⁹⁹Ru with a t_1/2 of 2.1X10⁵ years. In addition to being a fission fragment,⁹⁹Tc is also associated with medical and research wastes where it is used in diagnostic testing. ¹²⁹I is an important fission fragment because it beta decays to stable ¹²⁹Xe with a long t_1/2 of 1.7 X10⁷ years. It is also used in medical diagnostic tests and can therefore be found in low-level waste as well as spent fuel.

¹³⁷Cs emits gamma and beta radiation while decaying to stable ¹³⁷Ba and has a t_1/2 of 30.3 years. Along with ⁹⁰Sr, ¹³⁷Cs is one of the high-activity isotopes in high-level nuclear waste for the first 200 years after fission ceases.