|NATURAL ATTENUATION OF INORGANIC CONTAMINANTS|
Inorganic contaminants, such as metals and radionuclides, are not readily digested and destroyed by microorganisms. Thus, with inorganics, other natural processes, such as chemical transformation, dilution, sorption, and radioactive decay, are responsible for natural attenuation, as broadly defined by U.S. EPA. The health and environmental risks posed by those contaminants may be reduced by changing the amount of exposure, the exposure pathway, or the toxicity of the chemical.
Though non-radioactive metals do not break down in the environment, sometimes chemical changes in the subsurface environment reduce their toxicity. Copper in certain forms is considered so safe that we use it to pipe drinking water, yet trace concentrations can be detrimental to aquatic life. Chromium in one form (Hexavalent, or Cr6+) forms compounds which are highly toxic and very soluble, yet under certain conditions it transforms into Trivalent Chromium (Cr3+), which is less toxic, as well as insoluble. The potential for such a change differs for each substance, but it can be estimated by conducting a chemical and biological analysis of the subsurface environment.
Dilution is the process in which a contaminant becomes less concentrated. It is similar for both organic and inorganic contaminants. It reduces risk because individual receptors (humans or animals) are likely to be exposed to lower, less toxic concentrations of the hazard. In fact, the chronic effects of most dilute contaminants are extremely difficult to measure. By itself, however, dilution does not reduce the contaminant mass; rather, it spreads the area of potential exposure. And some contaminants are believed hazardous even at levels too dilute to be detected with standard field characterization techniques.
Sorption occurs when contaminants attach to underground particles, immobilizing and thus limiting the availability of contaminants. The most common "host" materials for metals and radionuclides are iron hydroxides, clays, and carbonate minerals. Sorption is most effective for metals and radionuclides that are present in low concentrations. With sorption, the contaminants are mobilized, but not destroyed. While sorption is always reversable, it is conventionally termed "irreversible" if the soil encloses the contaminants. When this happens the sorbed contaminants are less likely to be affected by changes in the environment that might desorb or dissolve the material, such as changes in acidity or exposure to additional reactive chemicals.
Sometimes sorption is enhanced through chemical transformation, as described above, or chemical stabilization. That is, before the metals attach to soil particles, they form solid compounds with chemicals, such as oxygen and sulfur, that are found in the soil. This has two advantages: First, this dense material won't travel as well with the flow of groundwater. Second, the new compound is more likely to adhere to the soil. However, in the long run changes in acidity or the concentration of charged particles and reactive chemicals in the soil can destabilize these compounds.
Sorption is more difficult to measure than the degradation of organic substances. With organic degradation, one can measure the byproducts of biological and chemical reactions to estimate how much degradation has taken place. In contrast, inorganics are sorbed by displacing ions that are abundant to begin with. Measuring relatively small variations in the concentration of those ions is very difficult, and the results are often less reliable.
In evaluating sorption as a natural attenuation technique, one must consider the following three factors:
Sorption, like other natural attenuation processes, occurs whether or not it has been approved as a remedy. In fact, sometimes "irreversible" sorption makes active removal of the contamination-now bound to the soil-much more difficult.
Radionuclides are substances that emit radiation because their atoms are unstable. They disintegrate or decay as they release energy in the form of radioactive particles or waves. This decay is a natural process that happens spontaneously. As they decay, radioactive isotopes transform into other, often less radioactive isotopes of the same element or even sometimes into other elements.
Thus, unlike other inorganics, radionuclides degrade naturally, but the degradation process itself is hazardous to living things. The risk posed by radionuclides is a function of the type and amount of radiation, as well as exposure pathways.
Radionuclides with relatively short half lives, such as tritium-which has a half-life of 12.33 years-naturally attenuate at a rate that may decrease risks to human health over a relatively short span of time. If they remain isolated from receptors-that is, if no one is exposed to the radiation-then the risk is low. Active remediation does not affect the decay rate. Instead, it is designed to stabilize, prevent the migration of, or isolate the substance. In comparing the natural attenuation of radionuclides to active remediation, one usually considers sorption and dilution as the mechanisms for isolating the material and decreasing the concentration. Will the radioactive substance remain bound to the soil for the amount of time it takes for it to decay to levels deemed acceptably safe?
Some radioactive isotopes decay to form other, more hazardous radionuclides. As with the degradation of organic contaminants, therefore, it's important to consider the hazard posed by the daughter products as well as radiation from the original hazard when evaluating the effects of natural radioactive decay.
As the name suggests, the half-life of a radionuclide is simply the amount of time that it takes for a radionuclide to decay to half of its initial radioactivity. After two half lives, a quarter of the initial radioactivity still remains. After three half lives, one eighth remains. Thus, radioactive materials never disintegrate entirely.
The reduction in radioactivity is independent of how long the substance has already been decaying. For example, the amount of radioactivity after four half-lives is half that after three half lives, since the duration of decay between those two times is one half life.
The time it takes for a radioactive substance to decay to a level deemed acceptable by regulatory agencies depends activity, the magnitude of the radiation is still considered unacceptable if the initial contamination was more than a thousand times the acceptable level.
Radioactive substances range in half-life. Elements created in atomic laboratories sometimes have half-lives of a fraction of a second, while Uranium 238 has a half-life of 4.5 billion years. Tritium has a half life of 12.33 years.
The identity and chemical properties of an element are determined by the number of protons in its nucleus: the atomic number. Many elements, however, occur as different isotopes, which are defined by the number of neutrons in the nucleus.
The common isotope of hydrogen has one proton and no neutrons in its nucleus. Tritium, the radioactive isotope of hydrogen, has one proton and two neutrons. U-235 and U-238, two isotopes of the element Uranium, both have 92 protons, but they have 143 and 146 neutrons respectively.
August 30, 1998
Principal author: Luat Vuong
Center for Public Environmental Oversight