Environmental Geochemistry of Radioactive Contamination

Abstract

This report attempts to describe the geochemical foundations of the behavior of radionuclides in the environment. The information is obtained and applied in three interacting spheres of inquiry and analysis: 1) experimental studies and theoretical calculations, 2) field studies of contaminated and natural analog sites and 3) model predictions of radionuclide behavior in remediation and waste disposal. Analyses of the risks from radioactive contamination require estimation of the rates of release and dispersion of the radionuclides through potential exposure pathways. These processes are controlled by solubility, speciation, sorption, and colloidal transport, which are strong functions of the compositions of the groundwater and geomedia as well as the atomic structure of the radionuclides.The chemistry of the fission products is relatively simple compared to the actinides. Because of their relatively short half-lives, fission products account for a large fraction of the radioactivity in nuclear waste for the first several hundred years but do not represent a long-term hazard in the environment. The chemistry of the longer-lived actinides is complex; however, some trends in their behavior can be described. Actinide elements of a given oxidation state have either similar or systematically varying chemical properties due to similarities in ionic size, coordination number, valence, and electron structure. In dilute aqueous systems at neutral to basic pH, the dominant actinide species are hydroxy-and carbonato-complexes, and the solubility-limiting solid phases are commonly oxides, hydroxides or carbonates. In general, actinide sorption will decrease in the presence of ligands that complex with the radionuclide; sorption of the (IV) species of actinides (Np, Pu, U) is generally greater than of the (V) species.The geochemistry of key radionuclides in three different environments is described in this report. These include: 1) low ionic strength reducing waters from crystalline rocks at nuclear waste research 4 sites in Sweden; 2) oxic water from the J-13 well at Yucca Mountain, Nevada, the site of a proposed repository for high level nuclear waste (HLW) in tuffaceous rocks; and 3) reference brines associated with the Waste Isolation Pilot Plant (WIPP). The transport behaviors of radionuclides associated with the Chernobyl reactor accident and the Oklo Natural Reactor are described. These examples span wide temporal and spatial scales and include the rapid geochemical and physical processes important to nuclear reactor accidents or industrial discharges as well as the slower processes important to the geologic disposal of nuclear waste.Application of geochemical information to remediating or assessing the risk posed by radioactive contamination is the final subject of this report. After radioactive source terms have been removed, large volumes of soil and water with low but potentially hazardous levels of contamination may remain. For poorly-sorbing radionuclides, capture of contaminated water and removal of radi...

Lithological and hydrochemical controls on distribution and speciation of uranium in groundwaters of hard-rock granitic aquifers of Madurai District, Tamil Nadu (India)

“…It is rich in acid igneous rocks, whereas it is depleted in basic and ultrabasic rocks. The average concentration of U in Earth crust is 2.7 mg l -1 (Siegel and Bryan 2004). In groundwater, U distribution is mostly dependent on lithology, geomorphology and other geological conditions of the region (Michel 1991;Ortega et al 1996).…”

Radioactivity, Geochemistry, and Health

“…This chapter supplements and does not repeat the information presented in the corresponding chapter in the First Edition of the Treatise on Geochemistry Siegel and Bryan (2003). Much of the basic information describing methods in speciation, solubility and sorption experiments, and thermodynamic or reactive transport modeling is not repeated here.…”

“…This is about equal to the total activity of 40 K in the world ocean. Table 2 in Siegel and Bryan (2003) provides additional examples of large-scale sources of natural and anthropogenic radioactivity in the environment throughout the world.…”

“…In addition, due to limitations in space, no update is given on recent work concerning colloid transport of radionuclides and only a cursory overview of studies of natural analogs for candidate nuclear waste disposal sites is provided. Finally, in Siegel and Bryan (2003), considerable attention is devoted on how geochemical studies were used in support of performance assessment calculations for the Waste Isolation Pilot Plant (WIPP) for transuranic (TRU) wastes and the proposed high-level waste (HLW) repository at Yucca Mountain. That information is not repeated here and is only briefly updated; comprehensive updates to those approaches can be found in the Yucca Mountain license application submitted to the NRC (DOE, 2008) and supporting documents (e.g., SNL, 2007a,b).…”

Natural radioactivity levels (K, Th, U and Rn) in the Cecita Lake area (Sila Massif, Calabria, Southern Italy): An attempt to discover correlations with soil features on a statistical base

“…All the naturally occurring heavy radioisotopes, i.e. those with atomic numbers of 82 (lead) and larger, are members of a radioactive decay series commencing with one of the three radionuclides 238 (Nelson-Eby, 2004;Siegel & Bryan, 2005). …”