The Purpose of Geochemical Modeling

The ultimate question for the parties and regulators responsible for the site (and the question that geochemists and hydrogeologists are hired to answer) is what will be the concentrations of the regulated constituents at the point of compliance (POC) over the compliance period of200 years  

Uranium ore was mined in the USA in significant quantities by private companies to produce nuclear weaponry. After the 1950s, uranium was also needed as fuel for nuclear power plants to produce electricity. Ores are first crushed into centimeter to millimeter size. The ground ore is then disposed to the leach tanks to be dissolved and oxidized, respectively, by sulfuric acid (H2SO4) and sodium chlorate (NaC104). The dissolved uranium is extracted from the solutions. 

The barren sand and slimes left after the leaching processes are called tailings. They contain about 85% of the radioactivity present in the unprocessed uranium ore. The tailings, spent acid, and process water are often pumped to a tailings impoundment. The tailings fluids or leachate from the tailings piles contain concentrations of radioactive and toxic materials that might pose a public health hazard. 

The reclamation and remediation of many contaminated sites, and uranium mill tailings sites in particular, have relied on natural attenuation processes. A number of physical, chemical, and biological processes can reduce the concentrations of contaminants. 

A scientific basis must be provided before natural attenuation can be accepted as an effective remediation alternative, because the public and communities often suspect this to be just a do nothing or walk away strategy by the polluters (National Research Council, 2000). The specific natural attenuation processes must be identified and quantified, and their long term sustainability must be evaluated (National Research Council, 2000). The geochemical modeling work, described in this section, provides the conceptual framework, which can help document and interpret the footprints (National Research Council, 2000) of the natural attenuation processes that are actually occurring in the aquifer. 

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