SUFFICIENT AND NECESSARY: THE SITE-SPECIFIC CHARACTERIZATION OF GEOCHEMICAL PROCESSES

The long-term behavior of a potential hazardous or radioactive waste site is a critical factor in site evaluation, but this information is notoriously difficult to obtain. Mineralogical and geochemical analyses can provide data on properties of a site that are either difficult to model accurately or are not embodied in models that are based on incomplete or faulty concepts. For example, determination of long-term or cumulative fluid flux can be constrained where cation-exchanging minerals such as zeolites have acted over geologic time scales as efficient accumulators of cations from solutions with approximately known cation concentration. Another important but often ignored property, mineral surface-mediated heavy-metal redox reactions, can be demonstrated where Pu analogs such as Ce have been shown to be removed from solution as Ce4+ by Mn-oxides. Sites where Ce has accumulated can be mapped to determine specific horizons or features where other heavy metals may accumulate. Such maps can be critical in evaluating remediation approaches or repository design. In some instances, the effective distance over which metals are removed from solution can be demonstrated where a ‘passive’ accumulator such as calcite records diminishing concentrations of redox-sensitive metals in groundwater with distance from a dissolution source. Use of geochemical observations to establish a history of long-term site performance provides a level of understanding that can augment short-duration field tests and can be used to evaluate models of long-term performance. In addition, geochemical and mineralogical data can supplement the results of modeling, particularly emphasizing processes that have not been modeled (e.g., strong retention of heavy metals by trace fracture-lining minerals). Results from modeling can be tested when a predicted outcome is either supported or contradicted by geochemical evidence (e.g., predicted impermeability of zeolitized tuffs versus geochemical evidence for through-flow). Perhaps the greatest benefit of site-specific geochemical evaluation is the demonstration to analysts, regulators, and the public that a defensible data-based understanding of the long-term behavior of a site has been obtained using data obtained from the site.