Geochemical factors in the selection and assessment of sites for the deep disposal of radioactive wastes

Abstract

Abstract Geochemical parameters which are important to waste isolation include (with optimum conditions in parenthesis): groundwater pH (10 > pH > 8), redox ( Eh < −200 mV), P co 2 (<10 −4 bars), and ionic strength (< 0.5 M); the colloid (< 0.01 ppm), organic (< 5 ppm) and inorganic ligand (Cl − > HCO − 3 , low abundances of phosphate) content of groundwaters; and the mineralogical composition and structure of groundwater flowpaths (zeolites, clays, iron oxyhydroxides with appreciable diffusive porosity). pH, redox, and in some environments, P co 2 , will be controlled by water-rock reaction; other parameters are influenced more by geological history or environment. pH and redox buffer intensities of likely mineral assemblages and groundwater compositions under possible physical conditions of deep disposal have been calculated. For pH, reactions between aluminosilicates, carbonates and water should dominate over those in the fluid phase alone. The reaction of small amounts of ferrous iron-bearing minerals (≥ 5 vol %) is sufficient to maintain chemically reducing conditions under likely hydraulic conditions at depth. Consideration of laboratory data suggest that pH and redox reactions should not be hindered by kinetics under likely physical conditions for deep disposal, but further field and laboratory experimental data are required to verify these conclusions. The most suitable geochemical environments for waste isolation are basic or intermediate igneous rocks in tectonically stable, low-lying terrain away from sedimentary basins. Geochemical criteria should not be used in isolation of other factors for the selection of sites, but suitable geochemical conditions may help relax the requirements for optimum performance dictated by other geological and hydrogeological factors.