Incorporation of Metals into Calcite in a Deep Anoxic Granite Aquifer

Understanding metal scavenging by
calcite in deep aquifers in granite
is of importance for deciphering and modeling hydrochemical fluctuations
and water–rock interaction in the upper crust and for retention
mechanisms associated with underground repositories for toxic wastes.
Metal scavenging into calcite has generally been established in the
laboratory or in natural environments that cannot be unreservedly
applied to conditions in deep crystalline rocks, an environment of
broad interest for nuclear waste repositories. Here, we report a microanalytical
study of calcite precipitated over a period of 17 years from anoxic,
low-temperature (14 °C), neutral (pH: 7.4–7.7), and brackish
(Cl: 1700–7100 mg/L) groundwater flowing in fractures at >400
m depth in granite rock. This enabled assessment of the trace metal
uptake by calcite under these deep-seated conditions. Aquatic speciation
modeling was carried out to assess influence of metal complexation
on the partitioning into calcite. The resulting environment-specific
partition coefficients were for several divalent ions in line with
values obtained in controlled laboratory experiments, whereas for
several other ions they differed substantially. High absolute uptake
of rare earth elements and U­(IV) suggests that coprecipitation into
calcite can be an important sink for these metals and analogousactinides
in the vicinity of geological repositories.