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Incorporation of trace elements into calcite precipitated from deep anoxic groundwater in fractured granitoid rocks

Paper i proceeding
Författare H. Drake
F. A. Mathurin
Thomas Zack
T. Schafer
Publicerad i Procedia Earth and Planetary Science. 15th Water-Rock Interaction International Symposium (WRI). Evora, Portugal, OCT 16-21, 2016.
ISSN 1878-5220
Publiceringsår 2017
Publicerad vid Institutionen för geovetenskaper
Språk en
Länkar dx.doi.org/10.1016/j.proeps.2017.01...
Ämnesord Calcite, trace element incorporation, deep groundwater, granite, radionuclide analogue, partition-coefficients, seawater solutions, coprecipitation, fractionation, 25-degrees-c, carbonates, systems, ree
Ämneskategorier Geovetenskap och miljövetenskap

Sammanfattning

An extensive microanalytical study of calcite precipitated from groundwater flowing into boreholes at >400 m depth in the Aspo Hard Rock Laboratory, Sweden, has been carried out. Hydrochemical variations in packed-off sections, isolating water-conducting fractures intersected by two boreholes, were documented over a period of 17 years. The extraction of the borehole equipment revealed calcite precipitation on the equipment. This mineral material enabled unique assessment of uptake of different trace elements by calcite during precipitation from granitoid fracture groundwater, at anoxic, low-temperature (c.a 14 degrees C), and neutral (pH: 7.4-7.7) conditions, under variable salinity (Cl: 2500-7000 mg/L) prevailing at these depths. Temporal hydrochemical variations could be traced by detailed micro-analytical transects in the calcites and the influence of metal speciation and complexation on partitioning into calcite could be assessed (e.g. explaining unexpectedly low incorporation of REEs). The resulting environment-specific partition coefficients for a large number of metals are relevant in models of radionuclide retention around proposed deep nuclear waste repositories in this kind of environment, particularly because 1) elements such as REEs act as natural analogues to actinides, and 2) existing coefficients established in laboratory or in other natural environments cannot be unreservedly applied to conditions in deep crystalline rocks.

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