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Title: Rare earth element geochemistry of groundwaters from a thick till and clay-rich aquitard sequence, Saskatchewan, Canada

Abstract

Owing to their exceptionally low hydraulic conductivities and extensive a real distribution, clay-rich deposits are of interest because of their potential as repository sites for the disposal of high-level radioactive wastes and heavy metals, as well as their use as protective covers for regional aquifer systems. Consequently, understanding the geochemical processes controlling solutes, such as transuranics and other heavy metals, in clay-rich deposits is of particular concern in order to properly assess the suitability of these deposits as potential waste repository sites. Rare earth elements (REE) were determined in groundwater samples collected from a thick till and clay-rich aquitard sequence located in southern Saskatchewan, Canada. The groundwaters are Na-Mg-SO{sub 4}-type waters that range from highly concentrated brines near the ground surface to relatively dilute waters at depth. The majority of these groundwaters have pH values between 7 and 8, although the deepest samples are more alkaline. Groundwater REE concentrations are relatively constant in the overlying till but increase by up to a factor of 50 in the underlying clay bedrock. Shallow groundwaters have heavy REE (HREE)--enriched shale-normalized patterns, whereas the REE patterns of the deep groundwaters are relatively flat. Solution complexation modeling indicates that variations in REE patterns reflect differencesmore » in solution complexation across the REE suite. In the shallow groundwaters, strongly adsorbed, positively charged carbonate complexes, sulfate complexes and free metal ion species dominate the speciation of light REEs (LREE), whereas HREEs occur chiefly as more stable, negatively charged dicarbonato complexes. For the deepest groundwaters, however, all of the REEs are predicted to occur in solution as dicarbonato complexes. The large HREE enrichments of the shallow groundwaters reflect the greater affinity of the positively charged LREE solution species to adsorb to clay minerals or coatings on clay minerals in the aquitard sequence compared to the more stable, negatively charged HREE dicarbonato complexes. On the other hand, the flat REE patterns, of the deep groundwaters reflect the dominance of the negatively charged dicarbonato complex for all REEs. The solution complexation model along with the strong positive correlation between REEs and to a lesser extend pH, indicates that carbonate ion concentrations, and thus pH, exert important controls on aqueous REE concentrations in these groundwaters.« less

Authors:
;
Publication Date:
Research Org.:
Old Dominion Univ., Norfolk, VA (US)
OSTI Identifier:
20062612
Resource Type:
Journal Article
Journal Name:
Geochimica et Cosmochimica Acta
Additional Journal Information:
Journal Volume: 64; Journal Issue: 9; Other Information: PBD: May 2000; Journal ID: ISSN 0016-7037
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; 12 MANAGEMENT OF RADIOACTIVE WASTES, AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES; SASKATCHEWAN; GROUND WATER; GEOCHEMISTRY; RARE EARTHS; ECOLOGICAL CONCENTRATION; PH VALUE; CLAYS; RADIOACTIVE WASTE DISPOSAL; SITE CHARACTERIZATION

Citation Formats

Johannesson, K.H., and Hendry, M.J. Rare earth element geochemistry of groundwaters from a thick till and clay-rich aquitard sequence, Saskatchewan, Canada. United States: N. p., 2000. Web. doi:10.1016/S0016-7037(99)00402-0.
Johannesson, K.H., & Hendry, M.J. Rare earth element geochemistry of groundwaters from a thick till and clay-rich aquitard sequence, Saskatchewan, Canada. United States. doi:10.1016/S0016-7037(99)00402-0.
Johannesson, K.H., and Hendry, M.J. Mon . "Rare earth element geochemistry of groundwaters from a thick till and clay-rich aquitard sequence, Saskatchewan, Canada". United States. doi:10.1016/S0016-7037(99)00402-0.
@article{osti_20062612,
title = {Rare earth element geochemistry of groundwaters from a thick till and clay-rich aquitard sequence, Saskatchewan, Canada},
author = {Johannesson, K.H. and Hendry, M.J.},
abstractNote = {Owing to their exceptionally low hydraulic conductivities and extensive a real distribution, clay-rich deposits are of interest because of their potential as repository sites for the disposal of high-level radioactive wastes and heavy metals, as well as their use as protective covers for regional aquifer systems. Consequently, understanding the geochemical processes controlling solutes, such as transuranics and other heavy metals, in clay-rich deposits is of particular concern in order to properly assess the suitability of these deposits as potential waste repository sites. Rare earth elements (REE) were determined in groundwater samples collected from a thick till and clay-rich aquitard sequence located in southern Saskatchewan, Canada. The groundwaters are Na-Mg-SO{sub 4}-type waters that range from highly concentrated brines near the ground surface to relatively dilute waters at depth. The majority of these groundwaters have pH values between 7 and 8, although the deepest samples are more alkaline. Groundwater REE concentrations are relatively constant in the overlying till but increase by up to a factor of 50 in the underlying clay bedrock. Shallow groundwaters have heavy REE (HREE)--enriched shale-normalized patterns, whereas the REE patterns of the deep groundwaters are relatively flat. Solution complexation modeling indicates that variations in REE patterns reflect differences in solution complexation across the REE suite. In the shallow groundwaters, strongly adsorbed, positively charged carbonate complexes, sulfate complexes and free metal ion species dominate the speciation of light REEs (LREE), whereas HREEs occur chiefly as more stable, negatively charged dicarbonato complexes. For the deepest groundwaters, however, all of the REEs are predicted to occur in solution as dicarbonato complexes. The large HREE enrichments of the shallow groundwaters reflect the greater affinity of the positively charged LREE solution species to adsorb to clay minerals or coatings on clay minerals in the aquitard sequence compared to the more stable, negatively charged HREE dicarbonato complexes. On the other hand, the flat REE patterns, of the deep groundwaters reflect the dominance of the negatively charged dicarbonato complex for all REEs. The solution complexation model along with the strong positive correlation between REEs and to a lesser extend pH, indicates that carbonate ion concentrations, and thus pH, exert important controls on aqueous REE concentrations in these groundwaters.},
doi = {10.1016/S0016-7037(99)00402-0},
journal = {Geochimica et Cosmochimica Acta},
issn = {0016-7037},
number = 9,
volume = 64,
place = {United States},
year = {2000},
month = {5}
}