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Oxygen, hydrogen, and helium isotopes for investigating groundwater systems of the Cape Verde Islands, West Africa

Stable isotopes (d super(18)O, d super(2)H), tritium ( super(3)H), and helium isotopes ( super(3)He, super(4)He) were used for evaluating groundwater recharge sources, flow paths, and residence times of three watersheds in the Cape Verde Islands (West Africa). Stable isotopes indicate the predominan...

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Bibliographic Details
Published in:Hydrogeology journal 2009-07, Vol.17 (5), p.1157-1174
Main Authors: Heilweil, Victor M, Solomon, DKip, Gingerich, Stephen B, Verstraeten, Ingrid M
Format: Article
Language:English
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Summary:Stable isotopes (d super(18)O, d super(2)H), tritium ( super(3)H), and helium isotopes ( super(3)He, super(4)He) were used for evaluating groundwater recharge sources, flow paths, and residence times of three watersheds in the Cape Verde Islands (West Africa). Stable isotopes indicate the predominance of high-elevation precipitation that undergoes little evaporation prior to groundwater recharge. In contrast to other active oceanic hotspots, environmental tracers show that deep geothermal circulation does not strongly affect groundwater. Low tritium concentrations at seven groundwater sites indicate groundwater residence times of more than 50 years. Higher tritium values at other sites suggest some recent recharge. High super(4)He and super(3)He/ super(4)He ratios precluded super(3)H/ super(3)He dating at six sites. These high super(3)He/ super(4)He ratios (R/R sub(a) values of up to 8.3) are consistent with reported mantle derived helium of oceanic island basalts in Cape Verde and provided end-member constraints for improved dating at seven other locations. Tritium and super(3)H/ super(3)He dating shows that Sao Nicolau Island's Ribeira Faja Basin has groundwater residence times of more than 50 years, whereas Fogo Island's Mosteiros Basin and Santo Antao Island's Ribeira Paul Basin contain a mixture of young and old groundwater. Young ages at selected sites within these two basins indicate local recharge and potential groundwater susceptibility to surface contamination and/or salt-water intrusion.Original Abstract: Les isotopes stables (d super(18)O, d super(2)H), tritium ( super(3)H), et helium ( super(3)He, super(4)He) ont ete utilises pour caracteriser les origines de recharge, les trajectoires d'ecoulements et les temps de sejour dans trois aquiferes distincts des Iles du Cap Vert (Afrique de l'Ouest). Les isotopes stables indiquent la predominance de precipitations importantes subissant une faible evaporation avant recharge de l'aquifere. A la difference d'autres points chauds oceaniques actifs, les traceurs environnementaux montrent que la circulation geothermale profonde n'affecte pas fortement l'aquifere. De faibles concentrations en tritium sur sept sites de prelevement indiquent des temps de sejour de plus de 50 ans. Des valeurs plus elevees sur d'autres sites suggerent plutot une recharge plus recente.Une concentration super(4)He elevee et des ratios super(3)He/ super(4)He eleves excluaient la datation super(3)H/ super(3)He sur six sites. Ces r
ISSN:1431-2174
1435-0157
DOI:10.1007/s10040-009-0434-2