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

• Published in: Hydrogeology journal 2009-07, Vol.17 (5), p.1157-1174
• Main Authors: Heilweil, Victor M., Solomon, D. Kip, Gingerich, Stephen B., Verstraeten, Ingrid M.
• Format: Article
• Language: English
• Subjects: Aquatic Pollution; Basalt; Earth and Environmental Science; Earth Sciences; Environmental tracers; Evaporation; Geology; Geophysics/Geodesy; Groundwater flow; Groundwater recharge; Helium; Hydrogeology; Hydrology; Hydrology/Water Resources; Islands; Isotopes; Saline water intrusion; Stable isotopes; Tritium; Waste Water Technology; Water Management; Water Pollution Control; Water Quality/Water Pollution; Watersheds
• ISSN: 1431-2174; 1435-0157
• DOI: 10.1007/s10040-009-0434-2

Stable isotopes (δ 18 O, δ 2 H), tritium ( 3 H), and helium isotopes ( 3 He, 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 4 He and 3 He/ 4 He ratios precluded 3 H/ 3 He dating at six sites. These high 3 He/ 4 He ratios (R/R 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 3 H/ 3 He dating shows that São Nicolau Island’s Ribeira Fajã Basin has groundwater residence times of more than 50 years, whereas Fogo Island’s Mosteiros Basin and Santo Antão 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.