ENVIRONMENTAL TRACERS FOR ELUCIDATING VADOSE-ZONE AND GROUNDWATER PROCESSES IN THE VOLCANIC CAPE VERDE ISLANDS

Cape Verde is an archipelago of volcanic islands located off the west coast of Africa. Because of limited precipitation, steep topography, and permeable basalt, surface water resources are limited. Aquifers provide the majority of water for agriculture, industry, and consumption. There is a need to develop additional groundwater, but surface infiltration of contaminants or saltwater intrusion caused by increased pumping may cause water-quality degradation. An environmental tracer study assessed groundwater conditions in selected watersheds of Cape Verde to quantify groundwater recharge and residence times. Water sampling and analysis included major- and trace-ion geochemistry, tritium, carbon-14, stable isotopes of hydrogen, oxygen, and carbon, and dissolved CFCs, SF₆, and noble gases. These tracers add insight into vadose-zone and groundwater processes.

For the Mosteiros Basin on the Island of Fogo, both stable isotope and noble gas data indicate that the majority of groundwater recharge occurs along the upper slopes and within volcanic calderas in the watersheds. Groundwater moves toward the lower parts of each watershed, where it discharges to wells, springs, streams, tunnels, and as submarine outflow. Recently collected data generally support earlier published interpretations, which indicate groundwater ages from pre-1950s to modern. While CFC-11 appears to be degraded, CFC-12 and CFC-113 ages generally agree and indicate binary mixing of about 60 percent young (1980s) recharge with pre-1950s water. SF₆ concentrations, adjusted to account for a terrigenic excess, are consistent with CFC-12 and CFC-113 data. Low tritium concentrations at some sites, however, indicate gas exchange with modern air and long unsaturated zone travel times. This is confirmed by noble-gas thermometry, indicating that recharge to some wells entered the water table under warmer conditions than at other sites, consistent with a deeper vadose zone and associated geothermal gradient. Stable carbon isotope data separately indicate a mixture of magmatic and soil-gas CO₂ dominated by C-4 plants characteristic of arid environments. Although carbon-14 was not useful for dating the pre-bomb groundwater component, bomb-pulse carbon-14 provided a stable isotope tracer of recent recharge in conjunction with CFC and SF₆ data.