2007 Ground Water Summit

Wednesday, May 2, 2007 : 10:00 a.m.

Use of Environmental Tracers for Investigating Ground Water Systems in Cape Verde, West Africa

Victor M. Heilweil, U.S. Geological Survey

Environmental tracers were used to investigate ground-water basins on three islands of Cape Verde: Mosteiros (Fogo), Paul (Santo Antão), and Fajã (São Nicolau). The use of these tracers, including stable isotopes (oxygen/deuterium), radioactive isotopes (tritium), and dissolved gases (helium, neon, argon, krypton, xenon) facilitated the determination of ground-water recharge sources, recharge altitudes, and residence times. This information has been useful for evaluating susceptibility of wells and springs to anthropogenic contamination, the sustainability of planned ground-water development, and the feasibility of artificial recharge for enhancing water supplies. The similarity of stable isotope ratios in rain and ground water shows that most recharge occurs from direct infiltration of precipitation and is not affected by evaporation or geothermal processes. This indicates a minimal influence of infiltration from standing water bodies, including existing surface-water retention/recharge infrastructures. Dissolved-gas thermometry shows that most water-table recharge occurs at high altitudes (900 to more than 2,000 m) within each basin, which is relevant for the conceptual understanding of volcanic island hydrology. It also has practical implications related to the location and effectiveness of future artificial recharge structures. Tritium concentrations and tritium/helium age dating show that Fajã Basin has generally old (pre-1950s recharge) ground water, whereas Mosteiros and Paul basins contain some young (post-1950s recharge) ground water. This young recharge indicates active replenishment and potential for short-term enhancement of water resources through artificial recharge, as well as possible susceptibility to surface contamination. One unforeseen age-dating limitation was the presence of mantle-derived helium – a consideration when using the tritium/helium method in volcanically active terrains. This study illustrates how a relatively small environmental tracer data set can be cost-effective for evaluating ground-water conditions for sustainable water management solutions, particularly in areas where well penetration and conventional water-quality data are limited.

The 2007 Ground Water Summit