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

Noble gases and coupled heat/fluid flow modeling for evaluating hydrogeologic conditions of volcanic island aquifers

“…High N 2 / 40 Ar ratios, low pH, and elevated P CO2 (causing elevated total-dissolved gas pressure) have been previously associated with actively circulating geothermal fluids (Chiodini et al, 1995;Dallai et al, 2005;Wood, 2006). In Cape Verde, low N 2 / 40 Ar ratios of less than 45, relatively neutral pH, and ambient total-dissolved gas pressures confirm that dissolved noble gases are primarily of atmospheric origin, rather than from mantle outgassing (Heilweil et al, 2009). Because atmospheric dissolved gases are relatively inert and there is no evidence of in situ gas production or gas stripping in collected noble-gas samples, it is assumed these gases remain relatively unchanged during groundwater transit to well and spring sampling sites.…”

“…Groundwater stable-isotope concentrations indicate a relatively high-altitude precipitation source. Samples collected in both 2005 (Heilweil et al 2009) and 2009 (Table 2) have d 18 O values ranging from À5.1‰ to À3.4‰. These values are on the more depleted end of the À0.5‰ to À4.8‰ range of previously published Cape Verde precipitation data (Barmen et al, 1990;Heilweil et al, 2009).…”

“…Previous groundwater studies of Fogo have hypothesized the presence of relatively high-altitude regional water tables beneath the volcanic summits and calderas (Kallrén and Schreiber, 1988;Barmen et al, 1990;Heilweil et al, 2009). However, the steep topography, high drilling costs, and limited road access have precluded the drilling of deep exploratory wells in the upland parts of Fogo, so little is known about groundwater conditions in the interior of the volcanic edifice.…”

“…Samples collected in both 2005 (Heilweil et al 2009) and 2009 (Table 2) have d 18 O values ranging from À5.1‰ to À3.4‰. These values are on the more depleted end of the À0.5‰ to À4.8‰ range of previously published Cape Verde precipitation data (Barmen et al, 1990;Heilweil et al, 2009). Tritium concentrations range from 0.0 to 1.2 tritium units (TU), indicating a mix of pre-modern (prior to the 1950s) and modern groundwater.…”

Dating of ‘young’ groundwaters using environmental tracers: advantages, applications, and research needs

“…The application of environmental tracers for groundwater dating is well established as indicated by the large number of journal publications. Recent case studies using tracer-derived groundwater ages are focussed on a variety of applications including groundwater sources, flow paths, and mixing relations [17][18][19]; groundwater recharge processes [20][21][22]; groundwater flow and geochemical evolution [23]; groundwater resources evaluation [24,25]; groundwater contamination with nitrate [1,26], sulphate [27], arsenic [28,29], and organic compounds [30]; groundwater quality trends [3,31]; weathering rates [32]; groundwater/surface water interaction [33]; urban pollution [34]; physics of gas exchange in porous media [35]; and catchment run-off processes [36]. A statistical evaluation of these applications and their related tracers, based on published case studies over the last 10 years, is shown in Figure 2 (see figure caption for details).…”