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

SummaryUnderstanding groundwater conditions in the upland parts of volcanic island aquifers is critical for sustainable groundwater development in these resource-limited environments. Yet groundwater conditions in such settings are generally difficult to characterize because of sparse well drilling (high cost and/or limited access). Information needed for resource evaluation includes upland depth to water, recharge rate, and aquifer permeability. In this study, noble-gas recharge temperatures and coupled heat/fluid flow modeling are used to indirectly infer these groundwater conditions. Mosteiros Basin on Fogo Island of the Cape Verde archipelago was selected as a representative volcanic island aquifer. Simulation results are calibrated to water-table altitude and temperature data, along with indirect information provided by noble and dissolved gases. Results of numerical modeling are most sensitive to recharge rates and hydraulic conductivity, less sensitive to basal heat flux, and not sensitive to porosity and thermal conductivity. Simulation results show that only a relatively narrow range of combined recharge values (12-25% of precipitation) and hydraulic conductivity (10−8-10−7 m s−1) is consistent with observed data. The simulated recharge of 3-6 million cubic meters per year (Mm3 yr−1) is much higher than measured discharge from the basin (0.25 Mm3 yr−1), indicating the occurrence of significant amounts of submarine groundwater discharge. The modeling results suggest a very deep upland water table located 600-1000 m beneath the floor of the central caldera, although the water table becomes more shallow and accessible towards the coast. Perhaps most significant is the dominant role that modest amounts of recharge (mean rates of 70-140 mm yr−1) plays in decreasing the geothermal gradient, resulting in relatively cool temperatures in the deep vadose and at the water table. Noble-gas recharge temperatures, constrained by numerical simulation results, range from about 15 to 22 °C and indicate a large water-table temperature lapse of about −15 °C/km, much steeper than typical adiabatic lapse rates.

► Noble-gas thermometry shows cool water tables in volcanic island aquifers. ► Innovative use of noble gases and coupled heat/fluid flow modeling. ► Modeling indicates deep (about 1 km) inland water table. ► Simulations indicate that even low recharge rates can dampen geothermal gradient.