Residence time distribution in a large unconfined-semiconfined aquifer in the Argentine?s Pampas using 3H/3He and CFCs tracers

MARTINEZ D.E.; FOURRE E.; QUIROZ LONDOÑO, M.; JEAN-BAPTISTE, P.; GLOK GALLI, M.; DAPOIGNY, A.; GRONDONA S.

HYDROGEOLOGY JOURNAL

SPRINGER

Lugar: Berlin; Año: 2016 vol. 24 p. 1107 - 1120

1431-2174

The Pampa region in Argentina includes vast unconfined-semiconfined aquifers that local economies depend on, but knowledge of their water resources is still not thorough. The Pampeano aquifer in Argentina covers around 1.5 million km2 in the Pampa plain. In order to achieve a better understanding of the hydrogeological system through the estimation of mean residence times (MRT), water samples were taken and analyzed for environmental tracers in 12 monitoring wells, drilled at three different depths in four locations. The concentrations of 3H, tritiogenic 3He and CFCs can be explained by mixtures of young waters adjusted to Exponential Piston Flow Models (EPM) or Dispersion Models (DM), and different proportions of tracer-free waters (dead water). The site located very close to the water divide shows a dominance of young waters: 85% of water best represented by a DM model with a MRT of 3 years. For the shallow levels in other sites, best fitting models result in a DM with MRT between 20 and 35 years, and proportions of dead waters between 40% and 60%. These results lead to important updates in the conceptual model of the Pampeano aquifer. Large proportions of dead waters at few meters depths can be the consequence of upward flows in a multilayered aquifer or diffusive retardation in the inter-bedded clay layers.