Hydrochemical studies in ground water of the Federal District: subsidies to the flow conceptual model

Abstract:

This paper refers to hydrochemical studies for a flow model characterization proposed to the Federal District region, Brazil. The “Two Potenciometric Surfaces Model” has been proposed by Campos & FreitasSilva (1988) and studied by other authors. Apply hydrochemical analysis of ¹⁸O, ³H e CFC to groundwater dating is the main goal. Isotopic analyses of ¹⁸O, ³H and gases CFC has been developed in shallow and deep groundwater samples for tree points located on Jardim River watershed. The δ¹⁸O values vary from -5,71‰ to -5,19‰, which is related to global mean atmospheric vapor and ocean rains, it shows that the groundwater was not submitted to significant process able to modify the isotopic composition. The values in tritium units (UT) were relatively high for shallow groundwater, indicating young ages. Groundwater in fracture flow showed low values, indicating old recharge. The CFC-11 e CFC-12 values corroborate the tritium interpretations, and turn feasible the quantitatively evaluation. Analyzing CFC-12 for the first point (EL1 - fracture aquifer and EL2 - shallow groundwater) was obtained similar ages, associated to 1990-1997 period. For the second point, the sample EL3 (fracture) and EL4 (porous), showed temporal variation of 20 years, associated to 1969-1989 period respectively. The values in EL5 (porous) and EL6 (fractured) samples, showed 15 years interval for groundwater entrance in an aquifer, related to 1974-1989 period. The results corroborate the groundwater flow model, considering that two available places exposed time interval between 15 and 20 years for water infiltration. This result converges for an existence of two distinct potenciometric levels, considering the significant age interval related to the required time to shallow groundwater infiltrate to deeper aquifers.

Keywords:
isotopes; groundwater; flow model