Estudos isotópicos em águas subterrâneas do Distrito Federal: subsídios ao modelo conceitual de fluxo

Abstract

This paper refers to hydrochemical studies for the characterization of a flow model proposed to the Federal District region, Brazil . The “Two Potenciometric Surfaces Model” has been proposed by Campos & Freitas-Silva (1988) and studied by other authors. The main goal of this paper is to apply hydrochemical analysis of 18 O, 3 H e CFC to groundwater dating. Isotopic analyses of 18 O, 3 H and CFC has been developed in shallow and deep groundwater samples in tree points on Jardim River watershed. The δ 18 O values vary from -5,71 ‰ to -5,19 ‰ , which is related to atmospheric vapor global mean and ocean rains, what 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 younger ages. Fracture flow groundwater showed low values, indicating older recharge. The evaluation of CFCs corroborate the tritium interpretations, and turn feasible the quantitatively evaluation. For the first point (EL1 – fracture aquifer and EL2 – porous shallow groundwater) was obtained similar ages, associated to 1986 – 1987 period, resulted from water mixing. For the second point, the sample EL3 (fracture) and EL4 (porous), showed temporal variation of 17, associated to 1968 – 1985 period respectively. The samples EL5 (porous) and EL6 (fractured), showed values that vary in a 14 years interval for groundwater entrance time in an aquifer, associated to 1972 – 1986 period. The results corroborate the groundwater flow model, considering that two available places exposed time interval between 14 and 22 years for water infiltration. This result converges for an existence of two distinct potenciometric levels, considering the significant age difference related to the required time to shallow groundwater infiltrate to deeper aquifers.