INTERPRETATION OF OXYGEN –18 ISOTOPE IN SULPHATE FROM DEEP GROUNDWATER IN JAKARTA AREA
ABSTRACT: It has been done a
determination of δ 18O (SO4 2-) and δ 18 O (H2O) value from Jakarta deep
groundwater with depth 40-140 m. The aim
of this research is to know some procesess influencing the composition of
oxygen isotope in groundwater sulphate. A method commonly used to determine δ
18 O (H2O) value is according to Epstein-Mayeda. CO2 gas resulted from equilibration process between water sample
and CO2 gas standard in which oxygen isotopic reaction has occurred, is
injected to mass spectrometer. For determination of δ 18O (SO42-) value, Rafter
method is used. CO2 gas released from reducing sulphate of water sample with
graphite is injected to mass spectrometer. The results of δ 18O (H2O) values
obtained in this experiment have a narrow range from -5,04 0/00 to
–6,65 0/00 SMOW whereas their δ18O (SO42-) values have a wider range
from +8,3 0/00 to +17,4 0/00 SMOW. The
more constant values of δ 18O (H2O) performed that evaporation effects
might not occur. Based on the similarity between δ18O (SO42-) values of deep groundwater and
that of marine evaporite sulphate rocks, it is supposed that sulphate of
Jakarta deep groundwater was derived from dissolution of this rocks. There was
an indication of seawater intrusion around Pejagalan and Kamal Muara
Penjaringan area based on the similarity between their δ18O (SO42-) values and δ18O (SO42-) of modern seawater. The contribution of oxygen from water in
sulphide oxidation reaction ranged 0% to 12% suggesting that oxygen in deep
groundwater sulphate was mainly derived from atmospheric molecular oxygen
Author: E. Ristin Pujiindiyati,
Wandowo, and Zainal Abidin
Journal Code: jpkimiagg070004
