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Gas transport in a confined unsaturated zone during atmospheric pressure cycles
Author(s) -
Elberling Bo,
Larsen Flemming,
Christensen Steen,
Postma Dieke
Publication year - 1998
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/98wr02037
Subject(s) - vadose zone , atmosphere (unit) , atmospheric pressure , geology , aquifer , oxygen , groundwater , environmental science , atmospheric sciences , soil science , chemistry , geotechnical engineering , meteorology , oceanography , physics , organic chemistry
Gas transport induced by temporal pressure fluctuations in the atmosphere can be an important mechanism for transport of atmospheric oxygen within the unsaturated zone. Moreover, the presence of oxygen in the unsaturated zone may be a factor controlling oxidation of sulphide minerals and other redox processes. A field study was carried out in a glacial aquifer with a 10–12 m thick sandy unsaturated zone to explore gas exchange between the atmosphere and the unsaturated zone. The exchange occurs through a “geological window” in a till layer which covers the sandy unsaturated zone. Observed pressure distribution and oxygen concentrations within the unsaturated zone were compared to numerical simulations with SUTRA, a finite element and fluid density dependent groundwater flow model. The simulations were carried out by modeling the gas pressure distribution within the unsaturated zone based on atmospheric pressure time series. The spatial variation in permeability observed from borehole logging was implemented in the model. The analysis demonstrated a good match between the field observations and the numerical simulations. During an atmospheric pressure cycle, atmospheric oxygen migrated more than 10 md −1 horizontally in the capped unsaturated zone. The analysis shows that both the amplitude and the length of the period of pressure variations are important for the transport of oxygen, and it shows that the combined effects determine the extent of a subsurface zone where atmospheric oxygen can reach.