Multi-scale characterisation of coastal sand aquifer media for contaminant transport using X-ray computed tomography |
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| Authors: | R Dann M Turner M Close and M Knackstedt |
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| Institution: | (1) Institute of Environmental Science and Research (ESR), PO Box 29-181, Christchurch, New Zealand;(2) Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Australian National University, ACT, Canberra, Australia |
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| Abstract: | To be able to predict contaminant transport in groundwater, an accurate conceptual and physical understanding of aquifer properties
at multiple scales is required. In this study, physical and hydraulic properties of a coastal sand aquifer were derived using
micro and macro X-ray computed tomography (XCT) techniques. Qualitative and quantitative data improved conceptualisation of
the aquifer structure at micro and macro scale. At the macro scale (50-mm diameter by 1,500-mm long core) XCT images of undisturbed
drill core identified coarse grained laminae (not obvious to the naked eye) of increased porosity and permeability, variations
in mineral assemblage and particulate organic matter distribution within the core. Micro-XCT analysis (16-mm diameter cores)
of the three main aquifer layers provided quantitative micro-scale data on permeability, porosity, grain, pore and throat
size distribution statistics, and grain sphericity. Ratios of mean pore to grain diameter were ~0.65–0.75 and ratios of mean
throat to mean grain diameter were ~0.2. Multiple permeability and porosity values were derived from micro domains (~4.35 mm3) within each micro-XCT core. Permeability values varied between and within micro-XCT core samples reflecting the heterogeneity
at the millimetre core scale in these sediments. Sphericity values were similar for all three layers (average ~0.4) which
reflected SEM observations of the semi-spherical nature of the dominant quartz and feldspar grains. The results of this study
indicate that models based on the assumptions of homogeneity in depositional structure below centimetre scale may not suitably
address factors affecting flow and transport of contaminants. |
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