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Australia: The Land Where Time Began |
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Groundwater Geochemistry and Associated hardpans in Southwestern
Australia
The mineralogy and geochemistry of pedogenic and
groundwater silcretes
and ferricretes and associated groundwater geochemistry were
investigated for deeply weathered, highly kaolinised regolith in
southwestern Australia. Hardpans are formed by
silcretes and ferricretes
that impede drainage of the highly saline, acidic, shallow groundwaters
in lower valleys of the subdued, ancient landscapes. These valleys are
associated with groundwater discharge and the formation of saline seeps,
in which there are a variety of secondary minerals that include
amorphous silica [opal-CT], goethite [α-FeOOH], haematite [α-Fe2O3],
akaganeite [β-FeOOH], jarosite [KFe3(SO4)2OH6],
barite [BaCO4], gypsum [CaSO4.2H2O],
and halite [NaCl].
Results and Discussion
Abundant silicified and ferruginised pseudomorphs of plant roots are
present in the shallow hardpans that underlie the saline seeps and this
indicates that biogeochemical processes were involved in the removal of
groundwater and concentration of silicon and iron. It is common to find
roots in silcrete that have remarkable preservation of cellular
structures by SiO2 that is almost pure. It may be indicated
by the presence of silcrete lenses at great depths that deep root
systems (e.g. eucalyptus species) have penetrated to a water table that
was much lower than the depths at present or that subsequent
sedimentation has buried these lenses. Lee suggests the former is more
likely as a result of the possible root channel between depths of 20-30
m in the silicified granite pallid zone.
Al2O3/SiO2 ratios that were obtained
from SEM electron microprobe analyses were used for the quantification
of amorphous silica in the fine-gained kaolin matrix. This procedure
enabled mapping at the micrometre scale of amorphous silica in silcrete.
Observations of a broad opal-CT XRD band that was centred at
approximately 4Å and a brown, isotropic matrix that was observed from
optical microscopy supported the recognition of abundant amorphous
silica.
The homogeneous cluster of data points in the dissolved silicon vs pH
diagram is divided into a group of clusters when silicon is plotted
against EC, when the dissolved silicon data for all field sets are
combined. It was shown by multiple regression of EC and pH on dissolved
concentrations of silicon that pH and EC can explain most of the
variation in silicon. Higher
concentrations of dissolved silicon than the relatively higher pH and
more saline groundwater (pH 6 to7.18) are contained in the lower pH and
less saline groundwaters (between 2.8 to 3.99). It may be implied by
this association that the concentration in groundwater of silicon is
controlled by both pH and EC.
Prediction of the saturation state of minerals was enabled by
geochemical modelling using PHREEQC (Parkhurst & Apello, 1999) and
indicated dissolution and precipitation reactions occurring in the
regolith-groundwater environment. Groundwaters that were acid and
moderately saline were in near equilibrium with respect to amorphous
silica, while groundwaters that were near neutral pH and saline
corresponded to undersaturated conditions. For groundwaters that were
extremely acid (pH<3.5), PHREEQC predicted dissolves kaolin, goethite
and haematite, which is reflected in the concentrations of iron that are
elevated (up to 169 mg/L) and aluminium (up to 389 mg/L) in groundwater.
Summary
Silcrete in regolith that consists of diverse kaolin-quartz minerals
that are indurated by opal-CT occurs commonly in Southwest Australia.
According to Lee the occurrence of this material is related to the
groundwaters that are acid and rich in silicon, and which pervade the
groundwaters in regolith in this region. The micrometric voids between
kaolin crystals are filled by opaline silica and to some extent may have
replaced kaolin, though this has not been proven. Lee suggests that
water used by plants is likely to be responsible for lenticular bodies
of silcrete that are continuous and discontinuous. The presence of
silcrete has major influences on hydrology, plant growth and the
development of salinity.
Lee, S. (2001). Groundwater geochemistry and associated hardpans in
southwestern Australia.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |