Australia: The Land Where Time Began

A biography of the Australian continent 

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(SO_4­)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.

Sources & Further reading

Lee, S. (2001). Groundwater geochemistry and associated hardpans in southwestern Australia.