Environmental and anthropogenic influences on ambient background concentrations of potentially toxic elements in soils of Victoria Australia

Mikkonen, H 2018, Environmental and anthropogenic influences on ambient background concentrations of potentially toxic elements in soils of Victoria Australia, Doctor of Philosophy (PhD), Engineering, RMIT University.

Document type: Thesis
Collection: Theses

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Title Environmental and anthropogenic influences on ambient background concentrations of potentially toxic elements in soils of Victoria Australia
Author(s) Mikkonen, H
Year 2018
Abstract Estimation of background concentrations of potentially toxic elements  in soil is an essential part of contaminated land assessment for quantification of added contamination to soil. Due to the extensive number of sources of contamination to the environment and multiple environmental influences on soil variability, it is often difficult to distinguish areas of natural enrichment from areas of diffuse or point source contamination. In addition, current analytical methods have limited reliability for quantification of contamination verses natural enrichment. An understanding of the key contributing factors to enrichment of potentially toxic elements, in soil can support identification of areas of natural enrichment and therefore support land management decision making.

It is generally accepted that pristine background surface soils (i.e. comprising no added anthropogenic contamination and no evidence of anthropogenic impact) no longer exist due to the broad extent of low level diffuse anthropogenic contamination at the earth¿s surface. Therefore, the term ambient background is used to describe soil conditions in areas away from point sources of contamination. Ambient background is defined as the sum of the geogenic concentration of an element plus diffuse anthropogenic contamination that has been introduced to the environment from non-point sources. Non-point sources of contamination include wide-ranging deposition of metals associated with the burning of fossil fuels and broad application of fertilisers during typical agricultural practices.

Whilst low resolution soil surveys have been undertaken to assess the distribution of potentially toxic elements across much of the earth's land surface, few peer reviewed studies have assessed geochemical patterns and environmental controls of background concentrations of potentially toxic elements at the local scale (0.5-500 km2).

A survey of ambient background soils was undertaken in Greater Geelong, Ballarat, Mitchell and Greater Melbourne, Victoria, Australia. Samples were collected from 320 locations at 0.0-0.1 m and 0.3-0.6 m depth intervals, targeting soils overlying Quaternary aged basalt, Tertiary aged sediments and Silurian aged siltstones and sandstones. In addition, background soil data from open-source environmental assessment reports (n=5512) were collated to support assessment of the spatial distribution of natural enrichment and concentrations of potentially toxic elements at depths greater than 0.6 m.

A summary of expected ambient background concentrations of As, Cd, Cr, Cu, Co, F, Pb, Hg, Mn, Ni, V and Zn in key development areas of Victoria, is provided. Ambient background concentrations of As, F and Ni were naturally enriched at concentrations in exceedance of Victorian waste management guidelines for Clean Fill and greater than national median concentrations of these elements.

Factor analysis and spatial mapping, was used to identify potential environmental and anthropogenic controls of enrichment of potentially toxic elements in soils. Weathering during palaeoclimates (associated with laterization of soils) was identified as the key driver of enrichment of ambient background concentrations of potentially toxic elements in the assessed soils. The extent of soil weathering influenced geochemical ratios between metals and Fe; immobile elements such as Cr were enriched in highly weathered environments compared to Fe, whereas Ni and Cu were relatively depleted compared to Fe in highly weathered soils.

Multivariate regression equations and rule based regression tree models were developed for prediction of ambient background Cr, Cu, F, Ni and Zn concentrations in soil. Regression tree models were used to account for metal variability between different weathering units and allowed prediction of Ni concentrations in soils of Australia, beyond the bounds of the assessed soils. Whereas, a parent material specific model was most accurate for estimation of ambient background F and Cu, indicating that the variability in mineralogy between soils weathered from different parent materials was not captured by a single regression tree model. Models for estimation of ambient background Zn were poor in surface soils, which was suggested to be due to the presence of variable levels of diffuse Zn contamination and/or variability due to biological cycling of nutrients, including Zn.

Arsenic enrichment was identified in localised areas overlying Tertiary sediments and Silurian siltstones and sandstones. Enrichment of As and V in Fe rich Tertiary sediments of Melbourne, was assessed using chemical extraction methods, micro focused X-Ray fluorescence and X-ray Absorption Near Edge Spectroscopy. Arsenic was enriched in Fe stones at up to 60 times As concentrations in surrounding soils, and 1000 times average As concentrations in world soils. In both soil and Fe stones, As was distributed with goethite as arsenate. Iron bound As was concluded to be relatively immobile under the current oxic conditions.

Micro focused X-Ray fluorescence revealed the presence of V on the outer edge of the assessed Fe stone. The enrichment of V on the outer edge of the Fe stone, was inferred to reflect differences in As and V solubility, with immobilisation of As with Fe indicated to have occurred prior to V. This research highlighted the key role of Fe in metal enrichment and immobilisation of As and V in soils.

This research provides an understanding of the distribution of ambient background concentrations of potentially toxic elements in Victorian soils. This research has highlighted the natural enrichment of As, F, Ni and Cu in some Victorian soils, identified environmental processes underlying enrichment of these elements and provided methods for predicting soils likely to be enriched in As, F, Ni and Cu. Consistent with international studies, Zn and Pb enrichment was reported in surface soils. However, the cause of Zn and Pb surface enrichment (whether it is due to natural cycling or diffuse contamination) remains uncertain.

The findings of this research have been published in six peer reviewed papers, and on an interactive website. The results of this research can be used to support improved soil management decisions during environmental assessment including soil categorisation (for the purpose of waste management) and risk assessment.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Engineering
Subjects Soil Chemistry (excl. Carbon Sequestration Science)
Environmental Management
Environmental Engineering not elsewhere classified
Keyword(s) background
geochemical indices
natural enrichment
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