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Open File Report 160

Regional Geochemical Surveys: Riverina Pilot Project – Methodology and Preliminary

P. de Caritat, S. Jaireth, M. Lech and J. Pyke

During financial year 2003-04, we carried out fieldwork and preliminary analyses for a collaborative project between CRC LEME and Geoscience Australia aimed at testing methodologies and strategies for regional geochemical surveys in Australia. The Riverina bioregion, in southern New South Wales and northern Victoria, was selected for this pilot project. Overbank sediments were chosen as sampling media, with a near-surface sample (TOP: O horizon, from 0-10 cm below the humus layer) and a bottom sample (BOT: B-C horizon, ~10 cm interval between approximately 70-90 cm below the humus layer) being collected. River Red Gum (Eucalyptus camaldulensis) leaves were also collected at a small number of locations. The sample sites were selected to be near outlets or spill points of large catchments, so that overbank sediments there could reasonably be assumed to represent well-mixed, fine-grained composite samples of all major rock and soil types present in the catchment. Three field trips visited 143 sites in the Riverina. This report documents the methodologies used in this survey and presents results to date.

Sediment samples were subjected to a detailed description and the determination of bulk parameters in the field (texture, moist and dry colour, field pH). In the laboratory, the samples were analysed for moisture content, EC 1:5, pH 1:5; some were also subjected to laser particle size analysis. Bulk (<180 µm) composition was determined by X-Ray Fluorescence (XRF), Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Instrumental Neutron Activation Analysis (INAA), and the samples were also analysed by sequential digestion and prepared for heavy mineral separation. At the time of writing, bulk composition of the samples from the
first fieldtrip (eastern Riverina) only was available. All together the concentrations of 62 elements were determined, and maps are presented showing the spatial and statistical distributions in the TOP and BOT samples and of the TOP/BOT ratios. These results will be the basis for further interpretative work and the backbone of a forthcoming geochemical atlas of the Riverina.


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