Australian Capital Territory Geochemical Urban Mapping (ACTGUM): Project Manual 1 – Sample Collection, Preparation and Total Element Content Analysis

The Australian Capital Territory Geochemical Urban Mapping (ACTGUM) project, a collaboration between the National Centre for Forensic Studies at the University of Canberra, Geoscience Australia (GA) and the Australian Federal Police, aims to enable high-quality research investigating the provenancing of topsoil in a law enforcement/intelligence context. Although many geochemical soil surveys already exist, these are typically developed primarily for exploration and related earth sciences, limiting their use for forensic applications that have specific, and often differing requirements (e.g. sampling density, sampling medium and depth, and data treatment). An important part of the project is the development of a high quality, internally consistent, geochemical dataset of topsoil, covering a model area of interest at a relatively high sampling density. Initiated in 2017, a soil geochemical survey has been completed, covering the urban and surrounding suburban areas of Canberra, Australian Capital Territory (ACT), in inland southeastern Australia. At present, a total of 823 topsoil samples (including field replicate samples at 65 sites) have been collected from 686 sites, including 6 resampled sites, which together cover over 650 km2 at the average sample density of 1 site per 1 km2. Samples were collected from urban and suburban public parks, nature reserves, roadside verges, nature strips, and private agricultural land. Obvious areas of anthropogenically introduced, modified or contaminated soils were avoided. At each site, a 1 – 2 kg composite of five subsamples taken from the corners and centre of a 1m x 1m area at a depth interval of 0 – 5 cm was collected, after removal of surface vegetation. All composite samples were subsequently air dried, homogenised and split into two halves. One half was set aside for archiving. The remainder was dry sieved to <75 μm, consistent with existing geochemical surveys in Australia, including the GA National Geochemical Survey of Australia and GA Northern Australian Geochemical Survey. Any excess sample/fraction was kept for potential future use. All subsequent analytical protocols were applied to the <75 μm fraction, and those performed at the GA inorganic laboratory include: (1) X-ray fluorescence (XRF) analysis of fused beads and loss on ignition (LOI) (total geochemistry); and (2) Inductively coupled plasma – mass spectrometry (ICP-MS) analysis of total digestions of fused bead fragments (total geochemistry). Additional and complimentary measurements have been completed elsewhere, including magnetic susceptibility, ICP-MS analysis of aqua regia digests, Munsell soil colour, and X-ray diffraction (XRD) analysis. These measurements have been discussed elsewhere and/or will be published separately. In the present project manual, we describe the: (1) The synopsis underpinning the broader project and determination of sampling sites; and (2) Detailed protocols for sample collection, preparation and analysis of total element content. Release and interpretation of the project dataset, quality assessments and geochemical maps will be published separately.