John Wilford

The Radiometric Map of Australia

Geoscience Australia and the Australian State and Territory Geological Surveys have systematically surveyed most of the Australian continent over the past 40 years using airborne gamma-ray spectrometry to map potassium, uranium and thorium elemental concentrations at the Earth’s surface. However, the individual surveys that comprise the national gamma-ray spectrometric radioelement database are not all registered to the same datum. This limits the usefulness of the database as it is not possible to easily combine surveys into regional compilations or make accurate comparisons between radiometric signatures in different survey areas. To solve these problems, Geoscience Australia has undertaken an Australia-Wide Airborne Geophysical Survey (AWAGS), funded under the Australian Government’s Onshore Energy Security Program, to serve as a radioelement baseline for all current and future airborne gamma-ray spectrometric surveys in Australia. The AWAGS survey has been back-calibrated to the International Atomic Energy Agency’s (IAEA) radioelement datum. We have used the AWAGS data to level the national radioelement database by estimating survey correction factors that, once applied, minimise both the differences in radioelement estimates between surveys (where these surveys overlap) and the differences between the surveys and the AWAGS traverses. The database is thus effectively levelled to the IAEA datum. The levelled database has been used to produce the first ‘Radiometric Map of Australia’ – levelled and merged composite potassium (% K), uranium (ppm eU) and thorium (ppm eTh) grids over Australia at 100 m resolution. Interpreters can use the map to reliably compare the radiometric signatures observed over different parts of Australia. This enables the assessment of key mineralogical and geochemical properties of bedrock and regolith materials from different geological provinces and regions with contrasting landscape histories.

The pH of Australian soils: field results from a national survey

The pH is one of the fundamental soil properties governing nutrient availability, metal mobility, elemental toxicity, microbial activity, and plant growth. The field pH of topsoil (0–0.10 m depth) and subsoil (~0.60–0.80 m depth) was measured on floodplain soils collected near the outlet of 1186 catchments covering >6 Mkm2 (6 × 1012 m2) or ~80% of Australia. Field pH duplicate data, obtained at 124 randomly selected sites, indicate a precision of 0.5 pH unit (or 7%), and mapped pH patterns are consistent and meaningful. The median topsoil pH is 6.5, while the subsoil pH has a median of 7 but is strongly bimodal (6–6.5 and 8–8.5). In most cases (64%) the topsoil and subsoil pH values are similar; among the sites exhibiting a pH contrast, those with more acidic topsoils are more common (28%) than those with more alkaline topsoils (7%). The distribution of soil pH at the national scale indicates the strong controls exerted by precipitation and ensuing leaching (e.g. low pH along the coastal fringe, high pH in the dry centre), aridity (e.g. high pH where calcrete is common in the regolith), vegetation (e.g. low pH reflecting abundant soil organic matter), and subsurface lithology (e.g. high pH over limestone bedrock). The new data, together with existing soil pH datasets, can support regional-scale decision-making relating to agricultural, environmental, infrastructural, and mineral exploration decisions.

Hydrogeological Landscapes framework: a biophysical approach to landscape characterisation and salinity hazard assessment

In Australia, salinity has the potential to affect up to 17million hectares of agricultural and pastoral land. For many degraded sites, biophysical hazards are often poorly understood and consequently poorly managed. Attempts to remediate areas affected by salinity have met with varying degrees of success. The New South Wales (NSW) Office of Environment and Heritage, NSW Department of Primary Industries, University of Canberra and Geoscience Australia have collaborated to develop a biophysical expert-based approach for the assessment and management of salinity within landscapes. The Hydrogeological Landscape (HGL) framework provides a structure for understanding how salinity manifests in the landscape, how differences in salinity are expressed across the landscape and how salinity may best be managed. The HGL framework merges the flow dynamics of the groundwater flow system with the landscape elements of the soil landscape or regolith landform approaches. This is the first approach to specifically address all three manifestations of salinity: land salinity, in-stream salt load and in-stream salt concentration. The HGL framework methodology recognises the interplay between surface and subsurface flow systems, as well as the capacity for water to interact with salt stores in the landscape, and identifies biophysical landscape characteristics (e.g. amount and type of vegetation cover, typical land use practice) that affect these interactions. The HGL framework is an expert system that integrates the spatial variability of landscape characteristics and salinity processes to produce a salinity hazard assessment for any given area.