This report documents the findings of the 'Northern Territory Coastal Plain: Mapping Seawater Intrusion (SWI) in Coastal Plain Aquifers Using Airborne Electromagnetic (AEM) Data' project. The principal objective of this project is to carry out an assessment of the potential for SWI to impact on aquifers within the Darwin Rural Water Control District (DRWCD). The project was funded by the National Water Commission (NWC), with significant in-kind resources and funding provided by Geoscience Australia (GA) and the Northern Territory Department of Resources, Environment, Tourism, Arts and Sports (NRETAS). The project entailed acquisition of regional AEM data over the priority areas within the DRWCD, small sonic and rotary mud drilling programs, borehole geophysical logging, groundwater sampling, laboratory analysis of pore fluids and groundwater samples, and integration, analysis and interpretation of results.

Demonstrates the application of modelling gamma-ray spectrometry and DEM for mapping regolith materials and in predicting salt stores.

This study reports the findings of salt store and salinity hazard mapping for a 20-km wide swath of the Lindsay - Wallpolla reach of the River Murray floodplain in SE Australia. The study integrated remote sensing data, an airborne electromagnetics (AEM) survey (RESOLVE frequency domain system), and lithological and hydrogeochemical data obtained from a field mapping and drilling program. Maps of surface salinity, and surface salinity hazard identified Lindsay and Wallpolla Islands, and the lower Darling Floodplain as areas of high to extreme surface salinity hazard. In the sub-surface, salt stores were found in general to increase away from drainage lines in both the unsaturated and saturated zones. Beneath the Murray River floodplain, salt stores in both unsaturated and saturated zones are high to very high (100 to 300t/ha/m) across most of the floodplain. Sub-surface salinity hazard maps (incorporating mapped salt stores and lithologies, depth to water table and the hydraulic connectivity between the aquifers), identify Lindsay and Wallpolla Islands; the northern floodplain between Lock 8 and Lock 7; and northern bank of Frenchman's Creek as areas of greatest hazard. Overall, the new data and knowledge obtained in this study has filled important knowledge gaps particularly with respect to the distribution of key elements of the hydrostratigraphy and salinity extent across the Murray River and lower Darling floodplain. These data are being used to parameterise groundwater models for salinity risk predictions, to recalculate estimates of evapotranspiration for salt load predictions, address specific salinity management questions, and refine monitoring and management strategies.

The map shows regions favourable for calcrete-hosted uranium mineral systems. For a more detailed description of selection method see Jaireth et al. (2013).

The map shows salt lake regions favourable for potash deposits. For a more detailed description of selection method see Jaireth et al (2013)

Salinity of groundwater directly affects its suitability for different uses, including human consumption, stock water, agricultural use, and mineral or energy extraction. Traditionally, direct measurements of groundwater salinity at monitoring bores that intersect an aquifer have been used to map the spatial distribution of groundwater salinity. However, drilling is a logistically and economically challenging task, and we are usually left with a sparse set of measurements from which to infer groundwater salinity over large spatial extents. Airborne electromagnetic (AEM) sounding provides a solution to this problem. This is because AEM can be flown rapidly and cost-effectively over large swathes of land, and high subsurface bulk conductivities inferred from the AEM are well correlated with groundwater salinity in porous aquifers. We present here a methodology and case study from the Keep River Plains in the Northern Territory that provides information for land and watershed managers about the confidence with which salinity can be mapped over large areas using AEM. Extensive pore fluid sampling of the saturated zone, which lies beneath the watertable, enables this workflow to be used effectively. The results provided by our method can feed into decision making while accounting for uncertainty, enabling remote communities to manage their land and water resources effectively. <b>Citation:</b> Symington, N.,Ray, A., Harris-Pascal, C., Tan, K.P., Ley-Cooper, A.Y., and Brodie, R.C., 2020. Groundwater salinity estimation using borehole and AEM data: a framework for uncertainty analysis. In: Czarnota, K., Roach, I., Abbott, S., Haynes, M., Kositcin, N., Ray, A. and Slatter, E. (eds.) Exploring for the Future: Extended Abstracts, Geoscience Australia, Canberra, 1–4.

The map shows salt lake regions favourable for boron deposits. For a more detailed description of the selection method see Jaireth et al. (2013).

Groundwater is a critical resource for supporting human consumption, stock water, agricultural use, and mineral or energy extraction as well as the environment. However, the quality of groundwater varies enormously from potable to hyper-saline, particularly in the Australian context. To evaluate the suitability of a groundwater resource, the spatial distribution of salinity within an aquifer is typically estimated by measuring the electrical conductivity (EC) of groundwater samples from within boreholes. However, drilling is a logistically and economically challenging task, and hydrogeologists are usually left with a sparse set of measurements from which to infer groundwater salinity over large spatial extents. Airborne electromagnetic (AEM) surveying is a geophysical technique for estimating the bulk electrical conductivity of the near-surface. Where AEM bulk conductivity are well correlated with groundwater salinity in aquifers, AEM is a useful tool for modelling salinity in the data sparse areas between the boreholes. We present here a probabilistic method for modelling salinity and a case study from the Keep River Plains in the Northern Territory. Co-located probabilistic AEM inversions and EC measurements on pore fluids at coincident locations were fused to calculate an empirical joint probability density function. This function allowed us to estimate salinity away from the bores by sampling the ensemble of AEM conductivities. Unlike deterministic methods that provide a single estimate of salinity, our method generates an ensemble of estimates, which can be used to quantify predictive uncertainty. The results provided by our method can feed into decision making while accounting for uncertainty, enabling remote communities to manage their land and water resources more responsibly.

The role of palaeovalley evolution in the manifestation of salinity across the Australian continent P.M. English1 & J.W. Magee1, 2 1Geospatial & Earth Monitoring Division, Geoscience Australia, GPO Box 378, Canberra, ACT 2601 Pauline.English@ga.gov.au 2Department of Earth & Marine Sciences, Australian National University, Canberra, ACT 0200 jwmagee@ems.anu.edu.au Networks of palaeovalleys characterise much of arid, semi-arid and sub-humid Australia, including widespread rangelands and agriculturally important regions. These ancient river valley systems are very commonly the focus of salinity across our landscape, both primary salinity (Quaternary salt lakes and associated saline landforms), and secondary salinity in regions that have been cleared for agricultural development in the last 150 years. The preservation of palaeovalleys is a legacy of the continents tectonic stability, low relief and slow erosion and sedimentation rates. Palaeovalleys drain all the cratonic blocks and extend from Precambrian basement uplands across Palaeozoic, Mesozoic and Cainozoic sedimentary basins. They are now largely filled with Tertiary, or older, sediments and are commonly blanketed with Quaternary lacustrine, alluvial or aeolian sediments, including saline sediments and salts. While the palaeovalleys and their sedimentary infill are relatively ancient (early to mid Tertiary) the groundwater systems they host have evolved during latest Tertiary and Quaternary times. Salt began to concentrate in these palaeovalley networks and surrounding hinterlands from as long as 350 000 years ago, in response to glacial-interglacial cyclicity driven by global climate regimes. More recent salt accumulation in the historic period is the consequence of hydrologic disequilibrium wrought by land clearing which has caused broad-scale mobilisation of ancient salt stores into topographic lower areas and waterways in agricultural regions. Sodium chloride (NaCl) is the dominant salt in the Australian terrestrial environment, in both the arrays of ancient salt lakes and in secondary salinised agricultural regions. This reflects the dominance of marine aerosol source of salts in our rainfall and the longevity of salt accumulation in the near-surface environment. The marine signature is further pronounced in the case of primary salinity occurrences, where calcium sulphate, or gypsum (CaSO4.2H2O), is prevalent across the inland landscape. The ubiquity and solubility of NaCl and its mobility in shallow groundwater systems accounts for the recurrence and concentration of salinity in palaeovalley systems in particular. The persistence of ancient palaeovalley networks and of marine-derived salts in the salinised Australian landscape forms the basis of comparisons with southern Africa, for example, where tectonism and a significantly higher proportion of terrestrially-derived salts predispose contrasting manifestations of salinity. Such comparisons provide some insight and a broad perspective relevant to Australias contemporary salinity issues.

This study grew out of the GILMORE project, which is a pilot study designed to test methodologies and technologies for assessing mineral prospectivity and dryland salinity in areas of complex regolith cover (Lawrie, 1999; Apps et al., this volume). The project area is in central NSW, towards the eastern margin of the Murray-Darling Basin, straddling the catchments of the Lachlan and Murrumbidgee. The area can be divided into two parts, with creeks (eg. Bland creek) in the northern catchment flowing towrds the Lachlan River, and creeks in the southern catchments flow towards the Murrumbidgee River. The GILMORE project utilised a multi-disciplinary approach to generate a coherent picture of the various factors controlling the distribution of sediments and the saline groundwater. Thus, geomorphology and landscape processes (past and present), sedimentary environments, pedology, weathering and regolith formation, geophysical attributes of regolith materials and groundwater characteristics were all considered (Lawrie et al., 2000). The project has acquired a wide range of data, including airbone geophysics such as high-resolution magnetics, radiometrics and electromagnetics (AEM, TEMPEST system), as well as ground based (down-hole) geophysics, and information from 6600 drill holes of which 4000 have lithological and geological data (Apps et al., this volume). In addition, there is groundwater chemistry data of over 100 bore holes (Lawrie et al., 2000). Specifically, AEM surveys have been flown across areas 1 and 2 and AEM depth slice images have been utilised to establish the spatial pattern of conductivity and inferred distribution of saline groundwater and preferential groundwater pathways. This study aims to establish the association between mineral composition, lithology units and groundwater chemistry.