<div>This report presents key results from the Upper Darling River Floodplain groundwater study conducted as part of the Exploring for the Future (EFTF) program in north-western New South Wales. The Australian Government funded EFTF program aimed to improve understanding of potential mineral, energy, and groundwater resources in priority areas for each resource.</div><div><br></div><div>The Upper Darling River Floodplain study area is located in semi-arid zone northwest New South Wales is characterised by communities facing critical water shortages and water quality issues, along with ecosystem degradation. As such, there is an imperative to improve our understanding of groundwater systems including the processes of inter-aquifer and groundwater-surface water connectivity. The key interest is in the fresh and saline groundwater systems within alluvium deposited by the Darling River (the Darling alluvium - DA) which comprises sediment sequences from 30 m to 140 m thick beneath the present-day floodplain.</div><div><br></div><div>The study acquired airborne, surface and borehole geophysical data plus hydrochemical data, and compiled geological, hydrometric, and remote sensing datasets. The integration of airborne electromagnetic (AEM) data with supporting datasets including surface and borehole magnetic resonance, borehole induction conductivity and gamma, and hydrochemistry data has allowed unprecedented, high resolution delineation of interpreted low salinity groundwater resources within the alluvium and highly saline aquifers which pose salination risk to both the river and fresher groundwater. Improved delineation of the palaeovalley architecture using AEM, seismic, and borehole datasets has permitted interpretation of the bedrock topography forming the base of the palaeovalley, and which has influenced sediment deposition and the present-day groundwater system pathways and gradients.</div><div><br></div><div>The integrated assessment demonstrates that the alluvial groundwater systems within the study area can be sub-divided on the basis of groundwater system characteristics relevant to water resource availability and management. Broadly, the northern part of the study area has low permeability stratigraphy underlying the river and a generally upward groundwater gradient resulting in limited zone of freshwater ingress into the alluvium around the river. A bedrock high south of Bourke partially restricts groundwater flow and forces saline groundwater from deeper in the alluvium to the surface in the vicinity of the Upper Darling salt interception scheme. From approximately Tilpa to Wilcannia, sufficiently permeable stratigraphy in hydraulic connection with the river and a negligible upward groundwater gradient allows recharge from the river, creating significant freshwater zones around the river within the alluvium.</div><div><br></div><div>Hydrometric and hydrochemical tracer data demonstrate that the alluvial groundwater systems are highly coupled with the rivers. Results support the conceptual understanding that bank-exchange processes and overbank floods associated with higher river flows are the primary recharge mechanism for the lower salinity groundwater within the alluvium. When river levels drop, tracers indicative of groundwater discharge confirm that groundwater contributes significant baseflow to the river. Analysis of groundwater levels and surface water discharge indicates that the previously identified declining trends in river discharge are likely to produce the significant decline in groundwater pressure observed across the unconfined aquifer within the alluvium. Improved quantification and prediction of groundwater-surface water connectivity, water level and flux is considered a high priority for both the Darling River and the wider Murray–Darling Basin. This information will assist in understanding and managing water resource availability in these highly connected systems, and enhance knowledge regarding cultural values and groundwater dependent ecosystems (GDEs).</div><div><br></div><div>This study identifies several aquifers containing groundwater of potentially suitable quality for a range of applications in the south of the study area between Wilcannia and Tilpa and assessed the geological and hydrological processes controlling their distribution and occurrence. Potential risks associated with the use of this groundwater, such as unsustainable extraction, impacts on GDEs, and saline intrusion into aquifers or the river, are outside the scope of this work and have not been quantified.</div>

Groundwater-dependent ecosystems (GDEs) rely on access to groundwater on a permanent or intermittent basis for some or all of their water requirements (Queensland Government, 2018). Remotely sensed data from Digital Earth Australia (DEA) (Geoscience Australia, 2018) were used to map potential aquatic and other GDEs and enhance understanding of surface water – groundwater interactions in the Upper Burdekin region. Two Landsat TM satellite products (Water Observations from Space (WOfS; Mueller et al. 2016) summary statistic and Tasselled Cap Index (TCI) wetness summary)) were used to investigate the persistence of surface water and soil moisture in the landscape to identify perennial streams, springs and other parts of the landscape that may rely on groundwater discharge. The WOfS summary statistic represents, for each pixel, the percentage of time that water is detected at the surface relative to the total number of clear observations. Due to the 25-m by 25-m pixel size of Landsat data, only features at least 25 m wide are detected and only features covering multiple pixels are consistently detected. The WOfS summary statistic was produced over the McBride and Nulla Basalt provinces for the entire period of available data (1987 to 2018). Pixels were polygonised and classified in order to visually enhance key data in the imagery, such as the identification of standing water for at least 80% of the time. The TCI is a method of reducing six surface reflectance bands of satellite data to three bands (Brightness, Greenness, Wetness) using a Principal Components Analysis (PCA) and Procrustes' Rotation (Roberts et al., 2018). The published coefficients of Crist (1985) are applied to DEA's Landsat data to generate a TCI composite. The resulting Tasselled Cap bands are a linear combination of the original surface reflectance bands that correlate with the Brightness (bare earth), Greenness and Wetness of the landscape. The TCI wetness summary (or Tasselled Cap Wetness (TCW) percentage exceedance composite), derived from the Wetness band, represents the behaviour of water in the landscape, as defined by the presence of water, moist soil or wet vegetation at each pixel through time. The summary shows the percentage of observed scenes where the Wetness layer of the Tasselled Cap transform is above the threshold, i.e. where each pixel has been observed as ‘wet’ according to the TCI. Areas that retain surface water or wetness in the landscape during the dry season are potential areas of groundwater discharge and associated GDEs. The TCW threshold is set at -600 to calculate the percentage exceedance. This threshold is based on scientific judgment and is currently in the research/testing phase. It is based on Australian conditions and conservative in nature. The dry season, when surface runoff to streams and rainfall are minimal, is particularly useful for identifying and mapping groundwater-fed streams, springs and other ecosystems that rely on access to groundwater during periods of limited rainfall. The Upper Burdekin region was especially dry between May and October 2013, with low rainfall totals in the months preceding this dry season and overall below-average rainfall conditions (i.e. decline in rainfall residual mass). The TCW exceedance composite was classified into percentage intervals to distinguish areas that were wet for different proportions of time during the 2013 dry season. Field validation of the remote sensing data products would be required to confirm the preliminary identification of parts of the landscape where groundwater discharges to the surface and potentially supports GDEs. This release includes the classified WOfS summary statistic and classified TCW percentage exceedance composite (May-October 2013) data products for the McBride and Nulla basalt provinces in the Upper Burdekin region, North Queensland. <b>References: </b> Crist EP (1985) A TM Tasseled Cap equivalent transformation for reflectance factor data. Remote Sensing of Environment 17(3), 301–306. Doi: 10.1016/0034-4257(85)90102-6. Geoscience Australia (2018) Digital Earth Australia. Geoscience Australia, http://www.ga.gov.au/dea. Mueller, N., Lewis, A., Roberts, D., Ring, S., Melrose, R., Sixsmith, J., Lymburner, L., McIntyre, A., Tan, P., Curnow, S. and Ip, A. (2016) Water observations from space: Mapping surface water from 25 years of Landsat imagery across Australia. Remote Sensing of Environment 174, 341-352, ISSN 0034-4257. Queensland Government (2018) Groundwater dependent ecosystems, WetlandInfo 2014. Queensland Government, Brisbane, https://wetlandinfo.des.qld.gov.au/wetlands/ecology/aquatic-ecosystems-natural/groundwater-dependent/. Roberts D, Dunn B and Mueller N (2018) Open Data Cube Products Using High-Dimensional Statistics of Time Series. International Geoscience and Remote Sensing Symposium. Valencia, Spain: IEEE Geoscience and Remote Sensing Society.