Geology
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The Layered Geology of Australia web map service is a seamless national coverage of Australia’s surface and subsurface geology. Geology concealed under younger cover units are mapped by effectively removing the overlying stratigraphy (Liu et al., 2015). This dataset is a layered product and comprises five chronostratigraphic time slices: Cenozoic, Mesozoic, Paleozoic, Neoproterozoic, and Pre-Neoproterozoic. As an example, the Mesozoic time slice (or layer) shows Mesozoic age geology that would be present if all Cenozoic units were removed. The Pre-Neoproterozoic time slice shows what would be visible if all Neoproterozoic, Paleozoic, Mesozoic, and Cenozoic units were removed. The Cenozoic time slice layer for the national dataset was extracted from Raymond et al., 2012. Surface Geology of Australia, 1:1 000 000 scale, 2012 edition. Geoscience Australia, Canberra.
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A novel methodology has been developed to assess potential impacts of managed aquifer recharge (MAR) and/or groundwater extraction on groundwater dependent ecosystems (GDEs) in the Darling River floodplain. Potential negative impacts on GDEs include: (1) lowering of watertables in response to groundwater extraction, with the potential for enhanced leakage from lakes and rivers; and (2) raising of the watertables as a result of enhanced recharge. The latter has the potential to lead to saturation and/or salinisation of root zones through mobilisation of salts stored in the unsaturated zone. Depending on the magnitude of watertable fluctuations, type of vegetation, and degree of groundwater dependence, this can have significant implications for vegetation function. In this study, time-series Landsat data were used to identify groundwater dependent vegetation (GDV). Field-validated GDV maps were integrated with 3D sub-surface hydrogeological, hydrogeochemical and hydrodynamic data to characterise the hydrogeological system and understand controls on GDV distribution and condition. Particular attention was paid to the distribution and integrity of near-surface aquitards and identifying potential sites for inter-aquifer leakage and enhanced recharge. This paper reports on two sites: Jimargil and Lake Menindee. GDV at both sites (e.g. Eucalyptus camaldulensis (River Red Gum) and E.largiflorens (Black Box)), depends largely on groundwater in shallow unconfined aquifers. At the Lake Menindee site, remnant woody vegetation is concentrated along lake fringes, while the Jimargil site includes a narrow strip of riparian vegetation associated with the Darling River. At the Lake Menindee site there is a high degree of connectivity between the lake, shallow watertable, and target aquifer. Current management of lake levels impacts on GDVs at this site, and negative impacts are likely to be compounded by groundwater extraction and/or MAR. In contrast, connectivity between the shallow unconfined aquifer (and the Darling River) that sustain GDV communities at Jimargil, and the semi-confined target aquifer, is localised along faults and gaps in the intervening aquitards. Groundwater extraction and/or MAR in the deeper aquifer at the site would have minimal impacts on riparian and floodplain GDV.
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The keystone element of a system is one which is disproportionately important to the workings of that system relative to its size, abundance and/or distribution. In the Broken Hill Managed Aquifer Recharge (BHMAR) Project, previously unidentified faulting of the unconsolidated sediments beneath the Darling Floodplain, N.S.W., may be considered as such a keystone element, as it is a spatially discrete and subtle element of the regional hydrogeological system that is critical to the recharge of the underlying Pliocene aquifers, and consequentially vital to the viability of MAR and groundwater extraction options in the area. Initial inversions of a regional airborne electromagnetics (AEM) dataset revealed a multi-layered conductivity structure in the top 100m, broadly consistent with the hydrostratigraphy identified in a sonic drilling program. However, initial laterally and spatially constrained inversions showed only moderate correlations with ground data in the near-surface (~20m). As additional information from drilling and complementary hydrochemical and hydrodynamic studies became available, various horizontal and vertical constraints were trialled using a new Wave Number Domain Approximate Inversion procedure with a 1D multi-layer model and constraints in 3D. The resultant improved 3D conductivity model revealed that an important Pleistocene aquitard (Blanchetown Clay) confining the main target aquifer (Pliocene Calivil Formation), has an undulating top which is locally sharply offset. The interpreted top surface suggests that it has been affected by significant warping and faulting, as well as regional tilting due to basin subsidence or margin uplift. Overall, the aquitard top surface varies in elevation by 60m. Several of the sharp offsets in the conductivity layers are coincident with lineaments observed in LiDAR data, and with underlying basement faults mapped from airborne magnetic data. The recognition of neotectonics in this area was made possible through the acquisition of high resolution AEM data and the selection of appropriate horizontal and vertical constraints in inversion procedures. Prior to the structural features being mapped it had not been possible to explain apparently contradictory data, nor develop a plausible hydrogeological conceptual model.
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The Broken Hill Managed Aquifer Recharge (BHMAR) project has successfully mapped a multi-layered sequence of aquitards and aquifers, as well potential groundwater resource and managed aquifer recharge (MAR) targets, in the top 100m of the Darling Floodplain. Near-surface aquitards overlying the Pliocene target aquifers (fluvial Calivil Formation (CFm) and marine Loxton-Parilla Sands (LPS)), were identified initially as variably conductive layers in airborne electromagnetic (AEM) data, and validated by drilling and complementary borehole geophysical, textural, hydrogeological and hydrochemical studies. The stratigraphic unit underlying the Pliocene aquifers is the Miocene upper Renmark Group (uRG). Drilling and AEM data have confirmed this unit is present throughout the study area, deposited predominantly as thick muds. Facies and biofacies analysis suggests these muds were deposited on a low relief sedimentary plain with a high water table and numerous permanent water bodies, with relatively minor sand bodies deposited in narrow anastomosing fixed channel streams. Groundwater in the upper uRG is saline, and muddy sediments form a strongly conductive layer beneath the Pliocene aquifers. This is a much harder geophysical target than the upper confining aquitards, as the target lies at depths of 80-120m, which is near the depth resolution of the AEM system. Furthermore, there is little conductivity contrast between the Pliocene and uRG sediments except in areas where there is fresh groundwater in the former. Hydrochemical and hydrodynamic data shows that there is limited hydrological connection between the uRG and less saline Pliocene aquifers, except where the Pliocene is underlain by uRG channel sands. These channels are much narrower (10s to ~100m) and thinner (1 to 10m) compared with palaeochannels in the overlying CFm. Where the channels are connected, there can be a distinct salinity gradient from the Pliocene into the uRG sands, indicating localised mixing. Given the potential for up-coning of saline groundwater in these instances, a number of sites (e.g. Menindee Common), have been assessed as unsuitable for MAR. Overall, the uRG muds act as a good lower confining aquitard to the Pliocene aquifers over most of the project area, including a number of potential MAR and groundwater resource targets.
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Nuclear Magnetic Resonance (NMR) tools have been used for decades by the oil industry to study lithological properties in consolidated sedimentary materials. Recently, slimline NMR borehole logging systems have been developed specifically for the study of near-surface (<100m) groundwater systems. In this study of unconsolidated fluvial sediments in the Darling River floodplain, data were acquired downhole every 0.5 m using a Javelin NMR tool. A total of 26 sonic cored bores were logged to a depth of ~70 m. Hydraulic conductivity (KNMR) can be estimated from the NMR measurements using the Schlumberger-Doll Research Equation: KNMR = C x -2 x T2ML2, where is the NMR porosity, T2ML is the logarithmic mean of the T2 distributions, and C is a formation factor related to tortuosity. To this end, the NMR data were classified into five hydraulic classes ranging from clay to gravely-coarse sand using the core, geophysical, mineralogical, and hyperspectral logs. Borehole slug tests were conducted to provide constraints on the K and T of the aquifers. Least-squares inversion was used to solve for the optimum C values versus the slug test derived T for the aquifer material (medium to gravely sand). Laboratory permeameter measurements helped constrain the C values of fine textured sediment. Comparisons between the geophysics derived KNMR and slug test KSlug indicated correspondence within two orders of magnitude. Investigations were also carried out to compare measurements of water content between laboratory determinations (oven drying of wet sediment at 105 oC) and that derived from NMR bore log data. A systematic decrease in ratio between the NMR total water and gravimetric water with fining of texture is observed. This is in part due to the inter-echo spacing of the NMR instrument (2.5 ms), which may be too large to detect hydroscopic moisture. Differences observed between NMR free water and gravimetric water within the sands requires further investigation, including the potential influence of iron phase coating of grains on fast relaxation responses. Overall, the borehole NMR method provides logging of near-continuous variations in K through a saturated sedimentary sequence, providing useful K estimates at increments not achievable using traditional aquifer testing, as well as K estimates for aquitard material.
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The Murray River is known to display great complexity in surface-groundwater interactions along its course, with 'gaining' sections of the river identified as sites of regional saline groundwater system discharge to the river and the adjacent floodplain. 'Losing' reaches of the river occur where river water infiltrates through the base of the river and recharges underlying aquifers and/or where adjacent aquifers are recharged through lateral bank infiltration. Recent studies have shown that recharge is not-steady state, with surface-groundwater processes promoted after river bank scouring during major flood events. 'Losing' reaches of rivers are hard to identify hydrochemically, while only airborne electromagnetic (AEM) methods provide 3D spatial mapping of salinity and hydrostratigraphy at depth beneath the river and across the floodplain. In 2007 a regional airborne electromagnetic (AEM) survey (24,000 line km @ 150m line-spacing in a 20 km-wide swath) was acquired along a 450 km reach of the Murray River in Victoria from Gunbower Island in the east to near the South Australian border. The AEM survey was calibrated and validated by drilling and complementary field mapping, and lithological and hydrogeochemical investigations. Holistic inversions of the AEM data were used to map key elements of the hydrogeological system and salinity extent in the shallow sub-surface (top 20-50 m). The survey successfully mapped key elements of the hydrogeological system including previously unmapped salinity discharge zones and significant losing 'flush' zones. Significant 'flush' zones to depths of 25m and up to 1.5 km in width have been identified at Turrumbarry Weir, with other significant zones identified in parts of Gunbower Forest, and between Liparoo and Robinvale. Elsewhere, flush zones are smaller, and occur at depths of 5-10m in narrower zones associated with locks, weirs and irrigation districts. Salt mobilisation associated with the flush zones at weir pools may be an issue in terms of salt load delivery to the River Murray and floodplain. Reaches of the river where the flush zones are absent and /or significantly constricted, and similar zones in tributary creeks in the adjacent floodplain, are at higher risk of saline groundwater inflows.
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