groundwater
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Legacy product - no abstract available
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A major concern for regulators and the public with geologically storage of gas is the potential for the migration of gas (e.g. CO2) via a leaky fault or well into potable groundwater supplies. Given sufficient CO2, an immediate effect on groundwater would be a decrease in pH which could lead to accelerated weathering, an increase in alkalinity and the release of major and minor ions. Laboratory and core studies have demonstrated that on contact with CO2 heavy metals can be released under low pH and high CO2 conditions (particularly Pd, Ni and Cr). There is also a concern that trace organic contaminants could be mobilised due to the high solubility of many organics in supercritical CO2. These scenarios could potentially occur under a high CO2 leakage event but a small leak might be barely perceptible yet could provide an important early warning for a subsequent and more substantial impact. Different approaches are required for the detection and quantification of these low level leaks and are the subject of this paper. A 3 year groundwater survey was recently completed in the Surat Basin, which forms part of the Triassic-Cretaceous, Great Artesian Basin (GAB) aquifer sequence. In addition to a comprehensive water and isotopic analysis of samples from groundwater wells, gases were collected from groundwater samples and analysed for composition, '13CCO2, '13CCH4 and '2HCH4. Methane is prevalent in the major aquifers in the Surat Basin (e.g. Mooga, Gubbermunda and Hutton sandstones) and is invariably associated with a bacterial (methanogenic) carbonate reduction source, evident from its isotopic signature ('13CCH4 ~ -70', '2HCH4 ~ 220'). In addition to methane and low levels of CO2, trace levels of ethane are often detected.
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A new integrated, multi-scale geophysical and hydrogeological approach has been used to map shallow (<100m) aquitards in alluvial sediments beneath the Darling River floodplain. The 3D mapping approach uses a regional-scale airborne electromagnetics survey over an area of 7,500km2, integrated with targeted ground electrical surveys and borehole lithological, geophysical (induction, gamma and NMR), hydrogeological and hydrogeochemical data obtained from a 100 borehole (7.5km) sonic drilling program. This multi-scale mapping approach has confirmed the near-ubiquitous presence of a relatively thin (5-10m) Blanchetown Clay overlying Pliocene fluvial sediments containing the principal aquifer of interest (Calivil Formation). The aquitard properties of the Blanchetown Clay are demonstrated by hydrograph responses in overlying and underlying aquifers, by wetting profiles observed in drillcore, moisture data obtained from cores, NMR and gamma logging, laboratory permeameter measurements on cores, and hydrogeochemical data. This integrated mapping approach has revealed variations in aquitard extent and thickness, with a complex sub-surface distribution. Variations in the elevation of the top of the Blanchetown Clay (20-80m AHD ) are attributed partly to neotectonics. The study has also revealed that the aquitard forms a major barrier to recharge and discharge. Where absent (through erosion by the Darling River system, non-deposition, or facies change to fluvial sand), local recharge has resulted in previously unrecognised resources of fresh to slightly brackish water in the underlying semi-confined aquifer. Where present, it could form an effective cap for managed aquifer recharge schemes. More broadly, this study highlights the importance of understanding detailed floodplain sedimentology to alluvial groundwater management.
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Legacy product - no abstract available
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Legacy product - no abstract available