Authors / CoAuthors
Lawrie, K. | Brodie, R.S. | Somerville, P. | Hostetler, S. | Magee, J.
Abstract
Multiple lines of evidence were used to understand recharge processes in shallow (<100m) unconsolidated alluvial sediments of the Darling River floodplain, NSW. Major-ion chemistry highlighted a mixing signature between river waters, the shallow unconfined aquifer and the underlying semi-confined Pliocene aquifers. The hydrostratigraphy and groundwater salinities were mapped using airborne electromagnetics (AEM), validated by drilling. The fresh near-river shallow groundwater has a modern carbon signature. The mounding of groundwater levels near the river indicates the regional significance of losing river conditions. Stable isotope data show that recharge is episodic and linked to high-flow flood events rather than river leakage being continuous. This is also evident when groundwater chemistry was compared with river chemistry under different flow conditions. Critically, rapid and significant groundwater level responses were measured during flood events. Continuation of rising trends after the flood peak receded suggests that this is an actual recharge response rather than hydraulic loading. Mud veneers and mineral precipitates are evident along the Darling River channel bank when river flows are low. During low flow conditions these act as impediments to river leakage. During floods, high flow velocities scour these deposits, revealing lateral-accretion surfaces in the shallow scroll plain sediments. This scouring allows lateral bank recharge to the shallow aquifer. During flood recession, mud veneers are re-deposited while return flows from bank storage results in carbonate precipitation in river banks. Recharge to the underlying Pliocene aquifer occurs through mapped faults and via erosional 'holes' in the confining aquitard. Mapped depressions in the river bed ('cod holes'), are floored by indurated clays, and do not provide preferential connectivity to the underlying aquifer. Such flow-dependent recharge has implications for groundwater assessment and management. For example, an analysis of historic river flows suggests that active recharge to the groundwater system would only occur for about 17% of the time when flow exceeds about 9,000 ML/d. Recharge would be negligible with groundwater extraction during low-flow conditions.
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75733
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- External PublicationAbstract
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Geology
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2013-01-01T00:00:00
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Abstract prepared for submission to the International Association of Hydrogeologists (IAH).
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