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Accueil du site → Doctorat → Australie → 2013 → The distribution and mobilisation of salts in the Queensland Murray-Darling Basin

University of Queensland (2013)

The distribution and mobilisation of salts in the Queensland Murray-Darling Basin

Biggs, Andrew James Wyndham

Titre : The distribution and mobilisation of salts in the Queensland Murray-Darling Basin

Auteur : Biggs, Andrew James Wyndham

Université de soutenance : University of Queensland

Grade : Doctor of Philosophy (PhD) 2013

Résumé
Assessment of current and future salinity risk within the Queensland Murray-Darling Basin (QMDB) is essential to the protection of assets at both the local and Basin scale. This thesis explores aspects of salinity risk for the QMDB via using catchment salt balances and examines the level of complexity required in risk assessment.
The components of the salt mass balance and the balance itself (Export/Import ratio – E/I) are useful tools to encapsulate salinity processes within catchments. Salt inputs (atmospheric, anthropogenic, mineral weathering), salt stores (surface water, regolith, groundwater) and salt exports (streamflow, dust, deep drainage) have been evaluated for the QMDB in a level of detail never previously undertaken.
Rainfall salt accessions are more easily accounted than dust accessions, although the limited data available suggests the latter may be important in the very long-term. Rainfall salt inputs to the QMDB have been re-examined and a new generation of equations related to distance to coast have been developed. As with elsewhere in Australia, rainfall salt input declines exponentially with distance inland and is a major source of salts in the QMDB. Regolith/bedrock, groundwater, and streamflow ionic data suggest that mineral weathering is a relevant process creating salt inputs in some geologies in the QMDB, in particular eastern upland landscapes and to a lesser degree the Cretaceous geologies in the central and western QMDB. Further analysis of data is required however to achieve accurate quantification. Anthropogenic salt inputs have been calculated for the QMDB for the first time. Inputs via inter-basin water transfer, chlorination and fertiliser additions are negligible in the context of the QMDB, although inter-basin transfer is relevant in the Gowrie Creek catchment. Anthropogenic groundwater salt inputs are of a sufficient magnitude that they should be accounted for in catchment scale mass balance calculations – for example, in the Condamine catchment, groundwater salt inputs are potentially 4 times greater than atmospheric accessions. The expansion of the Coal Seam Gas (CSG) industry has the capacity to significantly increase non-cyclic groundwater salt accessions in the QMDB, in particular in the Condamine-Balonne catchments.
Salt stores in the QMDB have been evaluated using available data for surface water, groundwater and regolith materials. High regolith/groundwater salinity is common in the western two-thirds of the QMDB and large landscape salt stores are present. The conventional assumption that weathered zones in the landscape should be leached of salts is not borne out in either groundwater or regolith/bedrock data. Weathered zones may in fact have the highest salinity within a particular geological formation, although the relationship is not consistent. This likely reflects a change from historical (salt) transport unlimited conditions in the landscape during wetter climate phases, to the current transport limited conditions in a semi-arid climate i.e the accumulation of salts in weathered zones during drier climatic regimes post the period of weathering. Significant temporary surface water salt stores are held in the eastern half of the QMDB by virtue of large quantities of low salinity water being captured in dams, impoundments and other water bodies. While their role as a salt store is limited, these represent salt retained in the landscape that otherwise may have been exported via stream flow and can increase salt inputs to the landscape via increased recharge.
Streamflow is the primary salt export mechanism in the QMDB, although calculation of the salt load is dependent upon high quality data – a challenge in the episodic flow regime experienced in the region. Losses through groundwater recharge in clay dominated landscapes such as the QMDB are naturally small, although under post-clearing land uses such as irrigation they can be large and should be further investigated and considered in salt balance calculations. Dust may be a significant salt transport/loss mechanism out of the region, particularly over long time scales, but its magnitude remains very difficult to quantify. Fire-related and biomass salt export mechanisms have never previously been quantified, but example calculations suggest they are likely to only be minor components of the salt balance. The role of coal export as a salt loss mechanism requires further examination, as it is rapidly increasing in the QMDB and the coal is frequently washed with CSG water.
The salt balance within the QMDB is generally indicative of net accumulation of salts. This is particularly the case in larger, flatter, sedimentary rock catchments but less the case in small, eastern, hard-rock upland catchments. Many of these small catchments are net-exporters of salt although in a highly dynamic manner in response to climatic

Mots clés : Soils Salt balance Salinity risk assessment Groundwater Regolith Queensland Murray-Darling Basin

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Page publiée le 28 novembre 2014, mise à jour le 30 mai 2017