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Australian National University (1995)

Sources and chemical evolution of salts on the Australian continent

Thomas, Geoffrey Anthony

Titre : Sources and chemical evolution of salts on the Australian continent

Auteur : Thomas, Geoffrey Anthony

Université de soutenance : Australian National University

Grade : Doctor of Philosophy (PhD) 1995

Résumé partiel
This research investigates how the sources of salts in surficial waters and their chemical evolution are influenced by : distance inland from the coast, catchment lithology, catchment hydrology, climate, and agricultural and human activity. This project involved eight study areas in Australia : Acraman and Lake Eyre in South Australia, Yeelirrie and Disappointment in Western Australia, Natimuk and Colac in Victoria, Buchanan in Queensland and Kulgera in the Northern Territory. The hydrochemistry of surficial waters sampled from the eight study areas indicates that both marine aerosol and rock weathering are predominant hydrochemical processes that contribute to the chemical composition of these surficial waters. The ³⁶C1 and ³⁴S isotope hydrochemistry data indicate that the marine aerosol is the principal source of chloride and sulphate in surficial waters. The rock-weathering component is most evident from the Na/Cl, K/Cl, Mg/Cl, Ca/Cl and HCO₃/CI chemical ratios and the alkali and alkaline earth normative carbonate salts of dilute waters - in terms of their departure from a marine hydrochemistry and a marine salt norm, respectively. The chemical ratios and salt norms for the surficial waters also reveal that the highly-modified marine hydrochemistries for dilute groundwaters chemically evolve to hydrochemistries for saline waters which are marine-like (are similar to marine chemical ratios, and have vestiges of the halite-carnallite-bischofite-anhydrite-keiserite-magnesite marine salt norm). However, the trend in Cl/Br, Na/Cl, K/Cl, Mg/Cl and Ca/Cl chemical ratios (from dilute to saline waters) and the presence of complex alkali and alkaline earth normative sulphate salts in saline waters indicates that rock-weathering reactions (such as calcite and dolomite dissolution and ion-exchange reactions) have contributed solutes at early stages in the chemical evolution of these waters. As these waters chemically evolve, their hydrochemistry evolves in response to additional hydrochemical processes (such as carbonate-, gypsum- and halite-formation and ion-exchange reactions). The presence of clays in near-surface strata in all study areas contributes to the gradual decrease in cationic ratios (Na/Cl, K/Cl, Mg/Cl and Ca/Cl) due to ion-exchange reactions as dilute groundwaters chemically evolve to saline waters with marine-like hydrochemistries. The effect that ion exchange has on the chemical evolution of surficial waters is most evident for K+ ions. The findings of this project indicate that in the Colac study area, lake systems with the same basement lithologies but which have significantly different catchment hydrologies and hence significantly different groundwater contributions also have distinctly different hydrochemistries. This distinction is especially evident in the K/Cl ratios of their lake waters. In the more temperate study areas (Natimuk, Colac and Buchanan), the chemical precipitation of Ca- and Mg-bearing carbonates from surficial waters also decreases their Ca/Cl, Mg/Cl and HCO₃/CI ratios with increasing salinity. However, in the more arid study areas (Yeelirrie, Acraman, Kulgera, Disappointment and Lake Eyre), calcretes and other soil carbonates have formed in drainage channels.

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