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Australian National University (2009)

The geochemistry of alkaline salt-affected soils

Smith, Michael Shane

Titre : The geochemistry of alkaline salt-affected soils

Auteur : Smith, Michael Shane

Université de soutenance : Australian National University

Grade : Doctor of Philosophy (PhD) 2009

Résumé partiel
Alkaline salt-affected soils are very common globally, yet their geochemical evolution and the consequently available preventative and ameliorative landmanagement options are poorly defined in the literature. The essentiality of rock weathering and intense groundwater evaporation in the existence and geochemical evolution of seepage-related alkaline salt-affected soils in northern N ew South Wales was investigated. A study of hydrochemistry and stableisotope chemistry showed that basalt weathering primarily provides the Na, Ca, Mg, Si and carbonate alkalinity responsible for the Na-HCOs-Cl- to Na-MgHCOs-Cl-type chemistry of groundwaters of circumneutral pH ; the development of highly alkaline, highly sodic mud volcanoes and scalded (bare) soils ; and, the formation of high-Mg calcite, dolomite, natron and spherulitic silcrete. The pH of the discharging groundwaters increases to about nine by CO2 degassing and is then maintained by a constant near-unit molar Na/carbonate-alkalinity ratio. Regolith geochemistry and pore-water chemistry demonstrated that, subsequently, the mud volcanoes evolve hydrochemically like an open body of evaporating water to a Na-COs composition through the precipitation of near stoichiometric dolomite followed by natron and halite. Conversely, high alkalinity and dolomite formation in the salt-scalded soils are initially inhibited by cyclic salts that were previously concentrated in the vadose zone by evapotranspiration. Aerially derived SO4 has accumulated as soil gypsum whilst recharging waters that contain cyclic salts, mobilized from the vadose zone, have imparted a relatively low Na concentration, carbonate alkalinity and residual carbonate alkalinity to the groundwaters that subsequently discharge through the salt-scalded soils. Consequently, the scalded soils undergo gypsum dissolution and high-Mg calcite precipitation, followed by partial calcite dissolution and the concurrent precipitation of mostly far from stoichiometric dolomite. Moreover, dissolved concentrations of aerially derived K in the scalded soils are sufficient for the partial transformation of smectite to illite, through a transitional beidellitic phase of high permanent layer charge, and illite neoformation. Through the precipitation and depletion of carbonate alkalinity, and reverse cation exchange, pore waters at shallow depths in the scalded soils evolve with a decrease in pH to a Na-Cl composition. T

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