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Accueil du site → Doctorat → Australie → 2007 → The hydrogeochemistry of a saline aquifer system : central New South Wales, Australia

Australian National University (2007)

The hydrogeochemistry of a saline aquifer system : central New South Wales, Australia

Lenahan, Matthew James

Titre : The hydrogeochemistry of a saline aquifer system : central New South Wales, Australia

Auteur : Lenahan, Matthew James

Université de soutenance : Australian National University

Grade : Doctor of Philosophy (PhD) 2007

Résumé partiel
Within many semi-arid regions throughout the world, secondary salinization of groundwater and surface water represents a serious environmental threat to freshwater resources. However, there exists a limited knowledge of solute dynamics in these environments, in particular subsurface solute distributions and mobility. The origin, nature and mobility of solutes were determined for 25x25 km catchment area in central New South Wales through combined hydrogeological and hydrogeochemical techniques to better understand physiochemical processes occurring in the both the unsaturated and saturate zones of saline environments. This thesis aims to address the knowledge gaps in the current understanding of the nature and mobility of solutes in the subsurface environment. At present there is a lack of understanding of (a) the processes that change the relative solute proportions in rainwater to groundwater, (b) the timeframes of solute accumulation in the unsaturated zone and saturated zones, and (c) the nature and mobility of subsurface solute stores. Chloride concentrations of 155 unsaturated zone soil pore water solutions from four locations indicate long-term (>200,000 yrs) meteoric deposition and evapotranspiration as the dominant solute concentrating mechanism. However, meteoric water exhibits considerably different chemical and isotopic compositions compared to groundwater. Nutrient cycling is the dominant process controlling the K/Cl, Ca/Cl, Mg/Cl and NO3/CI ratios during vertical infiltration of rainwater through the unsaturated zone. Minor variations in Na/Cl and N/Ca ratios are the result of cation exchange. Biogeochemical processes, in particular organic adsorption and bioassimilation, influence the Br/Cl and SO4/CI ratios. Results from this study suggest the commonly observed discrepancies between local meteoric water and groundwater compositions in semi-arid regions are the result of geochemical and biogeochemical processes occurring in the near surface of the unsaturated zone.

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