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Australian National University (2013)

New environmental tracer methods for quantifying solute sources in semi-arid alluvial aquifers

Horner, Kyle

Titre : New environmental tracer methods for quantifying solute sources in semi-arid alluvial aquifers

Auteur : Horner, Kyle

Université de soutenance : Australian National University

Grade : Doctor of Philosophy (PhD) 2013

Description
Alluvial aquifer systems produce up to 60% of groundwater extracted in Australia. Identifying solute sources to these fresh water reservoirs is vital for their long-term sustainable management. In this thesis, new environmental tracer techniques are presented for quantifying the contributions of various processes to groundwater solute loads in semi-arid alluvial aquifers, extending the range of methods available for characterising solute sources in similar settings around the world. The methods are demonstrated in a surface water and groundwater study of the Lower Murrumbidgee Groundwater Management Area of south-east Australia’s Murray Basin. Equations derived using Cl/Br ratios are used to quantify the input of chloride salts to dissolved chloride in waters subjected to evaporation and transpiration. The equations are applied to assess the contributions of halite to groundwater salinity in the Lower Murrumbidgee, where Cl/Br ratios are heterogeneous. Low, uniform Cl/Br ratios suggest negligible halite dissolution in the catchment’s east while a systematic increase in Cl/Br ratios suggests up to 50% of dissolved chloride in the west could be from halite. Numerical simulations of the basin-scale "cyclic salt" conceptual groundwater salinisation model are used to quantify meteoric inputs to groundwater and to identify where additional processes not in the conceptual model contribute to aquifer salinity. Groundwater quantity in the study area is shown to depend on the leakage rate of the Murrumbidgee River to regional aquifers, but groundwater quality depends on the rate of cyclic salt input from the vadose zone. Solute distribution in irrigated areas deviates from the regional trend. Results indicate irrigation return flows have mobilised solutes in the unsaturated zone, but in-situ dissolution of soluble minerals does not significantly contribute to aquifer salinity. Stable silicon isotopes are used to examine spatial and temporal variations in silicate weathering in the Lower Murrumbidgee, where silica accounts for up to 30% of total dissolved solids in low-salinity groundwaters. A new conceptual model of silicon isotope fractionation during silicate weathering is presented and an existing mathematical model of isotope fractionation is adapted to calculate silicon isotope composition in solution. Fractionation is shown to be pathway-dependent, greatest in weathering processes that sequester high proportions of silica in secondary phases. Silicon isotope data from the Lower Murrumbidgee indicates that weathering intensity in the catchment may have increased over the last 20 000 years, and surface water 30Si values indicate a similar increase in the headwater catchments during the same period

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