Author: Mark Nicholas Keppel
Keppel, Mark Nicholas, 2013 The geology and hydrochemistry of calcareous mound spring wetland environments in the Lake Eyre South region, Great Arteisan Basin, South Australia, Flinders University, School of the Environment
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Mound springs of the Lake Eyre South region of South Australia (SA) are dome-shaped accumulations of largely calcareous precipitates associated with artesian springs sourced by groundwater from the Great Artesian Basin (GAB). These springs and associated wetlands provide unique habitats for a number of endemic flora and fauna species and hold great cultural and societal importance to indigenous and non-indigenous Australians. Few intensive studies of mound spring structures exist despite similar formations being found worldwide. Specifically, there has been little work with respect to either determining or verifying the hydrochemical and environmental factors important with respect to mound spring formation, or how spring flow is maintained once a mound structure has been built. Consequently, existing descriptions for the formation of mound structures described in the literature are inadequate with respect to the Lake Eyre South mound springs. Consequently, geological and hydrochemical investigations were undertaken to develop a conceptual model for the initial formation and evolution of mound spring environments. A reactive transport model was developed to provide a generic, quantitatively constrained framework for the analysis of spring hydrochemistry. This model was also adapted to afford insight into the hydrochemical conditions within the initial spring wetland from which mound structures develop (the maximum spatial extent of the mound base or mound "footprint"). This study found that mound spring-related calcareous deposits were largely composed of tufa, defined here as a terrestrial limestone precipitated from sub-ambient to ambient temperature springwater, with sedimentary textures synonymous with microbial activity and encrustation around hydrophytes. Evidence from experimental data obtained during this study highlighted the important role cyanobacteria have in mediating tufa precipitation. Carbon dioxide (CO2) degassing from calcium carbonate (CaCO3) laden water is the primary cause for CaCO3 precipitation. Degassing is caused by either physio-chemical processes, particularly in subterranean and near vent environments, or biomediation. CO2 degassing and CaCO3 precipitation were found to be time-dependent, with this dependence controlled by the rate at which CO2 is able to degas. The CO2 degassing rate was in turn dependent on flow turbulence and the surface area exposure of water to substrate and the atmosphere. Such rate-limited CaCO3 precipitation in mound spring environments is reasoned to have a critical stabilising effect on mound growth and spring flow; consequently rate-limited CaCO3 precipitation is central to a new conceptual model for mound spring formation and evolution. Reactive transport modelling was used to infer the previously unrecognised roles infiltration and heterotrophy have in controlling wetland hydrology and water chemistry respectively. Conversely, evapotranspiration was not found to significantly affect the rate of CaCO3 precipitation. The mound footprint predicted by the model provided a reasonable approximation of the measured mound, with the prediction being primarily a product of the carbonate precipitation rate. The location of mound springs are correlated with a tectonically active shear zone related to the Adelaide Fold Belt. Seismicity is evidenced through the propagation of deformation structures within spring-related sediments that are concordant with these regional structures and may have an important role in influencing the development of mounds. This study has demonstrated that a unique interplay between hydraulics, hydrochemistry and the environment is responsible for mound structure formation and the maintenance of spring flow after a mound structure has formed, while the developed reactive transport modelling methodology is novel with respect to terrestrial calcareous deposits and resultant morphotype development.
Keywords: mound spring,tufa,calcium carbonate,reactive transport modelling
Subject: Environmental Science thesis
Thesis type: Doctor of Philosophy
School: School of the Environment
Supervisor: Dr Adrian Werner