School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemNo Preview AvailableProcesses and implications of scald formation on the Eastern Dundas tableland : a case studyFawcett, Jonathon Duke (University of Melbourne, 2004)This thesis develops an understanding of the processes driving the formation of iron and saline rich scalds in groundwater discharge zones on the Eastern Dundas Tableland. The thesis tests the hypothesis that: The development of land degradation patterns around groundwater discharge zones on the Eastern Dundas Tablelands is not driven by salt concentrations, a consequence of water tables that have risen since European settlement and land clearing, but is the result of the disturbance to the soil environment of discharge zones since land clearing. The research proved the hypothesis correct and identified the transmission of hydrogen sulfide within the groundwater as a key factor in the severity of the degradation. The research also found land clearing may have altered the seasonal flow systems operating within the regolith, which are responsible for the spread of salinity and erosion within primary groundwater discharge zones. The Eastern Dundas Tablelands consist of a fractured rock aquifer, where the unweathered ignimbrites and lavas are hydrologically connected to the overlying regolith. Analyses of groundwater levels, pressures and chemistry indicate the groundwater flow paths to discharge zones increase in length with depth, such that discharge water is sourced from groundwater varying in depth of circulation (up to 150 metres) and flow path length (tens of metres to kilometres). A portion of discharge water flows through the unweathered volcanic aquifer and equilibrates with the rock, becoming reduced. The groundwater flow therefore contains hydrogen sulfide and has reducing capacity. This water then flows preferentially along large scale structures towards discharge zones. The location of groundwater discharge zones is controlled by the intersection between horizontal and sub vertical subsurface structures. Iron and saline rich scalding only occurs where groundwater flow with reducing capacity and containing hydrogen sulphide discharges. Iron and saline scalding is absent where only seasonal groundwater discharge not containing hydrogen sulfide occurs. Measurements of the discharge zones' soil and water EC, Ph and redox potential and the soil chemistry indicate degradation is the result of a combination of processes and is not solely the result of soil salting. Discharge water in wet months is covered by bacteria growth that reduces and oxidises iron and sulfur. During dry periods distinct iron precipitates form crusts on soil surfaces and clog soil pores. Salt efflorescence forms on soil surface and the oxidation of ferrous sulfide creates severe acidity. Evaporation accumulates salts in the discharge zones that are then spread downslope by seasonal break-of-slope discharge. Break of slope discharges erode highly sodic A horizon soils, radially expanding the area of degradation. Relatively constant groundwater levels, historical information and the carbon age of discharge water (2540 years) suggest discharge at the soil surface of the Eastern Dundas Tablelands occurred at least as early as the first records of European settlement. Little to no evidence exists suggest rising groundwater levels, caused by increased recharge since land clearing, have initiated the degradation of discharge zones. The pH, redox potential and measured quantities of ferrous material and sulfur in groundwater suggest iron and saline scalding is initiated by the interaction of the reduced groundwater of the regional groundwater flow and reducible material (iron) in the discharge zone. Prior to the clearing of native vegetation, reduced groundwater flow was able to discharge at the soil surface without coming into contact with reducible material (iron). Any hydrogen sulfide in discharge water was able to dissipate into the atmosphere, removing the reducing capacity of the water. Since clearing, iron has been redistributed into groundwater flow paths. Iron is reduced by the groundwater flow before the hydrogen sulfide is able to dissipate with ferrous and iron sulfide rich minerals forming within discharge zones. Upon oxidation, iron rich precipitates (ferrihydrite and schwertmannite) form in soil pores and along the soil surface with the oxidation of ferrous sulfide creating severe acidity (pH <4). The degree of degradation is directly related to the rate and volume of groundwater discharge. The least degraded areas coincide with point-flowing springs, the wettest areas of iron and saline scalds. Within point flowing springs, permanent saturation prevents the oxidation of ferrous sulfide material, the formation of iron crust and the accumulation of salts via evaporation. The most severe degradation coincides with areas of diffuse discharge, where the drying of the soil surface leads to iron precipitate formation, salt accumulation and severe acidity as ferrous sulfide material oxidises. The process of iron and saline rich scalding can be halted, and the area remediated if: � reducible material is removed from the groundwater discharge zone, preventing the reduced groundwater from mobilising iron and forming iron-sulfide material; and � the discharge zone is fully submersed in water, such as a dam. In this case, groundwater hydrogen sulfide will dissipate into the atmosphere, removing the reducing capacity of groundwater discharge.