The hydrochemistry of the Australian Victorian Alps drainage system : constraints on chemical weathering and the carbon cycle in temperate to semi-arid, silicate dominated river catchments

This dissertation uses river water geochemistry to constrain the role of chemical weathering on controlling the CO2 budget in temperate to semi-arid river catchments over multi-annual time frames. On a regional scale, this study provides a framework for improved water resource management in the region. The knowledge and understanding of geochemical processes discussed here provide essential insight into the application of different geochemical tracers to consolidate the various geologic, climatic and biologic parameters of the Australian Victorian Alps and allow evaluating the relative dominance of each of these parameters on chemical weathering and the carbon chemistry in river waters. This approach is applicable to other river systems and shows the importance of the use of multiple geochemical tracers to quantify solute sources and fluxes. Major ion and Rare Earth Element (REE) concentrations in Victorian Alpine rivers predominantly reflect those of rain and snow. Rain and snow samples differ in chemistry across the study area and, consequently, the major ion chemistry of the river water was corrected for atmospheric inputs according to the position of the sampling site. The major ion chemistry and 87Sr/86Sr values indicate dissolution of feldspar and calcite as the dominant weathering processes in Victorian Alpine rivers. The REE contribution from feldspar and calcite weathering, however, is low as most REE-enriched minerals (mainly biotite and titanite) remain in the streambed sediments as a residual phase of incipient rock weathering. There is a tendency of aqueous heavy REE (HREE) adsorption by suspended Al-oxyhydroxides. This may be a result of a high weathering-derived input of Al and the fact that the precipitation of gibbsite in the rivers happened to occur over the pH range (~5.40 to ~7.75) at which the less stable free HREE3+ and HREECO3+ ions dominate solution. Because of the non-conservative element behavior in Victorian Alpine rivers, the parameterizations of weathering processes, based on major ion and REE abundances alone, should be treated with skepticism. Chemical weathering of silicate rocks consumes ~0.026 x 106 mol/km2/yr CO2, with the highest values in the high runoff northern flanks rivers draining the basement outcrops rather than sedimentary rocks. This value is about one order of magnitude below the global average, which is probably due to an arid climate and low relief. Victorian Alpine rivers evade ~14.6 x 106 mol/km2/yr CO2 to the atmosphere, which is about half the global average, reflecting an arid climate, slow wind velocities and low heterotrophic activity. Respired CO2 efflux in Victorian soils is not balanced by combined in river photosynthesis, CO2 evasion, CO2 advection and rock weathering. This discrepancy implies that chemical weathering does not significantly neutralize respiration derived H2CO3 in river waters. Combined hydrogeological and hydrogeochemical data indicate long-term recharge of groundwater by river water along the entire reach of the Ovens River over horizontal distances of less than 1000 m normal to the river channel. Because recharge of high quality river water to the near-stream aquifers in the Ovens Catchment is localized and limited, groundwater resources are threatened by over-extraction due to irrigation. Also, excessive groundwater use from the near-stream bores could increase near-stream groundwater salinity due to increased leakage of distal poorer quality water.