Surface water and groundwater interactions in the Ovens River
thesisposted on 03.03.2017 by Yu, Matthew Chi Leung
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This thesis investigated surface water-groundwater (SW-GW) interactions in the Ovens River, southeast Australia. The Ovens River is hosted within a valley with coarse-grained Quaternary alluvial deposits in the upper catchment and flows across an alluvial floodplain with fine-grained and mature Quaternary to Tertiary sediments in the lower catchment. Electrical conductivity (EC), major ions and radon (²²²Rn) indicate that the Ovens River is dominantly gaining in the upper catchment and fluctuates between gaining and losing in the lower catchment. The groundwater inflow in the Ovens River, estimated through ²²²Rn mass balance, is 2 to 17% of the total discharge. The groundwater inflows in the upper catchment are higher during high flow periods. The spatial variation of SW-GW interaction is due to the differences in distribution of rainfall, topography and aquifer lithology across the catchment. The temporal variation of baseflow over 10 years in the Ovens River was studied by flow duration curve (FDC), graphical and filter-based hydrograph separation, and EC-derived chloride-based chemical mass balance (CMB). Baseflow fluxes derived from the hydrograph separation methods are significantly greater than those determined by the chloride-based CMB and FDC by 35% to 200%. The differences are greatest during and following high flow events. These differences suggest that discharge from transient water stores, such as river banks, the unsaturated zone, and pools or disconnected channels on the floodplain, contribute a significant proportion of baseflow in the Ovens Catchment. The differences in baseflow estimates are caused by the fact that baseflow estimated by hydrograph separation consists of both groundwater inflow and discharge from transient water stores, while CMB and FDC yield groundwater inflows only. Bank storage occurs along most of the river banks at the Ovens River in the middle and lower catchments, except for the steep mid-to-lower valley section of the upper catchment. The indications of bank storage in these areas include the reversed hydraulic head in river banks, the reduction of EC in river banks, the shift in the Na/Cl ratios, major ion concentrations and stable isotope values of the near-river groundwater toward those of the river water following high flow events, and the similarity in tritium concentrations between the water in river banks and the river water at high and moderate flows. Bank storage in the Ovens catchment occurs mainly in areas that have a relatively lower regional hydraulic gradient toward the river and high-to-moderate hydraulic conductivity river bank sediments. The knowledge gained from this thesis will improve our understanding of river-groundwater interactions and has management implications for the Ovens River and other similar rivers.