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Title: Tracing the age, origins and hydrodynamics of groundwater and surface water exchange in river banks
Authors: Unland, NP
Cartwright, I
Cendón, DI
Chisari, R
Keywords: Age estimation
Ground water
Surface waters
Electric conductivity
Issue Date: 17-Sep-2013
Publisher: International Association of Hydrogeologists
Citation: Unland, N. P., Cartwright, I., Cendón, D. I., & Chisari R. (2013). Tracing the age, origins and hydrodynamics of groundwater and surface water exchange in river banks. Paper presented to the IAH 2013, Perth, Australia : "Solving the groundwater challenges of the 21st century", International Association of Hydrogeologists 40th International Conference, Perth, Western Australia, 15-20 September 2013. (pp. 117).
Abstract: It is common for groundwater-surface water assessments to be conducted within streams at discrete time intervals in order to characterise the gaining or losing nature of a stream and the volumetric flux of water between the two reservoirs. While these studies offer sound scientific information for one point in time, they often overlook the dynamics of groundwater and surface water interaction under changing hydrologic conditions. This study couples discrete sampling for hydrochemical parameters with the continuous monitoring of physical parameters at multiple locations. in taking this approach the interaction between river water and groundwater stored in river banks can be assessed over space and time, allowing for both the qualitative and quantitative impacts of water exchange to be assessed. Continuous analysis of groundwater head levels and electrical conductivity indicates the presence of a semi conned aquifer of increased salinity underlying an unconfined aquifer of lower salinity in the region. Carbon-14 and tritium results indicate that groundwater in the underlying aquifer is significantly older than that of the unconfined aquifer, with variable mixing between the two resulting a range of intermediate ages. While discrete sampling and temperature profiling of river water indicates a predominantly gaining system, reversal of hydraulic gradients during periods of increased rainfall and river discharge indicates a change to losing conditions. Although this indicates the occurrence of bank infiltration, an initial increase in groundwater electrical conductivity during increased river discharge suggests that increased leakage from the semi-confined aquifer dominates groundwater chemistry at these times. The degree to which this occurs varies between locations and the scale of discharge events. This study illustrates the complexity and variability by which groundwater-surface water interactions can occur within river banks.
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