Author: Chani Welch
Welch, Chani, 2014 Evaluation of bank storage using pressure and solute propagation, Flinders University, School of the Environment
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Bank storage is the process of river water mixing with near-river groundwater as a result of an increase in river stage due to a flow event. Such mixing causes temporal and spatial variation in near – river groundwater chemistry. However, the extent of the interaction is poorly defined. The extent of the interaction has important ramifications for biogeochemical cycling, contaminant mixing and degradation, and resource assessment techniques that differentiate between surface water and groundwater reservoirs. Previous assessments of bank storage have primarily relied on hydraulic methods, particularly pressure propagation, and chemistry measurements with limited temporal resolution. This work aimed to evaluate the relative rates of solute and pressure propagation and develop new assessment techniques for bank storage in a variety of hydrogeological environments. In contrast to pressure propagation into homogeneous aquifers in response to river stage rise, the relationships between water propagation and aquifer properties were not well understood prior to this study. Practically, water movement is most readily measured using a conservative solute or tracer. Numerical assessment of a new analytical relationship between solute and pressure travel times and distances and aquifer and flow event characteristics determined that the solution may be used in variably saturated aquifers with errors generally less than 30%. In homogeneous aquifers the ratio of solute to pressure travel time is independent of hydraulic conductivity. Consequently, under certain hydrological conditions time series measurement of pressure and a solute (or proxy) and computation of pressure and solute travel times enables a first-order estimate of aquifer properties and the lateral extent of river water penetration into an aquifer. In homogeneous systems river stage rise causes pressure to propagate faster and further into an aquifer than water (or solutes). Numerical testing of two conceptual models of alluvial heterogeneity indicated that pressure and solute propagation are unequally affected by aquifer heterogeneity. Hence, under certain conditions, substantial solute change can be recorded in an aquifer before substantial pressure change. This may be identified by computing a solute travel time less than a pressure travel time. Flux estimates obtained from solute travel times using homogeneous solutions were determined to be more accurate than estimates obtained from pressure data. The error in estimates derived from pressure data was proportional to the contrast in hydraulic conductivity in a system. Theoretical investigations of bank storage have not systematically quantified the influence of the hydraulic gradient between aquifer and river. In this work analytical and numerical techniques demonstrated that variation in the hydraulic gradient influences bank storage exchange, penetration distance and residence time, at a scale similar to substantial variation in hydraulic conductivity, wave height and period, dispersivity, and river partial penetration. Consideration of the hydraulic gradient is therefore integral to quantitative assessments of exchange. Simultaneous measurement of pressure and solutes at high temporal resolution within rivers and adjacent aquifers is a useful technique for improving understanding of the spatial and temporal extent of river – aquifer exchange during flow events. The utility of the theory relies on contrasting river and aquifer chemistries. Future work should consider the use of alternative tracers to test residence time distribution theories, and geostatistics, spatial imaging, and uncertainty techniques to further understand the influence of heterogeneity.
Keywords: bank storage,floods,surface water - groundwater interaction,heterogeneity,analytical solutions,numerical modelling,hydraulic gradient,rivers,variably saturated flow
Subject: Environmental Science thesis, Environmental Studies thesis
Thesis type: Doctor of Philosophy
School: School of the Environment
Supervisor: Peter Cook