HYDROGEOLOGICAL AND HYDROCLIMATIC CONTROLS ON SURFACE WATER-GROUNDWATER INTERACTIONS

Author: Edward Wallace Banks

Banks, Edward Wallace, 2011 HYDROGEOLOGICAL AND HYDROCLIMATIC CONTROLS ON SURFACE WATER-GROUNDWATER INTERACTIONS, Flinders University, School of the Environment

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Abstract

Growing awareness about the benefits from more sustainable management and allocation of water resources has highlighted the need to manage surface water and groundwater systems as one integrated system. Whilst there has been a significant contribution to the knowledge and understanding of hydrogeological research of surface water-groundwater interactions in the past few decades, there are still specific knowledge gaps on how different types of systems (i.e. connected gaining and losing, and losing disconnected) function and interact at different spatial and temporal scales and in different hydrogeological environments. This body of research addresses some of the complexities of surface water-groundwater interactions in fractured rock environments, at different spatial and temporal scales and investigates a number of the dominant controls (e.g. geology, topography and vegetation) that influence the exchange processes and dynamics between surface water and groundwater. Specifically, this work investigates: (1) the importance of groundwater from the fractured bedrock compared to groundwater from the saprolite zone in streamflow generation and why the bedrock interface cannot be considered a no-flow boundary. (2) surface water-groundwater interactions in a pristine catchment at a regional scale to determine the state of connection between surface water and groundwater along a river system from the catchment headwaters to the discharge point at the sea. (3) the vegetation controls on variably-saturated processes between surface water and groundwater and its impact on their state of connection. The first part of this research was a field-based study examining surface water-groundwater interactions along a gaining river reach. The study investigated the importance of groundwater from the fractured bedrock compared to groundwater from the saprolite zone in streamflow generation and examined why the bedrock interface cannot be considered a no-flow boundary. The hypothesis was to determine whether the saprolite zone is hydraulically more active than the deeper bedrock zone. The findings of this study suggest that hydrologic conceptual models, which treat the saprolite-fractured bedrock interface as a no-flow boundary and do not consider the deeper fractured bedrock in hydrologic analyses, may be overly simplistic and inherently misleading in some surface water-groundwater interaction analyses. The results emphasise the need to understand the relative importance of subsurface flow activity in both of these shallow saprolite and deeper bedrock compartments as a basis for developing reliable conceptual hydrologic models of these systems. The second part of this research was also a field-based study in a pristine catchment which investigated the state of connection between surface water and groundwater along a river system from the catchment headwaters to the discharge point at the sea. The relative source and loss terms of the river and groundwater systems were assessed, as were their relative magnitude changes along the river, and how a fresh water river system in a pristine catchment covered by native vegetation exists in an otherwise saline regional groundwater system. Many surface water-groundwater interaction studies of different types of systems are either undertaken at the local or river reach scale, however, catchments encompass multiple types of systems at a regional scale. There has been very little research investigating how multiple river reaches function in the context of the entire regional river system from the headwaters to the sea or discharge point, and this study demonstrates the benefits of doing so. The final part of this research used a fully coupled, physically based numerical model to demonstrate the vegetation controls on variably-saturated processes between a perennial river and an aquifer and its impact on their state of connection. By examining different conceptual models of catchments with different slopes and vegetation type (i.e. root depth) the research identified the conditions required for changes to vegetation to have the greatest effect on the flow regime and the presence of an unsaturated zone beneath a riverbed. The analysis also suggested that the flow regime and hydraulic response to the presence of vegetation and subsequent removal can be much greater in flatter catchments than those that are steep. Intuitively, this may appear plausible in a qualitative sense; however, it has not been demonstrated quantitatively. The results of the study therefore suggests that in addition to the well known influences of physical variables such as hydraulic conductivity or topography, the effects of vegetation need to be carefully considered when investigating surface water-groundwater interactions.

Keywords: groundwater-surface water interactions,fractured rock aquifers,gaining streams,losing streams,disconnected,hydraulics,stable isotopes,hydrochemistry,modelling,environmental tracers
Subject: Hydrology thesis, Environmental Science thesis

Thesis type: Doctor of Philosophy
Completed: 2011
School: School of the Environment
Supervisor: Professor Craig T Simmons