INTEGRATED APPROACH TO CHARACTERISATION OF COASTAL PLAIN AQUIFERS AND GROUNDWATER FLOW PROCESSES: BELLS

CREEK CATCHMENT, SOUTHEAST QUEENSLAND

Tim Robert Ezzy

Bachelor of Applied Science (Honours 1st Class) (Queensland University of Technology)

School of Natural Resource Sciences

A thesis submitted for the degree of Doctor of Philosophy Queensland University of Technology

2005

STATEMENT OF ORIGINAL AUTHORSHIP The work contained in this thesis has not been previously submitted for a degree or

diploma at any other higher education institution. To the best of my knowledge and

belief, the thesis contains no material previously published or written by another

person except where due reference is made.

Signed …………………………………… Tim Robert Ezzy Date ………………………..

i

Abstract

Low-lying coastal plains comprised of unconsolidated infill are internally complex

hydrogeological settings, due to the high level of heterogeneity in the infill material.

In order to resolve the hydrogeological processes active in these complex settings, an

integrated multi-disciplinary, geoscientific approach is required. This research

determines quantitatively, the effects of sedimentary aquifer heterogeneity on

groundwater flowpaths and groundwater processes within a heavily laterised, coastal

plain setting. The study site is the Bells Creek catchment in southeast Queensland,

Australia. The methodology developed in this study provides a new approach to

enable the determination of groundwater flowpaths and groundwater processes at

macroscale resolution within other shallow alluvial and coastal plain aquifers. The

multi-disciplinary approach utilises sedimentological, geophysical, chronological and

hydrogeological techniques (including hydrochemistry and groundwater flow

modelling) to develop a high-resolution aquifer framework, and to determine

accurately, both groundwater flowpaths and relative flow rates.

Sedimentary framework is confirmed to be the principal factor controlling the

distribution of aquifer permeability pathways in any given setting, and is therefore,

the dominant control over groundwater flow and processes. For the Bells Creek

catchment, interpretation of stratigraphic and sedimentary data allowed the

compilation of a detailed sedimentary framework. This interpretation demonstrated

that weathering of the low-lying arkose sandstone bedrock has developed thick

lateritic profiles. Within the weathering profiles, cemented, iron-rich horizons have

resisted erosion and developed raised and elongated ridges in the modern landscape,

while other clay-rich weathered layers have submitted to erosion and downgraded

around those iron-rich ridges. Consequently, alluvial deposition throughout the Late

Quaternary has been restricted to narrow, and relatively deep valleys containing sand-

rich channels, and thin floodplains at shallow depth.

From a hydrogeological perspective, there is significant macroscopic aquifer

heterogeneity between fine-grained lateritic mixed clay layers, floodplain clays, iron-

cemented ferricrete horizons, and permeable sand-rich alluvial aquifers. This

variability of aquifer material has created a complex subsurface arrangement of

permeability pathways. Application of Ground Penetrating Radar (GPR) in this

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setting enabled accurate definition of alluvial channel boundaries and the high degree

of connectedness within the channels themselves. Interpretation of a comprehensive

GPR dataset (that covered the entire catchment) allowed refinement of the

sedimentary framework previously established to develop a detailed three-

dimensional aquifer framework.

Finite-difference groundwater modelling and particle tracking analysis (using

MODFLOW and MODPATH) has clearly demonstrated that the macroscopic

heterogeneity within the various aquifer materials of the plain has marked impacts on

groundwater pathways, and especially groundwater travel times. The variability

between a maximum residence time of 18 months for groundwater within the

alluvium, compared to hundreds of years for groundwater within the mixed clay

layers of the laterite, clearly demonstrates the importance of accurately defining the

spatial distribution of the various aquifer materials in a groundwater flow

investigation. In this setting, the interconnection of the narrow alluvial channels

feeding into a deeper alluvial delta has provided an effective conduit for shallow

groundwater flow. The role of the alluvial delta in discharging the bulk of fresh

groundwater from the central plain into the coastal and estuarine aquifers to the east,

is certainly critical in preventing saline intrusion from encroaching further west.

Hydrochemical and isotopic indicators have identified the dominant recharge

processes and groundwater flowpaths within the plain, and indicated that the

processes are strongly related to sub-surface permeability distributions determined in

the aquifer framework (and groundwater modelling), as well as seasonal fluctuations

in rainfall. In the northwest of the plain, sandstone hills provide a delayed and

slightly mineralized component of groundwater recharge into adjacent highly

permeable, unconfined alluvial aquifers; these aquifers also recharge directly via

precipitation. Aluminosilicate weathering in the bedrock hills and eastern peripheries

of the laterised bedrock are a source of excess Na, SiO2, and HCO3 to the alluvial

groundwater. As this groundwater flows down-gradient to the east, however, its

chemical composition evolves by sulfate reduction, silica equilibrium and ion

exchange processes into a more mature Na-Cl type.

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Within the shallow coastal aquifers proximal to the eastern shoreline, sulfate

enrichment is occurring (associated with increases in Ca, HCO3, Fe and Al) resulting

in major deterioration in groundwater quality. The deterioration is produced by saline

intrusion from the adjacent estuary coupled with oxidation of sulfide materials in

shallow marine and estuarine clays. Reverses in salinity in those coastal aquifers have

been correlated with surges in fresh recharge waters from unconfined coastal dunes

and semi-confined landward alluvium, following significant rainfall events.

The multi-disciplinary methodology developed, provides an effective approach for

accurately defining the three-dimensional distribution of shallow aquifer material of

varying permeability via detailed stratigraphic interpretation and GPR analysis.

Utilising this aquifer framework, finite-difference groundwater modelling aided by

hydrogeological data and hydrochemical analysis, allows accurate determination of

groundwater flowpaths and groundwater processes. This research provides a new

hydrogeological analogue for alluvial channel aquifers within a laterised coastal plain

setting.

Key Words:

groundwater flow, aquifer heterogeneity, numerical modelling, hydrochemistry,

recharge, ground penetrating radar, coastal plain aquifers, weathering, alluvial

channels.

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LIST OF PUBLICATIONS

Refereed Journal Papers

PAPER 1

Title: “Influence of weathering and sea-level changes on evolution of

Late Quaternary sedimentary profiles within an east Australian coastal

plain.”

Authors: Ezzy, T. R., Cox, M. E., O’Rourke, A. J., Huftile, G. J.

Status: Submitted to Sedimentary Geology

PAPER 2

Title: “Groundwater flow modelling within a coastal alluvial plain

setting using a high-resolution hydrofacies approach.”

Authors: Ezzy, T. R., Cox, M. E., O’Rourke, A. J., Huftile, G. J.

Status: Accepted by Hydrogeology Journal

PAPER 3

Title: “Hydrogeochemical indicators of recharge processes and

groundwater flowpaths within shallow coastal plain aquifers, eastern

Australia.”

Authors: Ezzy, T. R. & Cox, M. E.

Status: Submitted to Environmental Geology Journal

Refereed Conference Papers

PAPER A (Appendix 1):

Ezzy, T.R. and Cox, M.E., 2003. Implications of land-use changes on

groundwater within shallow coastal plain aquifers, Bells Creek

catchment, southeast Queensland, Australia. In: J.A. Lopez-Geta et al.

(Editors), Coastal Aquifers Intrusion Technology: Mediterranean

Countries. Hidrogeologia Y Aguas Subterraneas No 8. Instituto

Geologico Y Minero de Espana, Alicante, Spain, pp. 439-444.

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PAPER B (Appendix 2):

Ezzy, T.R., O'Rourke, A.J., Huftile, G.J. and Cox, M.E., 2003.

Applying Ground Penetrating Radar (GPR) to improve

hydrogeological understanding and groundwater modelling within a

coastal plain setting. In: J.A. Lopez-Geta et al. (Editors), Coastal

Aquifers Intrusion Technology: Mediterranean Countries.

Hidrogeologia Y Aguas Subterraneas No 8. Instituto Geologico Y

Minero de Espana, Alicante, Spain, pp. 149-156.

vi

Acknowledgments

I acknowledge my supervisor Dr Malcolm Cox for his encouragement, assistance and

friendship throughout the term of this research project.

Thankyou to the following organizations for significant contributions to this project:

• Lensworth Group for funding and particularly Dr Ron Black and Peter Gust

for technical support.

• ANSTO Laboratory for 14C dating under AINSE Grant 02/030.

• Queensland Department of Natural Resources and Mining for provision of

aerial photographs and DEM data.

I thank the following people for their support and assistance:

Angela O’Rourke, Dr Tania Liaghatti, Dr Micaela Preda, Hayden McDonald

(Mipela), Dr David Noon, Dr Gary Huftile, Mark Gallagher, the Keating family,

Andrew Wilson, Dr Daniel Lack, Kerrilea Horskins, Luke Nothdurft, Steve Hayes,

Matt Gibson, Dr Greg Webb and Madeline Majajas.

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TABLE OF CONTENTS

Abstract i

Key Words iii

List of Publications by candidate iv

Acknowledgements vi

Table of Contents vii

List of Appendices viii

INTRODUCTION 1

LITERATURE REVIEW 4

FOCUS 5

1. SEDIMENT ARCHITECTURE AS A PROXY TO AQUIFER

FRAMEWORKS 6

Examples of Heterogeneity Controlling Groundwater Flow 10

Application of Ground Penetrating Radar to Define Aquifer

Architecture 12

Limitations of GPR 17

The Effects of Weathering on Sedimentary Profiles 18

2. GROUNDWATER PROCESSES WITHIN COASTAL AND

ALLUVIAL PLAINS 19

Permeability Zones in Shallow Coastal and Alluvial Aquifers 19

Hydrochemical Facies 20

Effects of Weathering on Groundwater Chemistry 24

Role of Wetlands 25

3. THE USE OF NUMERICAL MODELS TO QUANTIFY

GROUNDWATER FLOWPATHS WITHIN COMPLEX

AQUIFER SETTINGS 26

Theoretical Components of Modelling 26

Theoretical Components of Groundwater Modelling 27

The Variety of Hydrologic Boundaries in Groundwater Modelling 32

Comparison of Model and Parameter Uncertainties 34

Groundwater Models Calibrated with Chemical

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and Isotopic Data: Examples 36

Detailed Hydrofacies Descriptions Leading

to Effective Groundwater Modelling: Examples 36

4. SCIENTIFIC AND SOCIETAL SIGNIFICANCE 39

Societal Significance 39

SALTWATER INTRUSION 39

ASSESSMENT OF CONTAMINATED SITES 39

Scientific Significance 40

SUMMARY 41

REFERENCES 42

PAPER 1

Influence of weathering and sea-level changes on evolution of Late Quaternary

sedimentary profiles within an east Australian coastal plain. 52

PAPER 2

Groundwater flow modelling within a coastal alluvial plain setting using a high-

resolution hydrofacies approach. 95

PAPER 3

Hydrogeochemical indicators of recharge processes and groundwater flowpaths

within shallow coastal plain aquifers, eastern Australia. 132

GENERAL CONCLUSIONS 172

APPENDICES 176

APPENDIX 1 Conference Paper: Implications of land-use changes on

groundwater within shallow coastal plain aquifers, Bells Creek catchment, southeast

Queensland, Australia. 177

APPENDIX 2 Conference Paper: Applying Ground Penetrating Radar

(GPR) to improve hydrogeological understanding and groundwater modelling within

a coastal plain setting. 188

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1

INTRODUCTION

Background

Groundwater is both a major resource as well as an integral component of the

hydrological cycle. Groundwater supplies are a primary source of drinking water for

more than 1.5 billion people worldwide including more than 33% of the United States

of America (Clarke et al., 1996; Franke et al., 1998; Alley et al., 2002). With rapid

increases in groundwater contamination from industrial, residential and agricultural

activities, as well as the occurrence of salt water intrusion in coastal aquifers due to

over-exploitation, the condition of many groundwater resources are under threat.

Contaminants can be transported through aquifers along groundwater pathlines via

advection, diffusion and dispersion mechanisms. These same mechanisms of flow are

responsible for enriching and depleting “natural” ions present in groundwater via

chemical interactions with plants, soils, sediments and rocks. Therefore, groundwater

within an aquifer is constantly evolving chemically as it flows through the complex

pathways of sediments, and can be radically altered if contaminants are introduced

into the aquifer via anthropogenic intervention.

To ensure aquifers remain in good health, careful management of these resources is

required. Management can range from strict protection of sensitive aquifers through to

extracting sustainable quantities of water from large, competent aquifer systems. At

all scales, an aquifer must be understood in terms of recharge, discharge and all fluxes

occurring within. Unfortunately, heterogeneity and preferential flow paths which

control the flux of groundwater are difficult to identify in many systems, and many

hydrogeological investigations (e.g. Anderson, 1995; Webb and Davis, 1998;

Klingbeil et al., 1999) recognize that detailed field measurements of heterogeneity

are needed to establish the occurrence of interconnected paths in different types of

aquifer materials.

Thesis Aim

This current study assesses quantitatively, the effects of macro-scale sedimentary

aquifer heterogeneity on groundwater flowpaths within a Quaternary coastal

catchment.

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Thesis Overview

The setting is best described as a back-barrier, heavily laterised, coastal and alluvial

plain. Initially geological logging of a network of groundwater bores, mapping of

sediment boundaries, and chronological interpretation of drill cuttings developed the

broad hydrogeological framework. The geophysical method Ground Penetrating

Radar (GPR), coupled with stratigraphic interpretation was used to define the

subsurface architecture of the narrow and elongate alluvial channels that constitute

the major aquifer system within the laterised plain. Numerical finite-difference

groundwater modelling was then used to show the substantial impact that

interconnected alluvial hydrofacies have on overall groundwater flowpaths. In

addition, spatial and temporal hydrochemical and isotopic analysis of the

groundwater within the plain was examined to determine processes and

interconnection between different groundwater bodies.

Thesis Objectives

The thesis is structured around a series of publications, each of which represents a

component of the multi-disciplinary approach developed in this research to enable the

determination of groundwater flowpaths and groundwater processes at macroscale

resolution within shallow alluvial and coastal plain aquifers.

Paper 1 is titled: “Influence of weathering and sea-level changes on evolution of Late

Quaternary sedimentary profiles within an east Australian coastal plain”. This paper

utilises four individual stratigraphic techniques (namely lithostratigraphy,

morphostratigraphy, chronostratigraphy and radar stratigraphy) to interpret the

geometry and distribution of various sedimentary deposits within the coastal and

alluvial plain setting. Together these four stratigraphic techniques provide an effective

approach for interpreting the phases of evolution within the plain during the Late

Pleistocene as well as assessing the contributions of long-term weathering and sea

level changes on the final sedimentary profile present today. The final outcome from

this paper is a high-resolution three-dimensional sedimentary framework.

Paper 2, “Groundwater flow modelling within a coastal alluvial plain setting using a

high-resolution hydrofacies approach”, expands on the sedimentary framework

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developed in Paper 1. This paper uses hydrogeological data (including bore

hydrographs, pumping and slug tests and chemical and isotope analysis of

groundwater samples) to establish a macroscopic conceptual framework for the

aquifer systems. Finite-difference numerical groundwater modelling is then utilised to

develop the dominant groundwater flowpaths present in the plain. The flow modelling

demonstrates that there is interconnection of narrow unconfined alluvial channels with

a broad, semi-confined alluvial delta. This subsurface drainage system ensures that

most fresh groundwater that enters the plain in the form of precipitation or recharge

from lateral bedrock hills, is discharged to the eastern coastal wetlands via the alluvial

delta aquifer.

Paper 3 is titled: “Hydrogeochemical indicators of recharge processes and

groundwater flowpaths within shallow coastal plain aquifers, eastern Australia”. This

paper examines in detail the effects of subsurface permeability distributions and

seasonal fluctuations in rainfall on the chemical character of the major groundwater

bodies within the plain. Quantitative knowledge of the aquifer framework and

groundwater flowpaths proved to be valuable in analysing the subtle chemical

reactions that occur throughout the plain from recharge to discharge zones.

Thesis Outcomes

The general conclusions bring together outcomes from the separate components. This

research produced two main outcomes of scientific significance:

1. a thorough methodology that can be followed in sequence to enable the

determination of groundwater flowpaths and groundwater processes at a

macroscale resolution within shallow alluvial and coastal plain aquifers.

2. a hydrogeological analogue for alluvial channel and coastal aquifers within a

laterised coastal plain setting.

4

LITERATURE REVIEW

Coastal and alluvial plain aquifers: sedimentary aquifer architecture,

groundwater processes, and the role of numerical modelling.

5

FOCUS

This literature review provides a broad global coverage of topics related to and

supporting the research area. The review explores the role that sediment variability

and aquifer heterogeneity play in groundwater processes, and the importance of

physical influences (permeability) and geochemical impacts (sediment-water

interaction). The review considers three separate research components, each

representing an integral phase in defining groundwater flowpaths in unconsolidated

coastal plain aquifer systems. Additionally, a fourth component of the review

provides a brief overview of the scientific and societal significance of this research.

The subject areas covered in the review are:

1. Sediment architecture as a proxy to aquifer framework: a review of the

concept of heterogeneity; application of 2D and 3D assessment methods

(stochastic, statistical and geophysical) including Ground Penetrating

Radar (GPR) as deterministic aquifer architecture tools in shallow, fresh

water, sand-prone settings; the effects of weathering on sedimentary

profiles and groundwater flowpaths.

2. Groundwater processes in coastal and alluvial plain aquifers: a review

of coastal aquifer permeability zones, changes in hydrochemistry along

groundwater flow paths, the effects of weathering on water chemistry, and

the role of wetlands.

3. The application of numerical modelling to quantify groundwater

flowpaths in complex aquifer settings: use of finite-difference numerical

groundwater models to produce quantitative assessments of the impacts of

hydrofacies on groundwater flowpaths.

4. Scientific and societal significance of defining groundwater flowpaths

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1. SEDIMENT ARCHITECTURE AS A PROXY TO AQUIFER

FRAMEWORKS

“Future success in understanding the dynamic nature of groundwater systems

will rely on continued and expanded data collection at various scales,

improved methods for quantifying heterogeneity in subsurface hydraulic

properties…” (Alley et al., 2002).

Introduction

Sedimentary facies, reflecting original depositional environment, define the trend,

dimensions, connectivity, and internal heterogeneity of transmissive zones within

clastic aquifer systems (Galloway and Sharp, 1998a; Heinz et al., 2003). The

importance of sedimentary heterogeneity with respect to groundwater flow has been

recognised in numerous Quaternary settings (Anderson, 1989; Koltermann and

Gorelick, 1996; Flach et al., 1998; Stanford and Ashley, 1998; Klingbeil et al., 1999;

Gerber et al., 2001). The geometry of sediment boundaries, the connectivity of high-

and low-permeability units and the hierarchical nature of sedimentary features are all

of critical importance in accurately predicting subsurface fluid flow and solute

transport (Sugarman and Miller, 1997; Fisher et al., 1998; Webb and Davis, 1998;

Huggenberger and Aigner, 1999; Miller et al., 2000). Moreover, understanding the

quantitative relationships between sediment architecture and hydrogeologic

parameters enables parameter uncertainty to be diminished in detailed groundwater

flow modelling (Anderson et al., 1999; Bersezio et al., 1999).

Each particular sedimentary depositional system has an individual style regarding

sedimentation and erosion which produces a predictable range of external aquifer

geometries and internal heterogeneities. The strong contrasts in permeable,

transmissive channel fill facies and confining floodplain facies, effect flow patterns

which are controlled by channel connectivity, which in turn correlates to fluvial

system type and overall sand percentage (Fogg, 1990; Galloway and Sharp, 1998b).

Aquifers containing high permeabilities promote preferential pathways or conduits for

groundwater. Conversely, aquitards possessing low permeabilities retard water and

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create barriers to flow. From a broad perspective, the grouping of geologic material

into aquifers and aquitards, is an effective means of characterising groundwater flow.

Unfortunately, the complex nature of sedimentological and erosional processes that

build the subsurface framework of aquifers ensures that the mix of sediments is highly

heterogeneous, and therefore the distribution of hydrogeologic parameters is equally

heterogeneous. The variability of permeability or hydraulic conductivity (K) within

hydrofacies generally ranges over one to three orders of magnitude, whereas the

permeability contrast between hydrofacies generally is four or more orders of

magnitude (Stanford and Ashley, 1998). Once the hydrofacies architecture of the

aquifer system, and the resulting major K contrasts, are determined (and perhaps

successfully modelled), then the internal K variability within hydrofacies can be

investigated if necessary (Stanford and Ashley, 1998).

Heterogeneity can be considered over a wide range of scales from megascopic,

defined at the scale of a well field, through to microscopic, which is present at the

scale of individual grains and pores (Galloway and Sharp 1998a; Huggenberger and

Aigner 1999). Macroscopic heterogeneity reflects depositional facies features such as

sediment stratification, aquifer geometry and zones of internal grain size variability

(both within-facies and between-facies); this intermediate scale of heterogeneity

strongly influences regional and local groundwater flow systems (Dominic et al.

1998; Galloway and Sharp 1998a; Stanford and Ashley 1998; Bersezio et al. 1999).

Permeability of an aquifer can also be examined at many scales. At the micro-scale

the permeability is controlled by the size of the pore opening, and in unconsolidated

alluvial deposits four main factors are seen to control micro-scale permeability

(Fetter, 1994):

1. as median grain size increases, so does permeability, due to large pore openings;

2. as the standard deviation of the particle sizes increases the permeability

decreases, because finer material clogs pore space;

3. coarser samples show a greater decrease in permeability with an increase in

standard deviation than do fine samples; and,

4. unimodal samples (one dominant size) have a greater permeability than bimodal

samples

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Aquifer heterogeneity in fluvial systems is evident at many scales (Galloway and

Sharp, 1998a; Bersezio et al., 1999), such as indicated by the following:

1. external boundaries of sand bodies create discontinuous permeable units that

commonly have a well-defined orientation and trend (Figure 1a);

2. individual permeable units are variably interconnected;

3. within permeable units, porosity and permeability may exhibit lateral or vertical

trends or spatial partitioning (Figure 1b);

4. permeable units may be internally stratified (Fig. 1b, zone I);

5. variably continuous low-permeability layers (referred to as baffles) may occur

within permeable units; and,

6. permeability is commonly anisotropic. Typically, vertical permeability is

substantially less than horizontal permeability in fluvial and alluvial aquifers, due

to the alignment of beds or baffles in a quasi-horizontal fashion.

Braided river (bedload) regimes commonly produce homogenous sand and gravel

aquifers, with minor heterogeneity only attributed with cases of cementation or mud

baffles (Webb, 1994; Miall, 1996). Sand isoliths are typically broad, continuous belts,

with multi-lateral channel fills commonly volumetrically exceeding overbank

deposits. Meandering (mixed load) and anastomosing (suspended load) rivers leave

more heterogeneous and complex “Labyrinth-type” aquifers (Fisk, 1944; Fogg, 1986;

Galloway and Sharp, 1998a). Mixed load aquifers have beaded sand isolith

geometries and their multi-story channel fills are generally subordinate to surrounding

overbank deposits (Miall, 1996). Suspended load aquifers possess shoestring or pod

sand isoliths, and have thin, multi-story channel fills enclosed in abundant overbank

mud and clay (Galloway and Sharp, 1998b).

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a)

b)

Figure 1: Types of fluvial aquifer heterogeneity: a) external geometry and

facies control of permeability pattern in an idealised sand-bed meandering

river depositional system; b) permeability zonation and heterogeneities in a

single point bar. Zone I includes relatively fine-grained lateral accretion beds,

bounded by discontinuous clay baffles, of the upper point bar. Zone II consists

of massive, cross-stratified sand of the middle to lower point bar. Zone III

coincides with the poorly sorted heterogeneous basal lag of gravel, mud clasts

and plant debris. Arrows indicate relative permeability within each zonation

(Galloway and Hobday, 1996; Galloway and Sharp, 1998a).

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Examples of Heterogeneity Controlling Groundwater Flow

Groundwater flowpaths within alluvial systems is known to be hydrogeologically

complex. Preferential groundwater flowpaths were identified within shallow alluvial

abandoned channels in the Nyack floodplain of the Middle Fork Flathead River in the

USA (Poole et al., 2002). It was established from that study, albeit in a qualitative

sense, that the permeability contrast between narrow belts of alluvium (up to 10 m

thick) and the encasing low-permeability floodplain material is the predominant

control on groundwater flow.

Variations in permeability in such settings can be substantial. A geophysical survey

coupled with an injection tracer test, conducted on a shallow delta aquifer in Gray,

Maine, USA, has provided sufficient evidence that anisotropy within the delta is

exerting a strong control on the direction of groundwater flow (Sandberg et al., 2002).

Previous groundwater flow models developed for that area do not account for the

anisotropic structural features within the aquifer which are transporting contaminants

at a vector which is completely different from the regional hydraulic gradient vector.

Teles et al. (2004) describe a new genesis model for replicating subsurface

heterogeneity in a large, single channel, alluvial aquifer in the Aube River alluvial

floodplain in France. This genesis model, which simulates depositional processes to

create a geological framework, was input into a groundwater flow model and

compared with the same groundwater model which utilised a common geostatistical

method (sequential Gaussian simulation) to capture the heterogeneity. Both methods

produced comparable calibrated groundwater heads across the model domain,

however, velocity fields and contaminant concentrations were compared and were

completely different, highlighting the importance of considering the heterogeneity

when looking at flowpaths.

For two synthetic examples and one field example (in the Denver Basin, near Golden,

Colorado), Poeter and McKenna (1998) demonstrated that enhanced geological

information regarding hydrofacies distributions and hydraulic conductivity of units,

improved groundwater flow models significantly. They concluded that better

conceptual models can be retained and poorer models eliminated by using geologic

11

rules regarding hydraulic conductivity relationships between units. Of additional

value is the appropriate characterisation of hydraulic conductivity surrogates, being a

combination of grain size, shape, sorting, packing, degree of cementation etc, and

comparison with K values at enough locations to develop rules on relative values

(Poeter and McKenna, 1998).

Weissmann and Fogg (1999) showed that large-scale palaeosols, imaged using both

geostatistics and sequence stratigraphic principles, are regional confining beds that

impact on overall groundwater flowpaths within an alluvial fan depositional system

located southeast of Fresno, California.

Unconfined aquifer properties (mainly horizontal hydraulic conductivity, saturated

aquifer thickness and head gradient) have proven to be very influential variables in

delineating capture zones for contamination plumes. It has been shown that by

adequately defining flow paths in unconfined aquifers, plantation areas for

phytoremediation techniques (deep rooted trees such as poplar can degrade

groundwater pollutants such as trichloroethylene as they take up and transpire water

from shallow contaminated aquifers) can be effectively designed (Matthews et al.,

2003).

Pumping tests have show that the average conductivity of bed-load channel fill

sequences is about twice that of mixed load sand bodies, with both much more

conductive than bounding mud and splay facies (Galloway and Sharp, 1998b). An

example of groundwater flow variation within fluvial architectural elements was

highlighted in the Sierra Ladrones Formation of New Mexico, where at least three

scales of permeability variation were observed within set, cosets and facies

assemblages of channel deposits (Davis et al., 1993; Webb and Davis, 1998).

A quantitative sedimentological and petrophysical analyses of fluvial depositional

systems by Hornung and Aigner (1999) clearly shows the vast differences in

permeability and porosity that exist within particular architectural elements. Mixed

load channel and bed load channel sands have the highest permeability values and are

potentially the best aquifers. The well sorted nature of the mixed load channel sands is

the likely reason it has a higher permeability than the poorly sorted sands and gravel

12

of the bed load channel. Lateral accretion and sheet flood deposits are also found to

highly permeable units, although their connectivity and bulk size may hinder their

ability to transport and store any reasonable quantity of groundwater.

Application of Ground Penetrating Radar to Define Aquifer Architecture

For subjective-based interpretations of aquifer architecture to be realistic, geophysical

methods are important as they provide subsurface information linking the direct

measurements made at drill holes and outcrops. The utility of particular geophysical

techniques is strongly dependent on geological setting (e.g. clay- or sand-prone), the

depth of data required, and the chemical character of the groundwater.

Ground Penetrating Radar (GPR) involves the transmission of high-frequency (10-

500 MHz) electromagnetic pulses and the recording of reflected signals, which are

processed similarly to seismic signals. Reflections occur at the interface of beds with

contrasting electrical properties, and the depth of penetration depends upon the

attenuation of the signal (Miall, 1996). Greatest penetration and lowest reflectivity are

yielded by unconsolidated sands, gravels and dry sandstone, while attenuation (loss of

signal) increases with saturation, decreasing grain size and the presence of cations (in

both water and clays). The water table will typically generate the most energetic

reflection “echo” (Nobes et al., 2001).

Radar sequence boundaries represent non-depositional or erosional bounding

surfaces, while radar sequences theoretically represent sediment packages between

these boundaries (Dominic et al., 1995; Neal and Roberts, 2000; Knight, 2001; Nobes

et al., 2001). Characteristic reflection strengths and configurations allow construction

of radar facies, which bundle series of similar reflectors into sequences that represent

individual architectural elements within sedimentary depositional systems (Knight,

2001; Nobes et al., 2001). Changes in electrical properties often correlate with

changes in lithology or intrapore water content (Szerbiak et al., 2001). A

consideration during interpretation is that if there is no or negligible contrast in

electrical properties across an interface, no reflection will be generated (Szerbiak et

al., 2001) and therefore some bounding surfaces will not be located. Interpreted radar

sequence boundaries (after Gawthorpe et al., 1993) recognised are not an ideal

13

example of sedimentological facies boundaries as they are only a representation of

boundaries between facies of contrasting electrical properties.

The two main applications for this geophysical technique are, firstly, the evaluation of

sand-body architecture for the study of reservoir/aquifer heterogeneity (Gawthorpe et

al., 1993; Dominic et al., 1995), and secondly, the evaluation of the flow of

groundwater through shallow aquifers (Hubbard et al., 1997; Doolittle et al., 2000;

Endres et al., 2000; Goes, 2000; Corbeanu et al., 2001; Schmalz et al., 2002; Bevan et

al., 2003; Oldenborger et al., 2003). The geophysical or sedimentological approach

used should always depend upon the scale of the problem, with a useful conceptual

model of an aquifer being a simplified version of the real-world complexity. The level

of simplification should always be at a minimum level appropriate to the scale of the

exercise (Armstrong, 2001).

GPR has been utilised in a number of modern and ancient fluvial settings to define

sedimentary element architecture for both groundwater- and petroleum-based

investigations (Davis and Annan, 1989; Stephens, 1994; Bridge et al., 1995; Harari,

1996; Bridge et al., 1998; Asprion and Aigner, 1999; Beres et al., 1999; Fielding et

al., 1999; Vandenberghe and van Overmeeren, 1999; Birken and Versteeg, 2000;

Bristow et al., 2000; Decker et al., 2001; Jol et al., 2002; O'Neal and McGeary, 2002;

Regli et al., 2002). For example, lateral accretion deposits within an ancient, broad,

sandy-braided river channel were interpreted within the Kayenta Formation of

southwest Colorado (Stephens, 1994). Lateral accretion bedding profiles have also

been reconstructed within a modern point bar deposit (Gawthorpe et al., 1993).

Three recent studies applied GPR to specific bedload fluvial systems in an attempt to

characterise both architectural elements and sand-rich, homogenous pathways that are

applicable to groundwater transmissivity (Asprion and Aigner, 1999; Beres et al.,

1999; Vandenberghe and van Overmeeren, 1999). The following is a summary of the

outcomes of each study, highlighting any strengths or limitations that were

encountered. Several figures are included (Figures 2-6) to highlight the powerful bed-

scale resolution of sedimentary features in sand-prone depositional environments in

these studies, and underlines the value of this method in defining shallow aquifer

architecture in alluvial channels.

14

Example 1: a site containing glaciolacustrine delta sediments (Asprion and Aigner,

1999):

• a ‘fines’ facies of delta silts and fine sands was clearly detected (Figures 2

and 3).

• the differentiation of a prograding delta front overlain by fluvial scour pools,

and horizontal gravel sheets.

• a tight grid (1 m spacing) provides pseudo-3-D reconstruction of

architectural elements (Figures 3 and 4).

• a tight grid only offer small scale of larger depositional systems. The vertical

scale was in the order of metres and lateral scale of tens of metres.

Figure 2: Outcrop analogue of Quaternary gravel deltas in the Singen Basin in

southwest Germany. GPR has detected a layer of finer deltaic sediments (unit

f ) indicated by higher amplitudes (darker colour) (Asprion and Aigner, 1999).

15

Figure 3: Fence diagram (incorporating the E-W radar section from Fig. 13b)

showing the three-dimensionality of the radar reflector patterns. Arrows

indicate reflector terminations (Asprion and Aigner, 1999).

Figure 4: Pseudo-3-D diagram with a single timeslice showing the orientation

of the ‘fines’ element in relation to delta foreset beds shown in the western

georadar profile and the topset beds. This type of diagram displays the

enormous potential of this technique for aquifer interpretation (Asprion and

Aigner, 1999).

16

Example 2: a site containing glaciofluvial sediments of a former braided river

environment (Beres et al., 1999):

• 2-D profiles determined the geometry and types of elements present (Figure

5).

• 3-D data (time slices) provided morphological and structural details,

including strikes of cross-bedding and depicting the connectivity and spatial

relationships amongst the main elements (Fig. 5).

• high resolution up to 10 m depth.

Figure 5: Schematic interpretations of georadar facies of Rhine gravels within a

former braided river environment. The facies are presented in A) vertical profiles

(cross-section), and B) horizontal time slices. Typical corresponding architectural

elements and lithofacies are also shown (Beres et al., 1999).

17

Example 3: numerous sites containing selected fluvial deposits (meandering and

braided) in southern Netherlands (Vandenberghe and van Overmeeren, 1999):

• distinction between channel and floodplain patterns in braided, meandering

and transitional river systems to a maximum depth of 10 m (Figure 6).

Figure 6: Radar facies chart of characteristic reflection patterns from fluvial

depositional systems of the Maas Valley in southern Netherlands. Channel and

floodplain patterns in braided, meandering and transitional river systems are

distinguished. Note that the vertical scale is twice the horizontal scale

(Vandenberghe and van Overmeeren, 1999).

Limitations of GPR

A major limitation to using GPR in coastal unconsolidated aquifers is the effect saline

groundwater from nearby estuaries, shorelines and tidal creeks has on the ability to

acquire subsurface data. Cations present in the saline water causes attenuation of the

electromagnetic signal (Knight, 2001; Van Dam et al., 2002). Attenuation means that

depth of penetration is limited and, therefore, the level of subsurface information is

equally limited. An example of this limitation was seen in a GPR study on the Tuross

Valley, in southeastern New South Wales (Nobes et al., 2001), where a loss of signal

18

at shallow aquifer depths was encountered in a line conducted proximal to an

estuarine wetland. Nobes et al. (2001) attributed the saline porewater in that area of

the survey as the contributing factor for poor penetration.

The Effects of Weathering on Sedimentary Profiles

The most obvious factors responsible for valley morphology are geology, catchment

size and fluvial competence (Nichol and Murray-Wallace, 1992). Near surface

weathering processes also have a major influence on the properties of the regolith.

Long-term ‘lateritic’ weathering processes such as duricrust formation additionally

have the potential to control stratigraphic evolution through inversion of relief (see

Twidale, 1980; Ollier and Galloway, 1990; Summerfield, 1991; Pain and Ollier,

1995). This process can enforce strict widths and depths onto valley floors and

interfluves, which in turn control the valley gradient and, therefore, the type,

magnitude and frequency of depositional environments within the coastal valley fill.

There is much disagreement within the literature on studies of laterite and its correct

classification (e.g. Bourman, 1993; Pain and Ollier, 1995; Eggleton and Taylor, 1999;

Bourman and Ollier, 2002). The major components within the laterite unit typically

are:

1. ferricrete: heavily indurated (iron-rich) coarser grained material;

2. saprolite: the slightly weathered zone above the fresh bedrock. Saprolite

classification is dependant on the alteration of low resistivity minerals to clays

and oxides, while much of the parent material, fabric and structural

characteristics are retained (Taylor and Eggleton, 2001); and

3. mixed clay layer: the extensive secondary clay horizons from which all

primary minerals and physical characteristics have been removed.

In a study in southeast Queensland, Ezzy (2000) noted that the laterite profile in the

Meldale Coastal Plain, contains a thick sequence of ferricrete and mixed clay layers,

which have implications for groundwater occurrence and chemical character:

1. the hematite-cemented ferricrete layers that are visible on the ridges fringing

the coastal plain constitute a low-permeability zone, due to secondary infilling

of pore spaces with cement;

19

2. the entire laterite cover constitutes not only a catchment-divide, but also a

groundwater divide for the plain, thus isolating all of the Quaternary aquifers

present above the laterite cover; and

3. a mixed clay layer sample taken from drillcore was found to display very low

permeability values during falling head permeameter tests, in the order of 0.1

to 0.01 m/day.

Complete laterisation, within which strong hydrolysis leads to the neo-formation of

goethite and gibbsite, is a very slow process, and takes millions of years, even in

tropical regions with rapid weathering (Bjorlykke, 1989). A recent investigation into

the age distribution of weathering profiles throughout Australia suggests that

weathering has been relatively continuous across the continent since the Permian,

rather than cyclic in distinct climate episodes such as the Tertiary period (Taylor and

Shirtliff, 2003). For this reason, thick laterite sequences are expected in most of the

exposed Mesozoic sedimentary bedrock units throughout Australia, which has

significant implications for groundwater flow in many of these basins.

2. GROUNDWATER PROCESSES WITHIN COASTAL AND ALLUVIAL

PLAINS

Coastal alluvial plain settings are zones typically formed on stable bedrock. Because

of the seaward-sloping nature of coastal plain strata, deeper aquifers are usually

recharged from inland sources, with fresh water flowing down-gradient and

discharging through a variety of mechanisms to the coastal waters (Fetter, 1994).

With saline water encroaching landward in most coastal aquifers, groundwater is

constantly in a state of flux, which is reflected in the diverse hydrochemistry and

fluctuating processes of groundwater-surface water interaction. This section outlines

all of the principle components that play a role in the overall groundwater occurrence

and character within coastal and bordering alluvial plains.

Permeable Zones in Shallow Coastal and Alluvial Aquifers

Within the unconsolidated infill of many coastal plains, the coarsest sediments with

the highest hydraulic conductivity values lie above the valley floor; these sediments

usually form a water-table aquifer locally confined by a thin layer of floodplain and

20

soil deposits. Terraces may act as secondary aquifers, although pedogenic processes

may restrict their permeability (Galloway and Sharp, 1998b). Hydraulic boundaries

often take the form of valley walls, and strongly focus the flow paths within buried

river channels in the middle of plains (Belknap et al., 1994; Zaitlin et al., 1994). Even

where permeable facies are juxtaposed, weathering profiles, diagentic clay, and mud

drapes may retard flow from uplands into the valley fill (Galloway and Sharp, 1998b).

Deeper aquifers (bedload sediments with high primary permeabilities) may be

partially confined, while shallow aquifers (typically suspended load sediments with

low permeabilities) tend to be hydraulically connected to active streams (Galloway

and Sharp, 1998b). The general orientation of the permeable facies, however, is

subparallel to the orietation of the valley. In some cases this may create a zone of

down-valley underflow (Larkin and Sharp, 1992). In other cases flow is directly

toward the stream as base-flow (Galloway and Sharp, 1998b). The proportion of

underflow is greatest in coarse, mixed load to bed-load systems characterised by

shallow, broad channels and steep valley/channel gradients.

Hydrochemical Facies

The chemical composition of groundwater provides an insight into the actual flow and

mixing of groundwaters. The term ‘hydrochemical facies’ classifies different

groundwater bodies on the basis of their chemical character (e.g. Cox et al., 1996;

Atwia et al., 1997; Lavitt et al., 1997; Malcolm and Soulsby, 2001). The normal

hydrochemical evolution of groundwater in the direction of flow is summarised in the

following sequence (Stuyfzand, 1984; Stuyfzand, 1999):

1. Development of stable water quality by means of longitudinal and

transverse dispersion.

2. From polluted (SO4, NO3, K, heavy metals, tritium) to unpolluted,

through: (a) elimination processes, such as filtration, acid buffering,

sorption, breakdown, and decay; and (b) increasing age of the water, which

results in a lower pollution load before the onset of industrialisation.

3. From acidic to basic water, by weathering reactions with the porous

medium, which consume acids like H2CO3, H2SO4 and HNO3 and produce

alkalinity.

4. From oxic to anoxic-methanogenic conditions, by continued oxidation of

organic matter in a system closed from the atmosphere. This process

21

results in a typical order of consumption of oxidants in water and an

increase in alkalinity.

5. From fresh to brackish or saline water, by means of dispersion across the

boundaries of adjacent water bodies or by continues evapotranspiration.

Hydrogeochemical indicators can distinguish between different bodies of groundwater

and are useful indicators of recharge processes and groundwater flowpaths within

both alluvial and coastal clastic aquifers. This usefulness is mainly due to how the

chemical composition of groundwater reflects both its history of recharge and

chemical reactions that occur between groundwater and unconsolidated aquifer

materials as that groundwater matures (Veeger, 1996; Schurch and Vuataz, 2000;

Garcia et al., 2001; McLean and Jankowski, 2001; Stimson et al., 2001). Importantly,

residence times, the physico-chemical conditions of the aquifer, the position of the

water table, anthropogenic impacts, and the mineralogy of the sedimentary material

dictate which hydrogeochemical reactions are occurring within particular aquifers, the

order of the reactions, and to what extent those reactions affect the overall

groundwater character.

Changes in the chemical composition of groundwater in Quaternary sediments of the

Atlantic Coastal Plain, USA, provide an example of the chemical evolution of

groundwater in a regional flow system (Figure 7). In the shallow regime, infiltrating

water comes in contact with gases in the unsaturated zone and shallow groundwater.

As a result, localised, short-term reactions occur that dissolve minerals and degrade

organic material. In the deep regime, long-term slower chemical reactions, such as

dissolution or precipitation of minerals and ion-exchange, add or remove solutes.

These natural processes and reactions commonly produce a predictable sequence of

hydrochemical facies (Fig. 7b) (Winter et al., 1998). Confidence in predictions of

chemical concentrations at a specific location can be very sensitive to minor

uncertainty in the spatial distribution of hydraulic properties (Alley et al., 2002).

In some instances the presence or absence of a particular ion can reveal a great deal as

to the history of recharge and chemical reaction with aquifer materials. A

hydrogeological study of Quaternary glacial aquifers on Block Island, Rhode Island,

USA (Veeger, 1996), found that bulk chemical differences between high- and low-

22

iron groundwater were due to stratigraphic controls. In that case, underlying the

Quaternary aquifers were two separate bedrock systems separated by a fault zone.

Groundwater seeping into the Quaternary aquifers from the individual bedrock zones

caused mixing of different waters to occur, one with iron, and one without.

Figure 7: In a coastal plain, such as along the Atlantic Coast of the United

States, the interrelations of different rock types, shallow and deep groundwater

flow systems and mixing with saline water (A) results in the evolution of a

number of different groundwater chemical types (B). (Back et al., 1993).

Hydrochemistry was also found useful in evaluating the groundwater processes active

in the La Plata coastal plain, 50 km southeast of Buenos Aires in Argentina. Here,

there is a shallow (50 to 80 m), elongate ( > 30 km) flow system, consisting of Plio-

23

Pleistocene fluvial sand overlain by Pleistocene aeolian and fluvial silt and Holocene

estuarine silty clay (Logan et al., 1999). Within this flow system a diversity of

hydrochemical facies is evident (Figure 8). Bicarbonate-type water includes back

plain recharge (Ca-Na-HCO3) that evolves through cation exchange and calcite

dissolution to a high pH, pure Na-HCO3 end-member. Similar Na-HCO3 water is also

located underlying recharge areas of the central coastal plain, and a lens of Ca-HCO3

water is associated with a ridge of shell debris parallel to the coast (Logan et al.,

1999). Mixed cation – Cl water near the coastline represents intruded sea water that

has undergone cation exchange. In the central coastal plain, Na-SO4,Cl water forms

plumes in the subsurface, and is chemically controlled by: 1) gypsum precipitated

during pyrite oxidation, 2) evapotranspiration, and 3) calcite precipitation.

Figure 8: Summary diagram showing the major hydrochemical facies and

hydrogeologic processes of the La Plata coastal plain flow system. R =

recharge, D = discharge, ET = evapotranspiration, and W = withdrawals. After

(Logan et al., 1999). It is clear from this figure that hydrochemical facies

analysis is an effective tool to develop conceptual models of coastal plain

aquifers.

Chemical reactions involving dissolution or precipitation of minerals commonly do

not have a significant effect on groundwater chemistry in sand and gravel alluvial

24

aquifers because the rate of water movement is relatively fast compared to weathering

rates. Instead, sorption and desorption reactions and oxidation/reduction reactions

related to the activity of microorganisms probably have a greater effect on the

chemistry of groundwater in these systems (Winter et al., 1998).

Hydrochemistry has proved to be an effective tool of discerning weathering and

recharge processes in fractured basaltic aquifers in the Atherton Tablelands in north-

eastern Australia (Locsey and Cox, 2002). By examining the weathering reactions of

minerals within the bedrock, and comparing this with the relative proportions of

chemical constituents present in the different water samples from within fractures, it

was possible to distinguish clear groundwater flow zones and recharge processes.

When seawater intrudes into a fresh-water aquifer, an exchange of cations occurs and

sodium is taken up by the exchanger (clay), and calcium is released; thus water

quality changes from Na-Cl-rich to Ca-Cl-rich water (Appelo and Postma, 1993).

Reverse ion-exchange, in which sodium reduces preferentially for calcium, is a

common feature of waters on the fringe of a saline intrusion (Lloyd and Heathcote,

1985).

Effects of Weathering on Groundwater Chemistry

Weathering of primary silicate minerals (e.g. albite) to form clays, or the weathering

of cation-rich clays (e.g. Na-montmorillonite) to form cation-poor clays (e.g.

kaolinite), causes the release of Na, SiO2, and HCO3 into solution, and is a common

chemical reaction in outcrop/alluvium recharge zones (Garcia et al., 2001; Sracek and

Hirata, 2002). Those hydrolysis reactions consume CO2 and therefore, cause a change

in pH conditions (Appelo and Postma, 1993). A weathering model proposed for an

analogous lateritic setting in the wheat belt in Western Australia (Salama et al., 1993;

Salama et al., 1999), suggests that forestry clearing can play a major role in

weathering reactions. The model suggests that prior to clearing forests, an open CO2

system stimulates weathering reactions, causing Na to be released in exchange for H

ions, resulting in high concentrations of Na and HCO3 in both shallow and deep

aquifers.

25

An understanding of the distribution of heavy metals and trace metals within different

coastal plain sediments and underlying/outcropping bedrock can provide important

information on why groundwater within particular zones of an aquifer is starkly

different to other zones. Metals such as iron and aluminium and the presence of acid-

producing pyritic sediments can drastically alter the quality of water that interacts

with them (Preda and Cox, 2002; Liaghati et al., 2003; Preda and Cox, 2004; Liaghati

et al., 2005). Sudden spikes in heavy metals in a water sample can show that a water

body has passed through a particular estuarine/bedrock material and thus distinguish a

groundwater flow path. In comparison, quartz-rich and granite material contain low

percentages of reactive minerals or heavy metals and thus contain a low TDS level.

Role of Wetlands

Wetlands are an important feature in low-lying coastal and alluvial plains. These

typically saturated areas are present where groundwater discharges directly onto the

land surface, or where surface water drainage is prevented due to low permeability

soils (such as organic-rich mats). Wetlands in riverine and coastal areas are

hydrologically complex owing to periodic water-level changes. Some wetlands in

coastal areas affected by predictable tidal cycles (Dyer, 1997; Malcolm and Soulsby,

2001).

Most wetlands comprise a mosaic of open water, with aquatic plant communities at

sites with permanent shallow water and terrestrial plant communities at sites that

experience no or only temporary inundation. Open water bodies in a wetland provide

extra water storage, and typically during rain periods, the water level in these open

bodies is higher than the groundwater table in nearby shallow aquifers, such as sand

ridges. As a consequence, there is a continuing flow of water from the wetlands into

the adjacent aquifers (infiltration), leading to elevated water tables (Spieksma and

Schouwenaars, 1997).

Reported values for the hydraulic conductivity of peat vary up to ten orders of

magnitude, indicating a site-specific need for flow values. An isolated marsh in the

pine flatwoods of a coastal plain near Jensen Beach, Florida, was utilised in a

pumping test experiment to define hydraulic resistance between the wetlands and

shallow groundwater levels (Wise et al., 2000). The basis behind the experiment was

26

to lower the wetland water level (by pumping piezometers) significantly below the

head of the underlying groundwater and observe the recovery phase. It was found that

the resistance was approximately 6 days, which corresponds to a hydraulic

conductivity of 0.083 m/day for the peat layer, which was 84-144 times less

permeable than the pump test values for the underlying sand aquifer (Wise et al.,

2000).

3. THE USE OF NUMERICAL MODELS TO QUANTIFY GROUNDWATER

FLOWPATHS WITHIN COMPLEX AQUIFER SETTINGS

“The major directions for groundwater modelers in the 21st century include

developing field techniques to help with geological characterisation…given

a complex geological setting, we are faced with the daunting prospect of

defining the nature and distribution of heterogeneities beneath the site.”

(Anderson, 1995)

Theoretical Components of Modelling

A model simulates spatial and temporal properties of a system, or one of its parts, in

either a physical or mathematical way (Waterloo Hydrogeologic, 2001). An example

of a physical model is a sand-tank experiment to define flow rates. Mathematical

models can be either empirical, probabilistic or deterministic (Bear and Verruijt,

1987; El-Kadi, 1995). Empirical models are derived from experimental data that are

fitted to some mathematical function. Probabilistic models are based on the laws of

probability and statistics (Anderson and Woessner, 1992). Deterministic models

assume that the stage or future reactions of the system studies are predetermined by

physical laws governing the system (El-Kadi, 1995). There are two large groups of

deterministic models depending upon the type of mathematical equations involved;

analytical and numerical models. Analytical models solve one equation of

groundwater flow at a time in the analysed flow field, while numerical models

describe the entire flow field of interest at the same time, providing solutions for as

many data points as specified by the user (El-Kadi, 1995). The two most widely

applied groups of numerical models include finite differences and finite elements

(Bear and Verruijt, 1987; Anderson and Woessner, 1992). Finite difference modelling

is mathematically simpler, while finite element modelling applies much more

27

rigorous, and less user-intuitive mathematical procedures to solve highly complex

polygonal networks. In hydrogeology, finite difference methods are more widely

used, as they allow a much higher level of user-intuition (Domenico and Schwartz,

1990).

Theoretical Components of Groundwater Modelling

A groundwater model is perhaps most simply defined as a representation of a real

aquifer system or groundwater process (Konikow, 1999; Reilly, 2001). A conceptual

model is a hypothesis for how an aquifer system or groundwater process operates

(Armstrong, 2001). Therefore, a groundwater model is a beneficial tool to test various

conceptualisations and hypotheses for an aquifer or series of aquifers (Domenico and

Schwartz, 1990; Anderson and Woessner, 1992). The distinction between modelling

(constructing a model to answer a particular objective) and simulation (constructing a

model to mimic as many aspects of a portion of the natural world as possible) needs to

be understood, before a model is designed and constructed (Anderson and Woessner,

1992; Haitjema et al., 2001).

Groundwater models attempt to represent an actual groundwater system with a

mathematical counterpart. The objectives of a study influence the size of the area of

interest, the depth of concern, the scale of discretisation (size of the model blocks or

elements), and the method used to represent the boundary conditions of the model

domain (Haitjema et al., 2001; Reilly, 2001). Computer simulations of groundwater

flow systems numerically evaluate the mathematical equation governing the flow of

fluids through porous media (Domenico and Schwartz, 1990). This is necessary

because analytical solutions of equation (1) are not possible in complex groundwater

systems, so various numerical methods are used to obtain approximate solutions

(McDonald and Harbaugh, 1988). The three-dimensional movement of groundwater

of constant density through porous earth material may be described by the partial-

differential equation:

(Equation 1) ∂/∂x (Kxx(∂h/∂x)) + ∂/∂y (Kyy(∂h/∂y)) + ∂/∂y (Kzz(∂h/∂z)) – W= Ss. ∂h/∂t

where:

28

Kxx, Kyy and Kzz are values of hydraulic conductivity along the x, y and z coordinate

axes (Lt-1);

h is the potentiometric head (L);

W is a volumetric flux per unit volume and represents sources and/or sinks of water (t-

1);

Ss is the specific storage of the porous material (L-1); and T is time (t) (Domenico and

Schwartz, 1990).

The finite-difference method constitutes a finite set of discrete points in space and

time, and the partial derivatives are replaced by terms calculated from the differences

in head values at these points (Bear and Verruijt, 1987). The industry-standard code

for finite-difference modelling is MODFLOW (McDonald and Harbaugh, 1988).

MODFLOW is designed to simulate aquifer systems in which: 1) saturated flow

conditions exist, 2) Darcy’s Law applies (discharge = hydraulic conductivity by cross-

sectional area by the groundwater gradient), 3) the density of groundwater is constant,

and 4) the principal directions of horizontal hydraulic conductivity or transmissivity

do not vary within the system.

Construction of a groundwater model includes development of a conceptual model,

creation of the model and execution of various trials, evaluation of the model results,

and either the compilation of new data or validation testing depending on the success

of the calibration. Once a model is successfully validated, it can be used predictively.

(Domenico and Schwartz, 1990; Anderson and Woessner, 1992; Armstrong, 2001;

Middlemis, 2001).

A conceptual model is created through the evaluation of the hydrogeologic data to

provide a picture of the hydrogeologic setting over some area of interest (Domenico

and Schwartz, 1990). Geologic investigations and pumping tests provide valuable data

that can be synthesised, according to hydrogeologic reasoning, to define the shape,

thickness and hydraulic properties of the major geologic units (hydraulic conductivity,

transmissivity, storativity), the distribution of hydraulic head, and the distribution and

rates of groundwater discharge (Domenico and Schwartz, 1990; Anderson and

Woessner, 1992).

29

Boundary conditions are defined along the simulation domain, including the top and

bottom. Their main function is to account for the influence of flow conditions from

outside of the domain (Anderson and Woessner, 1992; Chiang and Kinzelbach, 1998;

Reilly, 2001). Two commonly used boundary conditions include specified hydraulic

head boundaries (no-flow) and specified flow boundaries (constant-head boundary).

Model boundaries are usually placed along hydrogeologic boundaries such as rivers

(defined by constant hydraulic head boundary), or along a major watershed divide (no

flow boundary). A thick, low hydraulic conductivity unit is often selected as the

bottom of the simulation domain (Anderson and Woessner, 1992).

Discretisation is a process whereby the region to be modelled is subdivided using a

mesh or grid network (example in Figure 9). Finite difference methods apply a

rectangular meshing, consisting of a number of cells each containing a central node

(Chiang and Kinzelbach, 1998; Waterloo Hydrogeologic, 2001). Generally, every cell

or node must be supplied with information on hydraulic conductivity or

transmissivity, storativity, and fluxes due to sources and sinks (recharge, pumping and

evaporation) (Domenico and Schwartz, 1990).

Accurate representation of aquifers and their flow properties in groundwater models

stems from detailed analysis of the depositional systems and the variety of sediments

within each system (Scheibe and Murray, 1998). This is especially true when

considering the complex distribution of alluvial, fluvial and tidal architectural

elements in preserved coastal plain settings (Galloway and Sharp, 1998a; Scheibe and

Murray, 1998; Izbicki et al., 2004).

The scale of discretisation needs to reflect the detail of sedimentological and

hydrogeological understanding that exists. If field and laboratory analysis (drilling,

facies modelling, carbon dating, hydrochemical distribution modelling and

geophysics) have enabled sand-rich pathways to be constrained at a 10 m scale, then

discretisation should be at a scale of 10 m to best simulate groundwater flow

processes. Ideally, discretisation boundaries would trace bounding surfaces of

sedimentary elements, with internal model units being tightly or loosely packed,

depending upon the degree of heterogeneity and responding fluctuations in

permeability values.

30

Figure 9: A practical example of a geological domain that has been discretised

so that finite-difference groundwater modelling can be conducted. A) Cross-

sectional representation of the groundwater flow system at Long Island, New

York; and B) Discretised cross-section. (Buxton and Smolensky, 1999).

A numerical model should be able to simulate the observed field distribution of water

levels before the modeller attempts to simulate predictive scenarios. A model is

successfully calibrated when a good agreement between field and modelled water

levels has been achieved (Anderson and Woessner, 1992; ASTM Standards, 1993;

Armstrong and Narayan, 1998).

There is broad range of groundwater modelling guidelines available to provide overall

guidance and to bring awareness of the complexity of models, and how they may be

best utilised and best managed (ASTM Standards, 1993; ASTM Standards, 1999;

Middlemis, 2001). Due to the inherent uncertainty and non-uniqueness in modelling it

is very important that modellers follow guidelines so that end-users are aware of the

benefits/deficits of any partucular model.

31

In Australia, the Murray-Darling Basin Commission has developed the “Groundwater

Flow Modelling Guideline” (Middlemis, 2001), which is an all-encompassing

guideline that focuses on model conceptualisation, calibration, prediction, uncertainty

analysis, reporting and model review processes. The value of this guideline is that it

provides objective evaluation of all facets of the modelling process, scrutinises pre-

existing guidelines/methods, and provides a clear path to follow when initiating and

conducting a groundwater modelling investigation.

A long-standing guide to conducting and evaluating groundwater modelling is the

Anderson and Woessner (1992) publication “Applied Groundwater Modeling”. This

guide refers to equations and numerical methods, the conceptual model and grid

design, boundaries, sources and sinks within a model, special needs for transient

simulations, model execution and the calibration process, documenting a model, post-

auditing, particle tracking of groundwater flow, and advanced case studies for

example.

The US EPA (1997) have developed “Ground-Water Model Testing: Systematic

Evaluation and Testing of Code Functionality and Performance”. This report focuses

on the computer codes used in predicting groundwater processes and provides an

informative insight into the numerical processing that occurs within groundwater

models.

The US EPA (1995) have also developed “Ground Water Modeling for

Hydrogeologic Characterization”. This document provides guidelines for the

application of groundwater models to the characterisation of hazardous release sites. It

aims to aid model selection and provide recommended quality assurance and quality

control procedures.

Another valuable US EPA document is “Fundamentals of Ground-Water Modeling”

(Bear et al., 1992) which contains a critical review of model misuse, which is

summarised as: Improper conceptualisation; Selection of an inappropriate

code;Improper model application; and misinterpretation of model results.

32

The USGS (Reilly and Harbaugh, 2004) have recently developed “Guidelines for

Evaluating Ground-Water Flow Models”. This report is a critical evaluation of

groundwater models to ensure the appropriateness of model codes selected,

conceptual model construction, development of the model domain, and the accuracy

of the matrix solution.

American Society for Testing and Methods (ASTM) Standards are internationally

well-accepted standard practice guidelines, however, they are more focussed on solute

transport and resource assessment groundwater models. Figure 10 shows an adapted

version of the common modelling flow chart developed by ASTM (ASTM D5447-

93).

Figure 10: Flow chart of the groundwater flow modelling process. After (ASTM

Standards, 1993).

The Variety of Hydrologic Boundaries in Groundwater Modelling

Streams can gain water from the groundwater system or lose water. Losing streams

can be connected to the groundwater system by a continuous saturated zone, or can be

disconnected by an unsaturated zone. Some streams contain bank material of low

33

permeability that can cause a large head difference between the stream and the

aquifer, while other streams may be well connected to the aquifer system through

permeable material. Just as there are many types of streams, there are many ways to

represent a stream in a numerical model (Reilly, 2001). These include:

1. The model is simulated with a head that is unchanging, usually set at the

stage of the stream. This implies that there is no head loss between the

stream and the groundwater system. It is appropriate for large streams or

for systems in which the stream is well connected to the groundwater, and

the stream stage is not expected to change (Reilly, 2001).

2. A specified flow boundary (Type 2 or Neumann boundary): simulated by

specifying a flow rate at a node or location representing the stream. The

flow rate is independent of the head of the aquifer. It is appropriate for

systems that are disconnected from the groundwater system (Reilly, 2001).

3. Head dependent or leaky boundary (Type 3 or Cauchy boundary):

represents the stream as having a constant specified stage, but a layer of

material (the streambed) or some other resistance is present between the

stream and the groundwater system. Assumes that the stream and the

groundwater system are always connected, and the flow from or to the

stream is directly proportional to the head difference between the stream

stage and the head in the groundwater system (Reilly, 2001).

4. A specified head boundary (Type 1 or Dirichlet boundary): nodes in the

Wetlands can receive groundwater inflow, recharge groundwater, or do

both. A wetland that is known to recharge the underlying groundwater

system can be set as a specified inflow boundary (Koreny et al., 1999). A

wetland that gains water from the groundwater system can be represented

by a ‘drain’ conceptualisation in MODFLOW (Winston, 1996). This type

of boundary is the most commonly encountered in the current study of the

Bells Creek coastal plain.

Recharge via precipitation is an important source of water to groundwater systems

(De Vries and Simmers, 2002; Xu and Beekman, 2003). It is usually incorporated into

a groundwater flow model as a specified flow boundary condition along the top

boundary of the model (Reilly, 2001). A problematic issue is whether the recharge

should enter at the top layer of the model or the uppermost active layer. If recharge is

34

entered at the top layer, and ‘cell drying’ occurs (where the bottom of the cell lies

above the active groundwater surface, thus rendering that cell inactive, and ceasing its

productivity in the model until it is ‘re-wet’) then large quantities of recharge do not

enter the model itself and the model will not reflect ‘true conditions’. The best way to

constrain the rate of recharge is to obtain flux measurements (e.g. baseflow or

groundwater ages), however, distribution of recharge can be estimated without flux

data if the aquifer heterogeneity is characterised (Sanford, 2002)

Geological boundaries with low hydraulic conductivity are commonly treated as no-

flow boundaries. For example, in a valley-fill aquifer system, where the bedrock

contributes an insignificant amount of water to the valley-fill deposits, the bedrock

boundary could be conceptualised as an impermeable or no-flow boundary. This is

particularly useful in weathered bedrock valley walls composed of thick successions

of clay, like the Bells Creek plain. Anderson and Woessner (1992) suggest that

vertical permeability contrasts of greater than two orders of magnitude are sufficient

to merit a no-flow boundary

Comparison of Model and Parameter Uncertainties

Numerical modelling has enabled greater understanding of the effects of aquifer

properties on groundwater flow patterns (Alley et al., 2002). Uncertainty in model

accuracy is however, ironically, limited by the accuracy of measured field parameters.

The traditional approach to groundwater flow modelling is to continually modify the

hydrofacies boundaries and the hydraulic parameters until the model is calibrated

(Anderson and Woessner, 1992). In contrast, by developing a realistic (or as close to

reality as possible based on available hard data) conceptual model of aquifer

architecture, there is less margin to modify parameters and boundaries, forcing the

model to comply with reality (Artimo et al., 2003). By constraining parameterisation

in this manner, the potential for an unrealistic, yet at the same time calibrated model is

reduced dramatically. Failure to recognise the uncertainty inherent in both the model

input data and the calibration data can lead to ‘fine-tuning’ a model through

artificially precise parameter adjustments. These may be solely to improve the match

between observed and simulated heads, which may falsely increase the confidence in

35

outputs without producing an equivelent increase in its predictive accuracy (Konikow,

1999).

A groundwater model was developed to calculate groundwater flowpaths in glacial

moraine sediments near a landfill contaminant plume in western Cape Cod,

Massachusetts (Masterson et al., 1997). In this case a moderate revision to the general

hydraulic conductivity values in the model (from 15 m/day to 45 m/day) resulted in a

completely different set of model-calculated flowpaths (Masterson et al., 1997;

Franke et al., 1998). The sets of flowpaths were orientated 90 degrees apart and

situated several kilometres away from each other.

Scheibe and Murray (1998) compared various stochastic simulation techniques

(including sequential Gaussian simulation, sequential indicator simulation, and 2D

Markov Chain simulation) and demonstrated that different model conceptualizations

of the spatial structure of natural porous media can lead to quite different predictions

of groundwater flow. This proves that many non-unique, calibrated solutions can be

obtained for a given aquifer, without any of the solutions representing realistic

flowpaths within the aquifer. An excellent example of this was a small-scale synthetic

aquifer system consisting of unconfined glacial outwash gravels and sands, overlying

a shallow, flat-lying, low permeability bedrock was used to model total discharge and

travel time of groundwater (Massmann and Hagley, 1995). Five separate conceptual

models were used, with each model domain consisting of varying levels of

complexity relative to aquifer materials, whether these materials were homogenous or

isotropic, and whether any infiltration was present. Each of the five conceptual

models was simulated using four separate modelling methods, from Darcy’s Law to a

sophisticated finite-element numerical method. The variation caused by using

different models was compared to the variation caused by uncertainties in input

parameters via statistical analysis. In all cases, the variation between modelling

methods was less than that due to possible parameter uncertainty. Hydraulic

conductivity was found to be the parameter with the largest associated uncertainty

(Massmann and Hagley, 1995).

36

Groundwater Models Calibrated with Chemical and Isotopic Data: Examples

Izbicki et al. (2004) modelled groundwater flow paths and travel times for an alluvial

basin in Mojave River basin, southern California. Calculations were on the basis of

hydraulic data such as water levels and transmissivity, and agreed with flow paths

interpreted on the basis of the distribution of deuterium data and with groundwater

ages interpreted from carbon-14 and Tritium data. This study suggests that isotopic

data can act as an important constraint for groundwater flow model development

(Izbicki et al., 2004).

Dahan et al. (2004) applied a similar approach to groundwater modelling of the

Fernley Basin in western Nevada. A multi-variable (qualitative) mixing cell model

was constructed to represent the hydrochemistry, while a numerical model was

constructed to represent the groundwater flow paths. During calibration, the numerical

model was adjusted so that hydraulic conductivity values were changed to ensure

groundwater flowpaths honoured the interpreted basin geochemistry. This type of

calibration is very common in the modelling community and is certainly very

applicable and a powerful prediction tool in basin-wide investigations. The merit of

this approach at a macroscale is certainly arguable, particularly in alluvial aquifers.

Reilly et al. (1994) utilised environmental tracers in combination with numerical

MODFLOW simulation methods to provide an estimate of the flow rate and path of

water moving through a shallow aquifer near Locust Grove, in eastern Maryland,

USA. They concluded that the combination of these two methods provided feedback

on a more objective estimate of the uncertainties in the rates and paths of

groundwater. They did not resolve the heterogeneity present in the aquifer, and were

suffice to gain a first-pass estimate of flowpaths.

Detailed Hydrofacies Descriptions Leading to Effective Groundwater Modelling:

Examples

Long-term groundwater flow modelling was conducted for vertically-stacked aquifers

(St. Peter-Prairie and du Chien-Jordan aquifers) of varying hydraulic properties in

Rochester, Minnesota. The models displayed irregular groundwater flowpaths and

delineated recharge areas, with irregularities strongly influenced by the stacking of the

sedimentary beds (Delin and Almendinger, 1993; Franke et al., 1998).

37

Anderson et al. (1999) utilised hydrofacies distributions (from GPR and outcrop

analysis) as input into flow models (based on the code MODFLOW) to quantify

groundwater processes in two distinctly different braided stream deposits (a proximal,

Scott-type assemblage versus a medial, Donjek-type assemblage). After assigning

mean hydraulic conductivity (K-values) values for each hydrofacies (based on

literature), a hydraulic gradient was imposed and MODFLOW (McDonald and

Harbaugh, 1988) was used to solve the head distribution. Numerical experiments with

the particle tracking code PATH3D (Zheng, 1991) were then conducted to trace paths

of test-case particles through each medium. To isolate the contrasts in K-values a

constant effective porosity of 0.2 was assigned. High particle concentrations were

found to occur in zones with three things in common: 1) each contained a high-

conductivity hydrofacies (Figure 11), 2) each was laterally continuous, and 3) each

was constrained by relatively finer-grained facies (eg Fm or Fl) which form channel

boundaries that keep the particles confined within high-permeability flow paths

(Anderson et al., 1999).

This application clearly shows that an increased understanding of the aquifer

architecture improves the resolution and realism of calculated groundwater flowpaths

in a given sedimentary setting. This particular example is at a much finer scale than

‘bedding scale’ or macroscale (60m x 50m x 3m), however, the principles that

underpin the relationship between hydrofacies and groundwater flowpaths carry

across to much larger scales.

38

Figure 11: Three-dimensional image of the hydrofacies in the proximal Scott-type

deposit. The dimensions of the deposit are 60 m in the direction of flow, 50 m wide

and 3.3 m thick. Colours represent variable types of gravel and sand. The inset

displays the down-gradient or exit face through which particles left flow system

during particle tracking experiments. The colours in the inset represent particle

concentrations where red, yellow and blue represent high concentrations (areas C and

D). The particles within the area labelled D travelled through a tabular sheet-like body

of gravel shown near D1. (Anderson et al., 1999).

39

4. SCIENTIFIC AND SOCIETAL SIGNIFICANCE

Societal Significance

For groundwater to be managed as a natural resource, it must first be understood in

terms of a complex groundwater flow system. Essentially, each groundwater system is

a three-dimensional entity of sediments and earth material saturated with moving

groundwater that extends from areas of recharge to areas of discharge (Focazio et al.,

2002). With rapid increases in groundwater contamination from industrial, residential

and agricultural activities, as well as instances of salt water intrusion in coastal

aquifers, a demand for aquifer characterisation in all types of geological settings is

required at all scales.

SALTWATER INTRUSION

Coastal plain aquifer systems in particular are complex in nature owing to intricate

vertical and horizontal changes in sedimentary deposits over very small intervals. The

demand for fresh water in coastal regions is extremely high, and as a consequence

intensive groundwater extraction often causes sea water intrusion and a deterioration

of water quality (Foreman, 2003; Tulipano, 2003; Mantoglou et al., 2004; Milnes and

Renard, 2004; Park and Aral, 2004). Saltwater intrusion is basically caused by

extracting more groundwater than is sustainable via recharge, and as a consequence

adjacent bodies of salt water are drawn into the extraction zone of influence. The only

effective management of these coastal aquifers, therefore, is to develop simulation

groundwater models for calculating and predicting the maximum pumping rates while

still maintaining suitable water quality (Mantoglou et al., 2004). Detailed aquifer

characterisation is therefore a critical component in the development of realistic

conceptual models for input into numerical models.

ASSESSMENT OF CONTAMINATED SITES

Cleanup of groundwater contamination requires a detailed site characterisation, and

regardless of the disposal or remediation technology of choice for a contaminated site,

large uncertainties remain where information is lacking on aquifer heterogeneity

(Davis, 2001). Connected, high-conductivity hydrofacies form avenues for

contaminant movement while connected low-conductivity hydrofacies form barriers

40

to flow that promote channeling of contaminants into the high-conductivity pathways

(Anderson et al., 1999).

The risks faced (for contamination clean-up) are often much higher for heterogeneous

aquifers than for relatively homogenous aquifers (NRC, 1994; Davis, 2001). Without

proper aquifer characterisation based on the sedimentary geology of an area, the

conceptualization of the aquifer hinges on statistical analysis to calculate an

appropriate equivalent medium. This type of equivalent medium analysis is presently

being considered for use in restoring thousands of contaminated Department of

Defence sites in the United States, the cost of which is estimated in the billions of

dollars (Peters et al., 1998). However, if alluvial or fluvial channels exist as aquifers

at any of these sites, then these channels will either form fast migration paths, if they

are filled with high conductivity material, or provide a barrier to the migration, if

they are filled with low conductivity sediments (Teles et al., 2004). In such cases, the

geostatistical approach used to characterise the aquifers will fail to infer the

connectivity properties of sedimentary structures and will generate disconnected

ellipsoids of either high or low conductivity (Teles et al., 2004). Without successful

subsurface characterisation, any attempt at remediation of these sites will inevitably

fail, and could lead to disastrous environmental degradation. At contaminated sites

where alluvial or fluvial channels do not occur, monitored natural attenuation has

recently received attention as a cost-effective remedial alternative but detailed

geological site characterization is still critical (Anderson et al., 1999).

Scientific Significance

A recent review by Miller and Gray (2002) outlined important research directions for

hydrogeology in the 21st century, with an aim to focus research into major

groundwater-related scientific problems requiring attention. Twelve key scientific

issues were raised in that paper, and of those, three issues directly related to resolving

aquifer heterogeneity: 1) the issue of scale, from molecular to regional; 2) parameter

uncertainty within stochastic subsurface systems; and, 3) development of different

characterisation techniques to describe subsurface aquifer heterogeneity.

41

SUMMARY

It is apparent that sedimentary aquifer architecture is a major control on groundwater

flow in shallow unconsolidated materials such as those found in coastal plains.

Sedimentary heterogeneity promotes both preferential pathways as well as barriers to

groundwater flow depending upon the distribution of materials of variable

permeability. However, quantitative characterisation of subsurface heterogeneity in

such areas has largely been conducted with a sedimentological goal, and not used to

define groundwater provinces. New methodologies, such as Ground Penetrating

Radar, provide significant potential in hydrogeological studies, and enable a

quantitative assessment of the subsurface distribution of different sediments. This

high-resolution definition of aquifer architecture is an effective approach to determine

realistic groundwater flowpaths with numerical modelling. This integrated approach

provides a robust hydrogeological conceptual model, within which hydrochemistry

can be applied to further confirm groundwater processes, such as recharge, flow and

mixing.

“Hydrogeologic understanding is limited currently by field measurements, not

by a lack of models. Increasing model complexity likely will be beneficial, but

only if commensurate data are collected to constrain these models”.

(Halford 2004)

42

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52

PAPER 1

Influence of weathering and sea-level changes on evolution of Late Quaternary

sedimentary profiles within an east Australian coastal plain

T.R. Ezzy, M. E. Cox, A. J. O’Rourke, and G. J. Huftile

School of Natural Resource Sciences

Queensland University of Technology

(QUT)

Submitted to Sedimentary Geology Journal

95

PAPER 2

Groundwater flow modelling within a coastal alluvial plain setting using a high-

resolution hydrofacies approach; Bells Creek plain, Australia

T.R. Ezzy, M. E. Cox, A. J. O’Rourke, and G. J. Huftile

School of Natural Resource Sciences

Queensland University of Technology

(QUT)

Paper Accepted by Hydrogeology Journal

132

PAPER 3

Hydrogeochemical indicators of recharge processes and groundwater flowpaths

within shallow coastal plain aquifers, eastern Australia

T.R. Ezzy and M. E. Cox

School of Natural Resource Sciences

Queensland University of Technology

(QUT)

Paper for submission to Environmental Geology Journal

171

Table 1: Chemical and isotopic analyses of groundwater sampled in August 2002 (following major recharge event). Piezometer locations are shown in Fig. 2.

Note: ionic concentrations in mg/L, EC values in mS.cm-1, isotope values in 0/00, Eh (redox potential) in mV, Electro Neutrality (E.N.) values in 0/0. ID: * denotes creek sample. Aquifer: B=bedrock, A=alluvial, WB=weathered bedrock, EC=estuarine clay, D/CS=dune & coastal sand, FC=floodplain clay. Type: U=unconfined, S=semi-confined, C=confined. Depth: S=shallow (0-6m), M=medium (6-12m), D=deep (12+ m), Tidal= tidal reaches of creek, Fresh= fresh reaches of creek.

ID Aquifer Type Depth pH EC Eh Na K Mg Ca Sr Mn Fe Al H2SiO3 Cl Br SO4 NO3 HCO3 E.N. δ18O δ2H

A1 A U S 7.1 0.35 +14 86.3 3.98 10.20 11.03 0.14 0.87 14.01 15.48 65.9 136.0 0.13 24.45 8.67 56 3.40 - -

B3 (2) A S M 7.61 0.45 -58 47.0 0.92 3.73 1.10 0.01 0.04 0.80 0.07 20.8 85.5 0.25 3.53 0.00 8 -3.10 -3.18 -13.4

B7 A U S 7.21 0.25 -40 39.6 0.98 3.20 1.65 0.03 0.02 0.83 0.43 41.3 47.2 0.18 6.52 0.28 29 4.10 -2.84 -12.7

B10 A U M 6.64 0.40 +33 73.3 0.65 2.83 2.63 0.04 0.05 2.52 0.31 32.2 44.4 0.12 11.52 0.12 106 6.10 -3.74 -17.3

GB1 A S D 6.40 11.00 -43 1753.8 80.28 270.06 218.82 2.39 0.62 17.74 0.76 22.8 2894.2 9.80 1145.20 0.00 334 0.50 - -

H6 A S M 5.68 1.43 +138 461.0 19.59 64.15 71.57 0.77 0.36 28.45 0.52 14.1 912.6 0.76 33.80 5.40 21 6.20 -3.84 -16.9

B1 B C D 7.01 0.10 +83 14.8 0.46 0.82 0.15 0.00 0.01 0.10 0.09 30.9 18.9 0.12 3.43 0.34 5 2.50 -3.91 -18.4

B3 (1) B C D 6.48 4.88 -34 440.0 8.56 124.78 319.25 2.76 0.54 7.61 0.27 44.0 1458.9 5.36 17.44 0.80 180 1.90 -3.22 -13.4

B12 B C D 6.60 6.57 -178 928.0 11.93 106.36 164.32 6.43 0.10 1.58 0.31 23.6 1792.1 3.80 0.90 1.20 633 -2.70 -2.82 -12.9

B2 WB S M 7.50 0.17 +54 17.2 0.76 2.16 0.85 0.01 0.01 0.27 0.81 16.0 26.4 0.76 5.39 5.56 0 2.20 -3.71 -17.4

B5 WB U S 6.80 0.15 +44 20.9 1.97 3.10 6.61 0.03 0.16 9.83 5.54 30.6 51.3 0.04 11.05 0.21 11 3.00 -2.93 -10.7

B8 WB S M 7.82 1.64 +61 158.3 1.18 20.24 21.19 0.44 0.41 8.73 1.78 113.6 183.8 0.99 19.20 0.45 109 14.80 -3.56 -15.7

B9 WB S M 6.80 0.25 +50 41.9 0.11 12.20 2.53 0.08 0.05 9.50 2.80 115.4 81.4 0.12 15.91 0.25 21 5.00 - -

H3 WB S M 6.86 0.24 +64 44.6 5.35 15.59 12.93 0.14 0.24 31.11 7.51 38.6 112.3 0.00 52.10 0.60 29 4.60 - -

S1 WB S D 6.34 1.00 +57 139.3 4.36 11.81 7.15 0.11 0.21 0.76 0.22 44.3 236.8 1.00 34.80 0.00 40 -3.40 - -

GB2 DCS S M 7.08 5.05 -108 479.7 25.27 140.88 368.40 2.52 0.68 41.73 2.94 59.9 896.8 3.51 945.27 0.00 299 3.00 - -

GB3 DCS S M 6.00 3.20 -27 276.6 21.12 100.42 210.80 1.53 1.63 22.68 2.61 18.2 291.0 1.68 944.28 0.00 225 1.00 - -

H1 EC S M 6.20 19.90 +49 3489.6 111.82 532.80 243.00 3.47 0.33 6.42 2.59 48.6 5960.8 16.40 1256.00 2.80 1169 -0.70 -2.81 -10.7

H4 FC S M 5.74 1.15 +144 222.6 7.88 6.42 2.95 0.05 0.02 1.41 0.83 53.6 80.9 0.00 405.54 0.50 5 -1.00 -4.68 -25.7

L1 FC S M 5.37 39.70 +81 6307.2 131.40 1009.20 577.68 10.31 11.55 98.47 2.17 33.6 13182.4 53.60 1976.00 8.80 56 -2.60 - -

Bc1* - - Tidal 5.67 54.40 +206 9675.0 358.80 1153.50 361.20 6.77 0.01 0.74 2.14 5.0 18769.5 64.80 2457.90 0.00 101 -3.50 - -

Bc3* - - Fresh 8.13 0.15 +110 12.3 1.98 2.34 2.44 0.02 0.03 1.14 0.92 10.4 16.1 0.00 6.04 1.24 16 4.50 -4.31 -25.0

Hc1* - - Tidal 6.74 3.49 +230 385.8 17.30 64.42 39.92 0.46 0.39 6.16 3.95 18.8 778.9 1.92 322.62 0.00 10 -7.40 - -

Hc2* - - Fresh 6.60 0.14 +71 17.7 1.94 2.66 1.90 0.02 0.03 1.13 1.47 12.7 26.7 0.14 10.74 0.30 11 0.50 - -

172

GENERAL CONCLUSIONS

This research project has confirmed the substantial influence of macro-scale

sedimentary aquifer heterogeneity on groundwater flowpaths and groundwater

processes within a heavily laterised, coastal plain setting in the Bells Creek

catchment. The geometry of individual aquifer boundaries, the inter-connectivity of

high- and low-permeability units and the complex nature of the evolution of

sedimentary features within this landscape, have all proven to be of critical

importance in accurately predicting subsurface fluid flow and solute transport.

A new approach using high-resolution aquifer characterisation was developed during

this research. The method follows a simple, sequential order in which: a) the physical

nature of the aquifers are defined; b) different groundwater bodies and groundwater

processes are identified; c) a conceptual hydrogeological model is established; and d)

the groundwater fluxes are calculated.

To define the physical nature of the aquifers, Paper 1 utilises multi-faceted

stratigraphic analysis (lithostratigraphy, morphostratigraphy, radar stratigraphy and

chronostratigraphy) to define the macro-scale sedimentary framework. The major

findings from the stratigraphic analysis were:

• Inverse relief in the laterite profiles has enforced alluvial deposition

throughout the Late Quaternary to be restricted to narrow, and relatively deep

valleys which now contain sand-rich channels, covered by thin floodplains nearer

to the surface.

• Interglacial sea level highstands have produced significant erosion of a relict

lowstand valley basin proximal to the east of the alluvial plain. That basin became

a coastal embayment during sea level transgression, and developed into a back-

barrier coastal plain during the ensuing stillstand period.

• The simple coastal plain valley fill contains three distinct phases of alluvial

deposition during the last glacial cycle: 1) during the Last Interstadial period; 2)

the Last Glacial Maximum (LGM); and 3) the Late Holocene. GPR was able to

image sediments from the LGM and the Late Holocene.

• GPR was very useful in imaging the subsurface architecture of the alluvial

channel deposits, as well as buried longshore bars on top of the Pleistocene

173

floodplain, and enabled development of a detailed three-dimensional isopach map

of the alluvial channels.

To identify different groundwater bodies and groundwater processes, Papers 2 and 3

utilised analysis of aquifer hydraulics (pumping tests and slug tests on individual

hydrofacies units), spatial and temporal measurement of groundwater levels, and

detailed analysis of the hydrochemical and isotopic character of aquifers throughout

the plain. The major findings from this analysis helped to define the characteristics of

the conceptual hydrogeological model. The major findings were:

• Hydrochemical and isotopic indicators have shown that recharge processes

and groundwater flowpaths within the plain are strongly related to sub-surface

permeability distributions as well as seasonal fluctuations in rainfall.

• Landward sandstone hills provide a delayed and slightly mineralized

component of groundwater recharge into adjacent highly permeable, unconfined

alluvial aquifers; these aquifers also recharge directly via precipitation.

• Aluminosilicate weathering in the bedrock hills and eastern peripheries of the

laterised bedrock provide excess Na, SiO2, and HCO3 to the alluvial groundwater.

As this groundwater flows down-gradient to the east, however, its chemical

composition evolves by sulfate reduction, silica equilibrium and ion exchange

processes into a more mature Na-Cl type.

• Within the shallow coastal aquifers proximal to the eastern shoreline, sulfate

enrichment is occurring resulting in major deterioration in groundwater quality.

• Reverses in salinity in those coastal aquifers have been correlated with surges

in fresh recharge waters from unconfined coastal dunes and semi-confined

landward alluvium, following significant rainfall events.

To calculate groundwater fluxes through the different hydrofacies units throughout

the plain finite-difference numerical groundwater flow modelling was used.

Modelling allowed all of the features of the conceptual hydrogeological model to be

tested in terms of recharge and interconnection of aquifers. The major findings from

this analysis were:

• The interconnection of narrow alluvial channels feeding into a deeper alluvial

delta has provided a significant conduit for shallow groundwater flow.

174

• Finite-difference groundwater modelling and particle tracking analysis has

clearly demonstrated that the macroscopic heterogeneity both within-facies and

between-facies, has marked impacts on groundwater pathways and especially

groundwater travel times in this plain.

• The variability between residence times of groundwater within the alluvium,

compared to groundwater within the mixed clay layers of the laterite, clearly

demonstrates the importance of accurately defining the spatial distribution of the

various aquifer materials in a groundwater flow investigation.

• The role of the alluvial delta in discharging the bulk of fresh groundwater

from the central plain into the coastal and estuarine aquifers to the east, is

certainly critical in preventing saline intrusion from encroaching further west.

The multi-disciplinary approach developed and applied here, has proven to be

effective in developing a fine-scale understanding of both groundwater flowpaths and

relative flow rates. By combining sedimentological, geophysical, chronological and

hydrogeological techniques the resultant conceptual hydrogeological model is

extremely robust in terms of both physical framework and dynamic water fluxes.

Through the establishment of a detailed conceptual model, numerical groundwater

modelling can be used to define high-resolution groundwater flowpaths and residence

times. A significant outcome of this research is the establishment of a new

hydrogeological analogue for alluvial channel and coastal aquifers within a laterised

coastal plain setting. Such a model was not previously available in the

hydrogeological literature.

175

176

APPENDICES

177

APPENDIX 1

PAPER A

Refereed Conference Paper: Implications of land-use changes on groundwater

within shallow coastal plain aquifers, Bells Creek catchment, southeast Queensland,

Australia.

Conference: TIAC 03: Coastal Aquifers Intrusion Technology: Mediterranean

Countries. Held in Alicante, Spain, March 2003

Reference: Ezzy, T.R. and Cox, M.E., 2003. Implications of land-use changes on

groundwater within shallow coastal plain aquifers, Bells Creek catchment, southeast

Queensland, Australia, in Lopez-Geta, J.A., de la Orden, J.A., de Dios Gomez, J.,

Ramos, G., Mejias, M., and Rodriguez, L., eds., Coastal Aquifers Intrusion

Technology: Mediterranean Countries, Volume 1: Hidrogeologia Y Aguas

Subterraneas No 8: Alicante, Spain, Instituto Geologico Y Minero de Espana, 439-

444.

187

APPENDIX 2

PAPER B

Refereed Conference Paper: Applying Ground Penetrating Radar (GPR) to

improve hydrogeological understanding and groundwater modelling within a

coastal plain setting.

Conference: TIAC 03: Coastal Aquifers Intrusion Technology: Mediterranean

Countries. Held in Alicante, Spain, March 2003

Reference: Ezzy, T.R., O'Rourke, A.J., Huftile, G.J., and Cox, M.E., 2003.

Applying Ground Penetrating Radar (GPR) to improve hydrogeological

understanding and groundwater modelling within a coastal plain setting, in

Lopez-Geta, J.A., de la Orden, J.A., de Dios Gomez, J., Ramos, G., Mejias, M.,

and Rodriguez, L., eds., Coastal Aquifers Intrusion Technology: Mediterranean

Countries, Volume 1: Hidrogeologia Y Aguas Subterraneas No 8: Alicante,

Spain, Instituto Geologico Y Minero de Espana, 149-156.