joanne m. jackson thesis

7/27/2019 Joanne M. Jackson Thesis

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HYDROGEOLOGY AND GROUNDWATER FLOW MODEL,

CENTRAL CATCHMENT OF BRIBIE ISLAND, SOUTHEAST QUEENSLAND

by

Joanne M. Jackson

Bachelor of Science (Honours)

SUPERVISOR

Assoc. Professor Malcolm Cox

Queensland University of Technology

A thesis submitted in partial fulfilment of the requirements for

the Degree of Master of Applied Science.

2007

School of Natural Resource Sciences

Queensland University of Technology

Brisbane, Queensland, Australia


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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: ..

Joanne Jackson

Date: ..


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ABSTRACT

Bribie Island is a large, heterogeneous, sand barrier island that contains

groundwater aquifers of commercial and environmental significance. Population

growth has resulted in expanding residential developments and consequently

increased demand for water. Caboolture Shire Council (CSC) has proposed to

increase groundwater extraction by a new borefield.

Two aquifers exist within the Quaternary sandmass which are separated by an

indurated sand layer that is ubiquitous in the area. A shallow aquifer occurs in the

surficial, clean sands and is perched on the indurated sands. Water levels in the

shallow water table aquifer follow the topography and groundwater occurs under

unconfined conditions in this system. A basal aquifer occurs beneath the indurated

sands, which act as a semi-confining layer in the island system. The potentiometric

surface of the basal aquifer occurs as a gentle groundwater mound.

The shallow groundwater system supports water-dependent ecosystems including

wetlands, native woodlands and commercial pine plantations. Excessive

groundwater extraction could lower the water table in the shallow aquifer to below

the root depth of vegetation on the island.

Groundwater discharge along the coastline is essential to maintain the position of

the saline water - fresh groundwater boundary in this island aquifer system. Anyactivity that changes the volume of fresh water discharge or lowers the water table

or potentiometric surface below sea level will result in a consequent change in the

saline water freshwater interface and could lead to saline water intrusion.

Groundwater level data was compared with the residual rainfall mass curve (RRMC)

on hydrographs, which revealed that the major trends in groundwater levels are

related to rainfall. Bribie Island has a sub-tropical climate, with a mean annual

rainfall of around 1358mm/year (Bongaree station). Mean annual pan evaporation

is around 1679mm/year and estimates of the potential evapotranspiration rates

range from 1003 to 1293mm/year.

Flows from creeks, the central swale and groundwater discharged from the area

have the potential to affect water quality within the tidal estuary, Pumicestone

Passage. Groundwater within the island aquifer system is fresh with electrical

conductivity ranging from 61 to 1018S/cm while water near the coast, canals or

tidal creeks is brackish to saline (1596 to 34800S/cm). Measurements of pH show

that all groundwater is acidic to slightly acidic (3.3-6.6), the lower values areattributed to the breakdown of plant material into organic acids.


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Groundwater is dominated by Na-Cl type water, which is expected in a coastal

island environment with Na-Cl rainfall. Some groundwater samples possess higher

concentrations of calcium and bicarbonate ions, which could be due to chemical

interactions with buried shell beds while water is infiltrating to depth and due to the

longer residence times of groundwater in the basal aquifer.

A steady-state, sub-regional groundwater flow model was developed using the

Visual MODFLOW computer package. The 4 layer, flow model simulated the

existing hydrogeological system and the dominant groundwater processes

controlling groundwater flow. The numerical model was calibrated against existing

data and returned reasonable estimates of groundwater levels and hydraulic

parameters. The model illustrated that:

The primary source of groundwater recharge is infiltration of rainfall for the

upper, perched aquifer (Layer 1). Recharge for the lower sand layers is via

vertical leakage from the upper, perched aquifer, through the indurated sands

(Layers 2 and 3) to the semi-confined, basal aquifer (Layer 4).

The dominant drainage processes on Bribie Island are evapotranspiration

(15070m3/day) and groundwater seepage from the coast, canals and tidal

creeks (9512m3/day). Analytical calculations using Darcys Law estimated that

approximately 8000m3/day of groundwater discharges from central Bribie Island,

approximately 16% less than the model.

As groundwater flows preferentially toward the steepest hydraulic gradient, the

main direction of horizontal groundwater flow is expected to be along an east-

west axis, towards either the central swale or the coastline. The central swale

was found to act as a groundwater sink in the project area.


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ACKNOWLEGDEMENTS

I would like to thank everyone who helped in the completion of this research project.

The successful completion of this study has been made possible through the

practical and professional support and advice of many people, institutions and

departments, in particular:

I appreciate the support, guidance and expertise of Associate Professor

Malcolm Cox (principal supervisor), School of Natural Resource Sciences,

Queensland University of Technology.

Queensland University of Technology Staff

Dr. Micaela Preda, Dr. Deliana Gabeva, Wathsala Kumar, Bill Kwiecien and Dr.

Les Dawes.

Other Students: John Harbison, Tim Armstrong, Ken Spring, Lucy Paul and

Genevieve Larsen.

Funding for this study was provided by:

Caboolture Shire Council, QM Properties and Pacific Silica.

I appreciate the assistance and data provided by:

Bureau of Meteorology

Caboolture Shire Council

Caloundra City Council

Department of Natural Resources, Mines and Energy

Forestry Plantations Queensland (previously DPI Forestry)

HLA Envirosciences Pty. Ltd

Matrix Plus Consulting Pty Limited

QM Properties

Queensland Parks and Wildlife


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TABLE OF CONTENTS

1. INTRODUCTION 1

1.1 Aims and Objectives 2

1.2 Scope of Work 3

1.2.1 Data Review 3

1.2.2 Field Work 3

1.2.3 Interpretation of Resul ts 4

1.3 Significance of Project 4

2. BACKGROUND 7

2.1 Location 7

2.2 Topography and Vegetation 8

2.3 Climate 8

2.4 Land Use 8

2.5 Geomorphology 10

2.6 Regional Geology 12

2.6.1 Landsborough Sandstone Formation 14

2.6.2 Quaternary Sand 17

2.6.3 Indurated Sandstone 18

2.7 Regional Hydrogeology 19

2.7.1 Aquifer Recharge 20

2.7.2 Drainage 21

2.7.3 Hydraulic Parameters 22

2.8 Previous Work, Bribie Island 22

2.8.1 Groundwater Studies 22

2.8.2 Groundwater Modelling 25

3. METHODOLOGY 27

3.1 Hydraulic Monitoring Network 27


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3.1.1 Climate 27

3.1.2 Monitoring Bore Network 27

3.1.3 Groundwater Quality 28

3.2 Modelling 29

3.2.1 Conceptual Model 29

3.2.2 Mathematical Modelling 30

4. RESULTS 44

4.1 Hydraulic Monitoring Data 44

4.1.1 Climate 44



4.2 Modelling 55

4.2.1 Conceptual Model 55

4.2.2 Mathematical Modelling 56

5. DISCUSSION AND SUMMARY 67

5.1 Hydraulic Monitoring 67

5.1.1 Climate 67



5.2 Modelling 70

5.2.1 Analytical Solut ion 70

5.2.2 Numerical Modelling 70

6. CONCLUSIONS AND FURTURE CONSIDERATIONS 75

6.1 Monitoring Bore Network 75

6.2 Groundwater Quality 76

6.3 Numerical Model 77

7. REFERENCES 80


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LIST OF FIGURES

Figure 1. Location map of Bribie Island 7Figure 2. Land use map of Bribie Island 9Figure 3. Sea level fluctuation in the Late Quaternary 11Figure 4. Sedimentary basins in Moreton region 13Figure 5. Lithology of Bribie Island showing Quaternary sedimentary deposits 15Figure 6. Hydrogeological cross section showing monitoring bores 16Figure 7 Maximum extent of the sea during the last inter-glacial 17Figure 8. Pumicestone Region Catchment showing Bribie Island subcatchment 20Figure 9. Mathematical models are based on a conceptual understanding 30Figure 10. Model configuration of central Bribie Island 33Figure 11. Cross section of model showing the four model layers. 35Figure 12. Topography for the whole of Bribie Island. 36Figure 13. Boundary conditions for a) Layer 1 and b) Layers 2, 3 and 4 38Figure 14. Location of 25 shallow monitoring bores used in the model (Layer 1) 39Figure 15. Location of 20 deep monitoring bores used in the model (Layer 4) 40Figure 16. Zones of hydraulic conductivities showing observation bores 41Figure 17. Evapotranspiration zones split according to dominant vegetation type 42Figure 18. Summary of the four types of sensitivity 43Figure 19. Mean daily temperatures for Caloundra, Cape Moreton and Redcliffe 44Figure 20. Average monthly rainfall on southern Bribie Island 45Figure 21. Mean monthly rainfall compared to mean monthly pan evaporation 45Figure 22. Location of monitoring bores used to build the geological framework 46Figure 23. Heterogeneous sandmass of Bribie Island 48Figure 24. Hydrograph of long-term groundwater levels and the RRMC 49Figure 25. Cross section through central Bribie Island showing grounwater 50Figure 26. Trilinear plot of groundwater chemistry samples 52Figure 27. Schoeller plot of groundwater chemistry samples 53


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Figure 28. Stiff patterns overlain on the cross section through central Bribie Island 54Figure 29. Hydraulic conductivities determined mathematically using WinPEST 59Figure 30. Simulated water levels from steady-state model 60Figure 31. Calculated verses observed water levels, steady-state model 62Figure 32. Mass balance for steady-state model 62Figure 33. Sensitivity analysis for steady-state model 66

LIST OF TABLES

Table 1. Stratigraphical succession 14Table 2. Results of hydraulic testing 22Table 3. Field parameters measured with a TPS meter 28Table 4. Parameters tested for during water chemistry analysis 29Table 5. Hydrogeological layers used in the model 35Table 6. Groundwater physico-chemical measurements from monitoring bores 51Table 7. Estimated groundwater discharge 56Table 8. Zone budget for steady-state model 64

APPENDICES

Appendix A Climate Records

Appendix B Mean Pan Evaporation

Appendix C Summary of Monitoring Bore Details

Appendix D Standing Water Levels and Physico-chemical Parameters

Appendix E Groundwater Chemical Analyses

Appendix F Steady-state Groundwater Flow Model

Appendix G Observed and Calculated Water Levels - Steady-state Model


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1. INTRODUCTION

Groundwater is a pervasive and vulnerable resource. Hydrogeological

investigations must be conducted to enable us to sustain and protect these

resources, the ecosystems that they support and to overcome problems water

quality issues such as salinisation and pollution. In order to achieve these goals,

we require an understanding of the fundamental processes that control groundwater

quantity and quality.

Coastal zones are often densely populated areas that experience high demand for

fresh water. In coastal aquifers, water quality degradation resulting from saline

water intrusion is a common issue of concern.

Growing demands from industry, energy production, urban population centres and

agriculture place an increasing strain on the quantity and quality of water resources.

In combination with traditional hydraulic monitoring methods, mathematical

modelling has emerged as an important tool used to understand groundwater flow

in aquifers. In the following examples, models are used to assess various

groundwater aquifers:

Aveiro Aquifer, Portugal - a Cretaceous coastal aquifer was modelled to give a

better understanding of the groundwater flow conditions and the existing

geochemical processes. Mathematical modelling confirmed a reduction of the

naturally occurring hydraulic gradient and limited aquifer recharge from natural

sources (Condesso de Melo et al, 1998).

Big Pine Key, Florida, USA a small oceanic island with several canal

developments. The study examined the types of canals that are most detrimental to

the fresh groundwater supplies. It was found that the effect of the canals depended

on the relative penetration and position of the development. Canals bisecting long,

rectilinear islands reduced the groundwater lens volume more than canal

developments at the ends of the islands (Langevin et al, 1998).

North Stradbroke Island, Queensland, Australia - a large sand island that extracts

surface and ground water for town supply and for mining operations. A whole-of-

island groundwater flow model was developed with MODFLOW and PEST-ASP to

assist with managing the long-term sustainability of these resources (Chen, 2002).


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Trinity Aquifer, Texas, USA - a multilayer, sedimentary aquifer. The groundwater

flow model was developed with MODFLOW to predict water level responses to

pumping and drought. This enabled the prediction of areas likely to be impacted

from declining water levels in the future scenarios (Mace et al, 2000).

1.1 AIMS AND OBJECTIVES

This study aimed to characterise the existing groundwater environment and to

conceptualise groundwater flow processes in the central catchment of Bribie Island,

near the Pacific Harbour canal estate and residential golf course developments.

The objectives of the hydrogeological study were to:

establish a geological framework for the area from existing drill hole data

and downhole gamma-ray logs.

evaluate the link between the upper and lower aquifers by monitoring

groundwater levels and testing groundwater quality.

integrate the data to develop a conceptual hydrogeological model for the

area.

calculate a preliminary estimate of groundwater discharge from the central

catchment of Bribie Island using Darcys Law.

simulate the dominant processes controlling groundwater flow and discharge

by developing a 3D groundwater flow model in the central catchment of

Bribie Island, using the Visual MODFLOW program (version 3.1 with

WinPEST). The purpose of the model is to assist in understanding

groundwater flows through the aquifer system in the central catchment of the

island.


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1.2 SCOPE OF WORK

1.2.1 Data Review

The data review process involved:

acquiring and reviewing available geological and hydrogeological

information. Data was sourced from the Department of Natural Resources,

Mines and Water (DNRMW) database of registered bores, HLA

Envirosciences Pty Ltd (on behalf of QM Properties) and Queensland

University of Technology (QUT). Information reviewed included lithological

logs, gamma-ray logs, results of groundwater quality and water level

monitoring and hydraulic testing within monitoring bores.

acquiring and reviewing climatic information. Data was sourced from the

Bureau of Meteorology (BOM), the DNRMW and University of Queensland

(UQ). Information included rainfall records, temperature and pan

evaporation data.

reviewing reports of previous studies undertaken in the Bribie Island region.

1.2.2 Field Work

The field program was designed to obtain site-specific information in the central

catchment of Bribie Island, near the Pacific Harbour residential golf course

development. The field program included:

monitoring of groundwater levels within existing monitoring bores to

determine static water levels; and

sampling and laboratory analysis of groundwater from selected monitoring

bores within each aquifer to acquire water chemistry information.

The data gathered aimed to assist with understanding groundwater quality,groundwater occurrence and flow processes within the system and to support the

development of the conceptual and numerical models.


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1.2.3 Interpretation of Results

The interpretation of results involved:

collating and analysing results from the field program;

interpreting geological and hydrological data to develop a conceptual model

for the central Bribie Island area; and

assessing groundwater occurrence and flow processes in the central

catchment of Bribie Island by developing a rudimentary 3D groundwater flow

model using the Visual MODFLOW computer program.

1.3 SIGNIFICANCE OF PROJECT

Bribie Island is a large, sand barrier island that contains groundwater supplies of

commercial and environmental significance. There are competing demands on this

groundwater system that have lead to an increased stress on the local groundwater

resources. These groundwater resources are finite and must be carefully managed.

Groundwater discharge from this island aquifer system is essential to

maintain the position of the saline water - fresh groundwater boundary and

thus protect the aquifer system. The quantity and quality of environmental

flows from creeks and groundwater discharged from the area has the

potential to affect water quality within the tidal estuary, Pumicestone

Passage.

Tidal wetlands and waters around Bribie Island are protected as part of

Moreton Bay Marine Park. The passage provides a breeding area for fish,

crabs and prawns and it contains a population of dugong that feed on its

seagrass beds. The region provides an essential habitat for many species

of migratory and non-migratory birds. Due to its extensive system of tidal

flats, mangroves, salt marsh and claypan, the passage has been listed

under the Ramsar Convention as an important site for roosting and feeding

for migratory species. The Ramsar Convention is an international treaty that

aims to preserve intertidal feeding banks in both hemispheres and along the

flight paths of migratory bird species (South East Queensland Regional

Strategy Group, 2000).


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The shallow groundwater system supports water-dependent ecosystems

including wetlands, native woodlands and commercial pine plantations.

Commercial pine plantations and National Parks cover a significant portion

of the island. Native vegetation within the National Parks largely consists of

heaths, paperbark wetlands, open forests and woodlands. The vegetation

on the island is phreatophytic (i.e. the plants send a root to groundwater)

and utilise the shallow, perched groundwater system.

Population growth has resulted in expanding residential developments,

including the Pacific Harbour canal and golf course residential estates.

Population growth across the southern portion of Bribie Island has led to an

increased demand for water for domestic, industrial and horticultural uses.

Caboolture Shire Council (CSC) has proposed to increase groundwater

extraction to make the island self-sufficient for water supply. In late

September 2006, CSC commenced test drilling and construction of

production bores on the island. The CSC estimates that the new borefield

could produce an environmentally sustainable yield of up to 10 megalitres of

water per day.

Areas of concern that relate to an excessive extraction of groundwater along coastal

zones include:

seawater infiltration into the island aquifer system. Saline water intrusion is

the most common type of water quality degradation that occurs in coastal

aquifers (Fetter, 2001). Saline water sources for Bribie Island include the

seawater surrounding the island and surface tidal waters in natural estuaries

and in artificial canals. The position of the saline water - fresh groundwater

boundary is a function of the volume of fresh water discharging from the

aquifer system. Any action that changes the volume of groundwater

discharge or lowers the water table or potentiometric surface below sea level

will result in a consequent change in the saline water freshwater interface

(Driscoll, 1986; Fetter, 2001).


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impact on native vegetation (including wetlands and native woodlands) and

commercial pine plantations resulting from changes to groundwater levels.

Phreatophytic vegetation on the island is supported by the fresh water in the

shallow groundwater system. Excessive groundwater extraction could lower

the water table in the shallow aquifer below the root depth of vegetation on

the island.

Exploitation of groundwater resources has the potential to significantly alter

groundwater levels and intrinsic processes. These processes may influence and

even control the health of associated ecosystems.

Groundwater level monitoring and water quality testing will assist with characterising

the existing groundwater environment and developing an understanding of the flow

processes involved. Building a conceptual model and a rudimentary mathematical

model will assist with conceptualising groundwater occurrence and flow processes

in the central catchment of Bribie Island, near the Pacific Harbour developments.


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2. BACKGROUND

2.1 LOCATION

Bribie Island is located on the east coast of Australia, approximately 65 km north of

Brisbane as shown in Figure 1. It lies parallel to the southern Queensland coastline

and forms the northwestern perimeter of Moreton Bay. Bribie is separated from the

mainland by the narrow, tidal estuary, Pumicestone Passage.

The island lies between 26o 49 South and 27o 06 South latitude and 153o 04 20

East and 153o 12 30 East longitude. Bribie covers an area of approx 150 km2, is

around 30 km long and ranges from 5 to 7.5 km wide.

Shornc liffe

Redcliffe

Ningi

Caloundra

Bribie

Island

Moreton

Isla nd

North

Stradbroke

Isla nd

UMICESTONE

ASSAGE

P

P

ECEPTION

AY

D

B

MORETON

BAY

Caboolture

River

Pine River

Brisbane

River

0 10 20

Kilometres

1533015300

2730

2700

Figure 1. Location map of Bribie Island


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2.2 TOPOGRAPHY AND VEGETATION

Bribie Island is a low lying, vegetated, sand barrier island. The topographic highs

occur on the beach ridge systems, with a maximum elevation of around 14m above

Australian Height Datum (AHD). The ridges slope gently down into the central

swale area and to the coastline.

The shallow groundwater aquifer on Bribie Island supports exotic and native,

phreatophytic vegetation. Exotic, commercial pine plantations cover a large portion

of the northern and central areas of Bribie. The rooting depth of mature aged pines

in unsaturated soil profiles could range from 3 to 5 metres (and use up to 150ml of

water a day) (K. Bubb, pers comm., 2005). Large areas of remnant native

vegetation occur on the island including Acacia scrub, Banksia woodland, softwoodscrub, Melaleuca forest, eucalypt woodland and heath communities. Dense stands

dominated by Melaleuca quinquenervia (broad-leaved paperbark) occur mainly in

the low, poorly drained areas, such as the central swale, along the western side of

the island (James and Bulley, 2004). This species of vegetation usually grows best

in swampy sites surrounded by open forest (Boland et al, 1992).

2.3 CLIMATE

The island has a sub-tropical climate and experiences a wet summer and a dry

winter. Mean annual rainfall from the Bongaree station is 1358mm/year (Bureau of

Meteorology).

Pan evaporation values fluctuate with the seasons with maximum values occurring

from October to January. The mean annual pan evaporation values recorded at the

University of Queensland, Bribie Island weather station were 1679mm/year (DNR,

1996). Potential evapotranspiration rates were estimated from water balance

models and range from 60% (~1003mm/year, Williams, 1998) to 77%

(~1293mm/year, Bubb and Croton, 2000) of pan evaporation.

2.4 LAND USE

Figure 2 displays the main land uses on Bribie Island which include native

vegetation, exotic pine plantations, residential and recreational areas (golf clubs,

parks and sports fields). Caboolture Shire Council administers the southern two

thirds of the island. Urban development is restricted to southern part of Bribie,

which is experiencing rapid population growth. Caloundra City Council manages

the northern one third of the island, which does not contain residential areas.


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Figure 2. Land use map of Bribie Island

showing the extent of vegetation and development on the island

(modified from Caboolture Shire Council, 2003)


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There are two conservation reserves on the island, the Bribie National Park

(4770ha) and the Buckleys Hole Conservation Park (87.7ha). All tidal areas and

waters around the island are gazetted as the Moreton Bay Marine Park (EPA).

Bribie Island currently uses two sources to supply urban water demands. Local

groundwater treated at the Bribie Water Treatment Plant supplies the southern and

eastern areas, while the mainland North Pine Dam Water Treatment Plant supplies

water to northern areas and meets demand above the capacity of the Bribie Water

Treatment Plant (CSC). Brisbane City Council (BCC) is responsible for the

operation and maintenance of the North Pine WTP.

Sewage is piped to the Sewage Treatment Plant, which is located in the

southwestern corner of the water reserve on southern Bribie Island. Treated

sewage is discharged into infiltration ponds south of the sewage treatment plant

(Isaacs and Walker, 1983; Marszalek and Isaacs, 1988).

Currently (October December 2006) Caboolture Shire Council is undertaking test

drilling and construction of production bores on the island. Pumping tests are being

conducted within the new bore field to determine yield capacities.

2.5 GEOMORPHOLOGY

Moreton Bay is formed by large sand islands on its eastern side. Sea level change

has dominated the geological history of Moreton Bay. Eustatic oscillations have

resulted in the emergence and submergence of the coastal lowlands within an

altitudinal range of approximately 150m since the beginning of the Pleistocene.

Figure 3 illustrates the amplitude of the sea level rise at the conclusion of the last

Ice Age, reaching a maximum height (+1.5m) around 6500 years ago. Sea levels

dropped to present levels around 3000 years ago (DEH, 1993; Jones, 1992a; Lang

et al., 1998).

These sea level oscillations created a series of differing environments that

controlled the deposition of sediment. During periods of low sea level, the floor of

Moreton Bay was exposed and rivers could incise channels and flow across the bay

surface. As sea levels rose, the sediments were submerged, but while the water

was still relatively shallow, waves were able to wash some sediments towards the

shore to accumulate on beaches and foredunes (DEH, 1993; Jones, 1992a; Lang et

al., 1998).


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Figure 3. Sea level fluctuation in the Late Quaternary

showing when Moreton Bay was dry

(modified from Jones, M.R. (1992a); Lang et al., (1998))

Bribie Island is best considered as a low lying, sand barrier island. The island

developed when a strandplain of prograded beach ridges bordering the coast was

separated from the mainland by the formation of Pumicestone Passage tidal

estuary. The sequence of sand dunes evolution extends from the Holocene period

(less than 10,000 BP) to before the last Pleistocene interglacial period (120,000-

140,000 BP) (DEH, 1993; Cox et al., 2000b).

The evolutionary classification of depositional coastal environments is based on the

relative roles of three main hydrodynamic processes: waves, tides and river outflow.

In this framework, coastal barriers can be considered the basic depositional element

on wave-dominated coasts. On these barriers the coastal dune, beach and

shoreface are sub-environments that make up large-scale coastal accumulation

features (Masselink and Hughes, 2003).

Barriers occur typically as elongated, shore-parallel sand bodies that extend above

sea level. A back-barrier environment, such as an estuary or lagoon, generallyoccurs between the barrier and the mainland (Masselink and Hughes, 2003). In the

case of Bribie Island, Pumicestone Passage formed as a passage-type estuary as a

result of the development of the Bribie Island barrier (DEH, 1993; Cox et al., 2000b).


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2.6 REGIONAL GEOLOGY

Bribie Island is located at the edge of the Late Triassic to Early Jurassic age

Nambour Basin in coastal southeastern Queensland as seen in Figure 4. The

Nambour Basin is a small, intracratonic basin with rock assemblages of less than

600m thick. The western boundary of the basin is adjacent to the Palaeozoic

basement rocks of the DAguilar Block to the northwest and the Beenleigh Block to

the southeast (McKellar, 1993; Cox et al., 2000b; Geoscience Australia, 2003).

Sediment for the Nambour Basin was derived from the erosion of mountains to the

south and west of the coastline. Sandy sediments with minor gravel and mud were

deposited on broad plains by braided rivers in the eastern part of the region. These

areas gradually subsided allowing a greater thickness of sediments to accumulate.These sediments consolidated to form the Landsborough Sandstone in the southern

Nambour Basin, which forms the bedrock for the Pumicestone catchment (Willmott

and Stevens, 1988; Cox et al., 2000b).

The regional basin experienced a Late Triassic Norian orogeny and the resulting

uplift exposed the newly stabilised continent to erosion. Ongoing erosion carved

the present landscape, depositing material in floodplains, as well as carrying

sediment out to sea (Cox et al., 2000b; Geoscience Australia, 2003).

Fluvial sediments of the Early Jurassic Landsborough Sandstone Formation form

the bedrock below Bribie Island, although no outcrop of this formation occurs on the

island. Quaternary age (Pleistocene and Holocene) sand deposits overlie this

sedimentary rock unit. Table 1 lists the stratigraphical succession for Bribie Island

and Figure 5 shows the Quaternary sedimentary deposits on the island. Through

the interpretation of geological logs and downhole gamma-ray logs of monitoring

bores, a lithological cross section of central Bribie Island was developed and is

shown in Figure 6 (Armstrong, 2006).


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Figure 4. Sedimentary basins in Moreton region

(modified from Geoscience Australia, 2003)


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Age Li thology

Holocene accretion ridges and swales

Holocene to Pleistocene undifferentiated sediments - mainly back barrierdeposits of sand and mud

Pleistocene accretion ridges and swales

Early Jurassic Landsborough Sandstone Formation

Table 1. Stratigraphical succession

(modified from Ishaq, 1980; Harbison, 1998; Spring, 2005)

2.6.1 Landsborough Sandstone Formation

The Landsborough Sandstone Formation consists of Late Triassic to Early Jurassic

fluviatile sedimentary rock units. McKellar (1993) details the stratigraphic

relationships of the Nambour Basin.

In the southern part of the Nambour Basin, the base of the Landsborough

Sandstone Formation contains pebble to cobble conglomerate together with

interbedded sandstone, siltstone, shale (partly carbonaceous) and minor coal

(McKellar, 1993; Cox et al., 2000b).

These lower beds are overlain by fine to coarse-grained, massive quartzose and

sublabile sandstone in the southern area. These beds correlate lithologically with

the basal-lower Landsborough Sandstone in northern Nambour Basin (McKellar,

1993).

The upper portion of the Landsborough Sandstone Formation consists of fine to

medium-grained and relatively less quartzose (labile to sublabile) sandstone, minor

conglomerate, siltstone, shale (partly carbonaceous) and coal (McKellar, 1993).


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Figure 5. Lithology of Bribie Island showing Quaternary sedimentary deposits

(modified from Department of Mining and Energy, 1999)


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Figure 6. Hydrogeological cross section showing monitoring bores

and gamma-ray logs (modified from Armstrong, 2006)


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2.6.2 Quaternary Sand

During the Quaternary, Australia was tectonically relatively stable. During the

Pleistocene (around 120,000 years ago), before the last Ice Age, the sea level was

1 to 5m higher than present day. The Pleistocene coastline which is shown in

Figure 7 lay further to the west, inland of the present coastline. This resulted in

seawater covering most of the low-lying coastal areas. Between the headlands and

islands of this time, sediment deposition produced low barrier sand spits. Shallow

tidal sand banks (tidal deltas) accumulated behind the spits from marine sediments

swept around into the calmer waters. Inland of the tidal deltas lay extensive bays of

open water, which were backed by mangrove estuaries and mud flats. The bays

gradually filled in with sediments of mud and sand (Willmott and Stevens, 1988).

Figure 7 Maximum extent of the sea during the last inter-glacial

approximately 120,000 years ago (modified from Willmott and Stevens, 1988)


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When sea levels fell during the last Ice Age, these bays and sandy tidal deltas were

exposed to become dry land. River channels that flowed eastward to the sea

consequently cut this area. Sea levels have not returned to this previous highstand

and consequently, the sediments are preserved and form the present coastline.

When the sea rose again to its present level, sands of the outer barrier spits were

redistributed, except in the southern area were remnant sand ridges of this age form

the core of Bribie Island (Willmott and Stevens, 1988).

Thompson (1992) delineated two types of sand deposit that typically occur along

the east coast of Australia:

a) Low sand ridges and swales that occur parallel to the coast. These formations

were widely distributed along the east coast.

b) Multiple systems of transgressive, parabolic dunes with the trailing arms of the

dunes open to the onshore winds from the southeast. These dunes can also be

influenced by local conditions such as bedrock morphology and smaller scale

local wind patterns.

2.6.3 Indurated Sandstone

Darker coloured layers of variable induration are common in the sediments of

coastal lowlands of subtropical southern Queensland. These induration layers

typically occur in coarsely textured, highly quartzose, base-poor parent materials

such as sands which generally lack minerals with the potential to weather to

crystalline clays (Thompson, 1992; Lundstrom et al., 2000). These layers occur on

remanent Pleistocene beach ridges and tidal delta deposits (Jones, 1992b) and on

sandy alluvial fans and floodplains along streams (Thompson et al., 1996).

Numerous processes that can result in induration include: pedogenic induration

within subsurface horizons of a soil profile; groundwater induration within a

sediment profile; and/or aquatic induration by direct precipitation of materials onto

floors of surficial water bodies (Pye, 1982).

Pye (1982) summarised the processes involved in induration as:

the formation of soluble and colloidal substances that are subsequently

leached by rainwater;

the vertical and lateral transport of substances by rainfall or groundwater to

areas where rates of water flow are low;


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the subsequent precipitation or flocculation of inorganic and/or organic

complexes on reaching an environment with different physical or chemical

conditions within the sediment or water; and

the irreversible drying of substances during periods of seasonal water table

lowering.

Cox et al. (2002a) found that induration within the Moreton Bay area and on Bribie

Island was mainly caused by organic carbon, Fe compounds and fine clays

precipitates. These substances were carried down sand profiles, coating grains and

partially filling in pore spaces. The resulting induration was both laterally and

vertically variable. Because of this process, the sediments would develop variable

porosity and reduced permeability. These indurated sands were found to have

hydrogeological significance as they can act as a semi-confining layer that

influences groundwater flows, separate groundwater bodies, and reduce storage

within the aquifer (Harbison, 1998; Cox et al., 2000a; Cox et al., 2002, Armstrong,

2006).

2.7 REGIONAL HYDROGEOLOGY

Bribie Island is a subcatchment of the coastal Pumicestone Region Catchment

which is shown in Figure 8. This catchment adjoins the catchments of Maroochy

Mooloolah to the north, Pine Rivers to the south and the Stanley to the west.

Bribie is a low sand island of approximately 150km2 that accommodates two

sandmass aquifers. Groundwater forms as a freshwater 'lens' that is stored within

the intergranular spaces of the porous, Quaternary sand deposits.

There are two distinct groundwater bodies occurring on the island: a shallow,

perched, unconfined aquifer; and a deeper, semi-confined, basal aquifer. A

hydrogeologically significant layer of more or less impervious indurated sands,

locally known as Coffee Rock, separates these aquifers (Harbison, 1998; Harbison

and Cox, 1998; Armstrong, 2006).


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2.7.1 Aqui fer Recharge

Rainfall is a diffuse source of recharge that replenishes Bribie Islands groundwater

reserves via direct infiltration into the porous Quaternary sand sediments. The

underlying, basal aquifer is recharged by water percolating down through the

Quaternary sandmass. Surface water and groundwater are fundamentally

interconnected. Localised aquifer recharge may occur within low-lying areas and

along the central swale where surface water can readily permeate into sediments

during and following rainfall events (Harbison, 1998; Armstrong, 2006).

Figure 8. Pumicestone Region Catchment showing Bribie Island subcatchment

(modified from Cox et al., 2000b)


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The amount of groundwater available in a system depends on numerous factors

including: the frequency of rain; the quantity, intensity and duration of rain; recharge

and discharge rates; the amount of water lost back to the atmosphere; and, the

amount of water used by water dependant ecology. Evapotranspiration and direct

seepage from the foreshore are the dominant drainage processes on the island

(Harbison, 1998; Harbison and Cox, 2000). Water quality in the Pumicestone

Passage depends on the quantity and quality of the water discharged into it; this

would include groundwater seepage as well as surface water flows.

Using the sodium accretion method (equivalent to the Cl accretion method in this

area), Harbison (1998) calculated an aquifer recharge of 7% of the average annual

rainfall for the whole island. Outside of the modelled area, on the southern,Holocene beach ridges this method gave a recharge estimated at around 13% of

the average annual rainfall.

2.7.2 Drainage

The primary mechanisms of groundwater discharge from Bribie Island are via

evapotranspiration, groundwater discharge to sea, evaporation and stream run-off

(Harbison, 1998; Harbison and Cox, 2000).

Streams are not well developed on Bribie Island and tend to be short and drain thelarge areas of wetland. On the western side of the island, direct drainage occurs

through two mangrove swamps and a number of small tidal creeks. Two tidal

creeks occur on the west coast, near the Pacific Harbour developments; Dux Creek,

which has been altered by canal development; and Wright's Creek, which drains the

southern portion of the central swale. In the east, Freshwater Creek in the south

and two freshwater lagoons in the north provide direct drainage. The lagoons

(Figure 2) are usually closed to the sea by sand deposits (Lumsden, 1964;

Harbison, 1998).

Surface drainage on the island is poorly developed due to the islands low

topography and the permeable nature of the sand. Surface flow occurs only after

periods of heavy rainfall when the sand becomes saturated. However, because of

these features, water can remain lying at the surface in the interior until it either

evaporates or percolates into the sand profile (Lumsden, 1964; Harbison, 1998;

Armstrong, 2006).


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2.7.3 Hydraulic Parameters

Hydraulic conductivity (K) is an important parameter in relation to the flow of

groundwater through an aquifer system, it is defined as the capacity of a porous

medium to transmit water (Driscoll, 1986). Hydraulic conductivity values determined

from hydraulic tests conducted in the central catchment of Bribie Island are

compared with literature values in Table 2.

Litho logy K (m/day) Reference

Fine to coarse sand 10-2 103 1

1.2 - 11 2

Sand (unconfined, perched aquifer)0.33 18.5 4

Sandstone, friable 10-3 1 1

0.09 0.25 2Indurated sand (Coffee Rock)

0.07 2.5 4

1 - 25 3Sand (semi-confined, basal aquifer)

0.13 4.7 4

1 Driscoll, 1986

2 Armstrong, 2006 Slug Test

3 Armstrong, 2006 Pumping Test

4 HLA Envirosciences Slug Test (unpublished data, 2005)

Table 2. Results of hydraulic testing

2.8 PREVIOUS WORK,BRIBIE ISLAND

Earlier groundwater studies of Bribie Island have investigated water supply and

wastewater disposal issues and focused on the developed, southern portion of the

island. Later studies have considered the whole island.

Previous investigations on Bribie Island are summarised below.

2.8.1 Groundwater Studies

In 1962, 6 production bores and a 2.2ML/day water treatment plant (WTP) were

installed to southwest of Woorim (Harbison, 1998). The Geological Survey of

Queensland conducted a hydrogeological investigation in 1963 1964, which


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included drilling 31 holes (typically to 14m) in southern Bribie Island (Lumsden,

1964). Water balance analysis estimated that groundwater seepage (45%) and

evapotranspiration (50%) accounted for the bulk of the total rainfall removed from

the system; a potential yield of 25% of total rainfall was estimated. That study

recorded average hydraulic conductivities of 4m/day and 13m/day from

permeameter tests and grain size distribution, respectively. Lumsden (1964)

recommended that an area of 2.6km2 be set aside as a water reserve. This area

was gazetted in 1970, in the southeast of the island, south of the Bongaree-Woorim

road (Harbison, 1998).

An additional 21 extraction bores were drilled within the water reserve in 1966

1967. In 1971, due to continued problems with iron fouling of production borescreens, groundwater extraction in the water reserve was changed to pumping from

a trench (approximately 3km long and 5m deep) (Isaacs and Walker, 1983;

Harbison, 1998).

John Wilson and Partners (1979) reviewed the performance and capacity of the

water reserve to supply an increased water treatment capacity of 6.6ML/day.

Recommendations from the investigation included extending the trench system

within the reserve and extending sewage disposal south of the water reserve to limit

groundwater flow out of the reserve. Water balance analysis estimated 42% of

rainfall recharged the aquifer and hydraulic conductivities within the water reserve

ranged from 13 to 30m/day.

In 1979 1980, the Geological Survey of Queensland conducted a second

hydrogeological investigation (Ishaq, 1980) in southern Bribie Island. As part of the

investigation, 26 holes were drilled and completed as observation bores. Ishaq

(1980) determined an average hydraulic conductivity of 17m/day from grain size

distribution. Analysis of pumping test data from two bores (from John Wilson andPartners, 1966) determined hydraulic conductivity results of 15 and 75m/day.

Water balance analysis suggested of the total rainfall, 13% recharged the aquifer,

82% was removed through evapotranspiration and 5% was lost through surface

runoff. Ishaq (1980) assumed that potential evapotranspiration was equal to 63% of

pan evaporation.


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The Department of Environment and Heritage (1993) completed an Integrated

Management Study of Pumicestone Passage and its catchment and groundwater

resources, which included Bribie Island.

In 1992, the Water Resources Commission completed 23 observation bores

(14100079 14100101) across the island. Aquifer stratigraphy was examined and

the base of the Quaternary aquifer was identified in drill logs and with downhole

gamma-ray logs. Estimates from the report include a specific yield of 0.17,

groundwater storage volume of 2.1x106ML, and a sustainable yield of

25,000ML/year. Regular monitoring of groundwater levels and water chemistry has

continued since 1992 (DNRMW database). The GSQ recovered 11 bores

(14100102 14100112) and installed 5 new bores (14100113 14100117) and agauge board (14100118) in the extraction trench in 1994.

In 1995, the Department of Natural Resources completed a report that aimed to

understand effects caused by changes in land use on Bribie Island. An additional 6

observation bores (14100119 - 14100124) were installed in northern Bribie Island

and a whole of island, groundwater model was constructed (DNR, 1996). A further

5 observation bores (14100125 - 14100130) were installed by DNR.

Harbison (1998) completed a research project with QUT investigating groundwater

occurrence and chemistry on Bribie Island. He developed a hydrogeological

conceptual model that recognised the significance of the indurated sands. The

indurated sand layer was found to control infiltration, the degree of aquifer

confinement and aquifer storage within the island aquifer system. Chemical

analysis of rainwater and groundwater recorded Na-Cl type water, with calcium and

bicarbonate enrichment in recent sand deposits (Harbison, 1998; Harbison and

Cox, 1998).

Paul (2003) as part of a research project with QUT studied the environmental

quality of ground and surface waters in the central catchment of Bribie Island. Paul

found that shallow groundwater and surface water were closely related and that

water chemistry of the different water bodies was linked through groundwater flow

processes.

Armstrong (2006) installed 21 single and nested, monitoring bores across an east

west transect in central Bribie Island as part of a QUT research project. He

investigated the affect of aquifer properties and heterogeneity on groundwater


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occurrence and migration. Hydraulic testing of the aquifer system confirmed that

the indurated sand layer had a lower hydraulic conductivity than the upper,

unconfined and the basal, semi-confined aquifers. The indurated sand layer

impeded groundwater migration, resulted in the elevated shallow water table

aquifer, and caused local semi-confinement of the basal aquifer. Water quality

analysis recorded a relationship between surface water and the shallow, unconfined

groundwater that is important to the wetland areas of the island (Armstrong and

Cox, 2002; Armstrong, 2006).

2.8.2 Groundwater Modelling

Isaacs and Walker (1983) built a finite-difference, numerical model for southern

Bribie Island. They assumed a constant hydraulic conductivity of 25m/day and a

recharge rate of 300mm/year (approximately 22%). Marsalek and Isaacs (1988)

conducted a field investigation to assess the effects of the treated effluent recharge

on groundwater quality and found that effluent tends to sink to the bottom of the

aquifer.

DNR (1996) constructed a whole of island, steady-state groundwater flow model

using the MODFLOW package (USGS) with the PMWIN graphical interface. The

aquifer was modelled as a single layered, unconfined aquifer. Calibration of the

model involved using the PEST package (inverse problem solver) to determine the

recharge and hydraulic conductivity values, to achieve the best match between

observed and calibrated water levels.

DNR developed steady-state and transient groundwater flow models to investigate

the removal of commercial pine plantations and for resource management

associated with current and proposed groundwater developments (Werner, 1998a;

Werner and Williams, 1999). The whole-of-island model was conceptualised as a

single unconfined aquifer layer. Werner (1998a) acknowledged that peaty layers

and clay lenses caused some semi-confined regions and isolated groundwater

perching. A block centred, finite difference, MODFLOW model was constructed.

Recharge, hydraulic conductivity and specific yield were mathematically calibrated

to historical groundwater levels using the PEST package. Zones of spatially

invariant hydraulic conductivity were assigned, calibrated and produced values that

ranged from 5 to 150m/day. Aquifer recharge was calibrated at 22% of annual

rainfall and potential evapotranspiration rates were estimated from a bucket model


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(Williams, 1998). The ratios of potential evapotranspiration rates to historical pan

evaporation rates ranged from 0.45 to 0.65. Steady-state analysis identified the

central swale as a region of net groundwater discharge and found that losses to

fixed head cells (coastline, canals and lagoons) was the dominate discharge

process for the modelled aquifer.

Werner (1998b) produced a supplementary report that investigated the effect of a

proposed groundwater extraction bore field. The MODFLOW model adopted most

of the basic model parameters from the principal groundwater investigation (Werner

1998a); alterations covered the bore field proposed by Caboolture Shire Council.

Evans et al. (2002) conducted an impact assessment of the bore field proposed by

Caboolture Shire Council. The evaluation considered factors including safe yield

with respect to security of supply and prevention of seawater intrusion, ecological

impacts and acid sulphate soil surveys. The groundwater model developed by the

Department of Natural Resources (Werner 1998b) was used and refined to optimise

the bore field arrangement. A sustainable groundwater extraction rate of 7ML/d

was suggested. This rate did not conflict with current forestry operations and

adjacent areas of national park.

As part of a research project with QUT, Spring (2005) developed a quasi three-

dimensional, steady-state, whole-of-island groundwater flow model of Bribie Island

using MODFLOW-96. The model was conceptualised as a two-aquifer system

separated by a heterogeneous, indurated sand layer. Hydraulic conductivity,

drainage and evapotranspiration parameters were calibrated using the PEST

package. The technique of pilot point parameterisation was used to mathematically

calibrate the hydraulic conductivities and VCONT layer across the island to achieve

a better fit of observed water levels. A difference in the hydraulic heads in the

upper, unconfined layer was reported as reflecting an increased movement ofgroundwater through the underlying indurated sands. Spring (2005) found that

evapotranspiration removed a significant amount of rainfall from the system before

recharge to the aquifer. The central swale was found to be a significant discharge

feature and as well as groundwater seepage (Spring et al., 2004; Spring, 2005).


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3. METHODOLOGY

Monitoring of groundwater levels, the analysis of groundwater quality and testing of

aquifer hydraulic properties is required to determine the performance of a

groundwater system in response to natural and induced conditions. These

parameters were used to assist with developing a conceptual model for central

Bribie Island and are summarised below.

3.1 HYDRAULIC MONITORING NETWORK

3.1.1 Climate

Climate averages were collected from the Bureau of Meteorology for three stations

in the area: Caloundra, Cape Moreton and Redcliffe. Rainfall records were

obtained from the Bureau of Meteorology for two weather stations on Bribie,

Bongaree Bowls Club and Bribie Island University of Queensland, both of which

have been decommissioned (Appendix A). Rainfall data was also acquired from the

Department of Natural Resources, Mines and Water (DNRMW) from an automatic

tipping bucket rainfall gauge located in east central Bribie at Bore 14100090.

Mean daily pan evaporation values were recorded from 1970 to 1993 at the

University of Queensland Bribie Island weather station (Appendix B).

3.1.2 Monitoring Bore Network

DNRMW maintain a groundwater monitoring bore network across Bribie Island. A

bore search of the DNRMW database for registered bores was completed on Bribie

Island and data from 52 bores (14100079 130) was found.

Data was also acquired from HLA Envirosciences (2002), who had installed a

groundwater monitoring network across the Pacific Harbour area on behalf of QM

Properties (MW1S 27S and MW 3D 19D). In 2001, as part of a QUT research

project, Armstrong (2006) installed 21 nested monitoring bores across central BribieIsland (14100131 - 151). Data acquired from the above sources included

lithological information, bore construction details, elevations, water levels and water

chemistry.


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Fieldwork was conducted on Bribie Island in May, July, September and November

2003. At selected locations across the central catchment area, standing water

levels were measured with a dipmeter (100m long) and physico-chemical

parameters were measured using a TPS 90 FL microprocessor, multi-probe, field

analyser.

3.1.3 Groundwater Quality

In May and November 2003, a groundwater sampling program was conducted at 27

monitoring bores in the central Bribie Island area. To ensure a representative

sample of the aquifer was collected, all monitoring bores were purged of three water

bore volumes using a submersible pump or a bailer (in low flowing bores) prior to

collecting a sample. Polyethylene sample bottles (500mL) had been prepared in

the laboratory with a wash of 1:3 diluted HNO3. Two sample bottles were used per

bore, one for anion analysis and the other for cation analysis. The cation sample

bottle was acidified with 1mL HNO3 to slow chemical reactions. Physico-chemical

parameters of the groundwater were measured in the field with a TPS meter and

parameters recorded are listed in Table 3.

Parameters Analysis

Physico-chemical EC (S/cm), Eh (mV), DO (ppm), pH and temperature

EC = Electrical Conductivity

Eh = Oxidation Reduction Potential (Redox Potential)

DO = Dissolved Oxygen

Table 3. Field parameters measured with a TPS meter

All samples were preserved at below 4oC by storing them with ice during the day

and in a refrigerator at night. Water quality analysis of samples for major ions and

metals was conducted in the School of Natural Resource Sciences (NRS) chemical

laboratory. Alkalinity was determined by acid titration. Cations were analysed with

the Varian Liberty 200 Inductively Coupled Plasma Optical Emission

Spectrometer (ICP-OES) and anions were analysed with the DX300 Dionex Ion

Chromatograph. Ions and metals tested for are listed in Table 4.


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Method Analysis

ICP OES Na, K, Ca, Mg, Fe, Al, Mn, Zn and SiO2

Acid titration HCO3Ion Chromatography Cl, F, Br, NO3, PO4 and SO4

Table 4. Parameters tested for during water chemistry analysis

3.2 MODELLING

3.2.1 Conceptual Model

A conceptual model is built on an understanding of how an aquifer system works.

The model must simplify the real world complexity to a minimum level that is

appropriate to the scale of the project, for example regional or local. Simplification

depends on the end product required, the amount of available data and the current

level of understanding. Building a conceptual model is an iterative process that can

identify gaps in the data which you can try to improve with further data gathering.

A conceptual model provides a simplified representation of a hydrogeologic system

and the flow processes present. The model describes factors including the system

geometry, physical and hydraulic boundaries and hydraulic parameters.

A complex geological model is simplified into a hydrogeological model which

recognises hydrostratigraphic units. The aquifer units and semi-confining layers are

portrayed in three dimensional space. The geological framework for the central

catchment of Bribie Island was established from analysis of drill hole data and

downhole gamma-ray logs, utilising cross sections and 3D cross sections with the

HydroGeo Analyst computer program.

It is necessary to identifying physical boundaries including faults, impermeable

strata and permanent bodies of water such as lakes and oceans within the

boundary domain. As Bribie is an island, the sea to the east and west of the model

area was used as a natural boundary. The less permeable indurated sands are a

hydrogeologically significant layer in the Bribie model.

Hydraulic boundaries such as groundwater divides can be used to limit the extent of

the model where available. Streamlines are essentially a boundary since flow can

only occur parallel to them, i.e. no flow can enter the model domain normal to a

streamline. This artificial barrier was used to the north and south of the model of


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the central area of Bribie Island as flow in this area is predominantly along an east-

west axis.

Results from hydraulic tests and well as monitoring of groundwater levels and

groundwater quality were taken into account when developing the conceptual

model. These factors assisted with understanding groundwater occurrence and

flow processes in the area. This helped to clarify the relationship between the upper

and lower aquifers and the impact of the indurated, sand layer, which lay between

the two aquifer systems.

3.2.2 Mathematical Modelling

Models simulate groundwater occurrence and movement in the subsurface

environment. A model represents a simplified form of the real-world aquifer system

and assists with understanding and managing a groundwater resource (Bear et al,

1992). Mathematical models are based on a conceptual understanding of the

aquifer system and they depend on the solution of basic mathematical equations as

shown in Figure 9. Analytical models provide the simplest approach to modelling

while numerical modelling can represent more complex systems.

Figure 9. Mathematical models are based on a conceptual understanding

of the aquifer system as expressed by mathematical equations

(modified from Mercer and Faust, 1981)


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Analytical Solut ion

The simplest mathematical model of groundwater flow is Darcys Law (equation 1)

which is an equation that describes the flow of groundwater. Groundwater flow

through a vertical section of an aquifer can be calculated using Darcys Law

(Driscoll, 1986):

L

hhKAQ

)( 21 = Equation 1

where:

Q = flow (m3/day)

K = hydraulic conductivity averaged over the height of the aquifer (m/day)

A = area (m2)

h1-h2 = difference in hydraulic head (m)

L = distance along the flowpath between the points where h1 and h2 are measured

(m)

An analytical solution of the aquifer system in the central catchment of Bribie Island

was used to assist with understanding the groundwater flow processes at a

rudimentary level. The results obtained by using Darcys Law were later compared

to the model results to verify the findings from numerical model.

Numerical Modelling

Numerical models are used to represent complex processes (Hill, 1998). Numerical

models are used when complex boundary conditions exist or where the value of

parameters varies within the model (Zheng and Bennett, 1995).

Due to the complicated subsurface environment, conditions can rarely be replicated

completely by mathematical expressions. Simplifying assumptions are usually

made to solve flow equations for appropriate boundary and initial hydrologic

conditions. Assumptions include; the aquifer being homogeneous; isotropic; and

infinite in areal extent. Simplification reduces the accuracy of the model (Driscoll,

1986).

The Visual MODFLOW (version 3.1.0) computer package was available for use to

build a groundwater flow model over the central catchment of Bribie Island. Visual

MODFLOW is a three-dimensional, finite-difference, Layer Property Flow (LPF)


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package built on the MODFLOW-2000 module (Harbaugh et al., 2000). MODFLOW

is a computer program developed by the U.S. Geological Survey (USGS) that

simulates three-dimensional ground-water flow through a porous medium by using a

finite-difference method (McDonald and Harbaugh, 1988).

Visual MODFLOW is built on the MODFLOW-2000 module, which requires the

direct definition of the complete geometry of the each cell (including vertical cell

geometry), unlike previous MODFLOW versions (Harbaugh et al, 2000). The

available version of Visual MODFLOW did not support all of the features and

analysis capabilities of MODFLOW-2000 including the Observation Process, the

Sensitivity Process and the Parameter Estimation Process. Visual MODFLOW

does support the PEST package (Doherty, 1994) which is a powerful and robustparameter estimation program.

PEST is an acronym for Parameter ESTimation. PEST optimises a set of user-

defined model parameters to minimize the calibration residuals from a set of user-

defined observations. PEST guides the model calibration process towards the most

reasonable set of parameter values in order to achieve a better calibration result.

Visual MODFLOW supports the optimisation of the model flow properties

conductivity, storage, and recharge (Waterloo Hydrogeologic, 1995). When

generating parameters using an inverse solution, exercise caution in order to

generate realistic values for aquifer parameters.

The average conditions within the central Bribie Island area were simulated by

Visual MODFLOW using the steady-state option. The model does not include

seasonal variability and does not attempt to model the fresh water-salt water

interface. These limitations are discussed in the sensitivity and uncertainty

assessment.

Spatial Discretisation and Boundary Conditions

Defining the physical configuration of the model involves delineating the areal extent

and thickness of the aquifers and defining the number of layers and the boundary

conditions within the aquifer systems (Fetter, 2001).

The model extends approximately 7.5km in a north-south direction and 8.5km in the

east-west direction. The co-ordinate system is MGA Zone 56 (GDA 94). The model

grid is aligned 16.9 degrees west of north to align the model grid with the dominant

direction of groundwater flow. Layers consisted of 75 rows and 85 columns of


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model cells the size of 100m x 100m. The model configuration for the Bribie model

is shown in Figure 10.

Figure 10. Model configuration of central Bribie Island

There are three ways of representing a semi-confining layer in multi-aquifer

simulations. The first and simplest is the quasi-three-dimensional approach. In this

situation, the semi-confining layer is not explicitly represented. It is simply

incorporated as a leakage term (VCONT) between adjacent layers. This effectively

ignores storage within the semi-confining bed and assumes an instantaneous

response in the unstressed aquifer. This analysis is appropriate for steady-state

simulations or systems with very thin semi-confining beds with limited storage

properties (Anderson, 1993).

Visual MODFLOW requires the top and bottom elevations for each grid cell in the

model and it requires hydraulic conductivity values (Kx, Ky and Kz) for each grid


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cell. Visual MODFLOW uses this information to calculate the interlayer leakage

(VCONT) values. As a result, a VCONT value could not be entered into the model

to simulate the leakance through the semi-confining layer between the two aquifers,

as Spring (2006) did in his regional model of the island.

A second approach is to discretise the semi-confining bed as a separate layer. This

considers the storage within the semi-confining layer but generally does not provide

a good approximation of the gradient within the confining bed (Anderson, 1993).

When this method was utilised for the Central Bribie Island area, the numerical

model would not converge.

The third method is to discretise several layers within the confining bed to

approximate the gradient. The modeller must weigh the benefits of including

gridding in an area where there is limited data and interest in hydraulic heads

(Anderson, 1993). The benefits for the central Bribie Island model of discretising

separate layers were convergence and stability of the model.

The model defines four layers: 1) the surficial sand; 2) and 3) the indurated sand

layer; and 4) the basal sand layer. Figure 11 shows a sample cross section through

the model of the island. The bedrock Landsborough Sandstone was not included in

the model because there was no hydrogeological information from this unit as none

of the piezometers penetrated to this depth. The bedrock contact was treated as a

no flow boundary as it is believed that no groundwater flows upward from this

stratigraphy.

The topography of the island (ground surface) was generated from topographic data

supplied by the Caboolture Shire and Caloundra City Councils combined with bore

hole elevations from DNRMW, HLA and QUT bores and is shown in Figure 12. The

surfaces representing the base of layers 1, 3 and 4 were gridded from data points

delineated by interpretation of drill log data and downhole gamma-ray logs.

Surfaces were contoured using the Surfer contouring software and imported into

Visual MODFLOW. Layer 3 was created by splitting the distance between the base

of Layer 1 and top of Layer 4 into two individual layers (Layer 2 and 3). The base of

the model represents the contact between Quaternary sediments and the

underlying Jurassic Landsborough Sandstone, which represents bedrock in the

area.


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Figure 11. Cross section of model showing the four model layers.The base of the model is the sandstone bedrock.

All layers were assigned as confined/unconfined variable S and T (Table 5).

Geological

unit

Model

layerAqui fer type

Model layer

type

Model layer

thickness

Surficial sand Layer 1 Unconfined 4 10 m

Indurated

sand

Layers 2

and 3

Semi-confining

layer1.5 7 m

Basal sand Layer 4 Semi-confined

Confined /

unconfined,

variable S,T

5 35 m

Table 5. Hydrogeological layers used in the model

There are three types of boundary conditions commonly used in groundwater

models: specified head, specified flow, and head dependent flow. In specified head

boundaries (Dirichlet Conditions), the head remains constant and water will flow into

and out of the model domain depending on the head distribution developed near the

boundary. Bodies of water, for example lakes and the ocean, are commonly

represented as constant head boundaries. Caution is to be used when applying

this type of boundary as it can act as an infinite source of water which may not

match the real world conditions. Specified flow boundaries (Neuman Conditions)

have a fixed flux of water assigned along the boundary. An example of this are no

flow boundaries, such as groundwater divides and impermeable barriers, which are


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given a specified flux that is set to zero. In head dependent flow boundaries

(Cauchy Conditions), flow across the boundary is determined by a prescribed head

outside of the model domain, heads calculated within the model, and some form of

hydraulic resistance to flow in between.

0

1

2

3

4

5

6

7

8

9

10

11

12

Figure 12. Topography for the whole of Bribie Island.Oblique view of elevation data created using Surfer contouring package.

The allocation of the boundary conditions attempted to correspond with natural

hydrogeologic boundaries in order to minimise the influence of model boundaries on

simulation results. The boundary conditions used in the model are displayed in

Figure 13.

m(AHD)


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Cells representing Pumicestone Passage and the Coral Sea were assigned as

inactive. Coastline cells, the Pacific Harbour canal system and tidal creeks were

assigned as fixed-head cells with a hydraulic head value of 0.3m (AHD), a typical

groundwater level along low-energy coasts (Harbison and Cox, 2002). Lagoons

were assigned as fixed-head cells with a hydraulic head value of 0.7m (AHD)

(Harbison, 1998).

The fixed head cells along the coast were assigned to give an approximation of the

interface between salt water and the less dense freshwater. This numerical model

was developed to simulate groundwater flow in central Bribie Island and does not

attempt to specifically map the fresh water - saltwater boundary along the coastline.

Artificial boundaries were created at the northern and southern boundaries of the

model, as there were no natural groundwater divides in the central catchment of

Bribie Island. They were assigned as general head boundaries as they were in full

hydraulic contact with the aquifer. The hydraulic head at the boundary was set at

0.3m and conductance values ranged from 0.012 to 0.5m2/day. Initial conductance

values were determined using equation 2; however, these values were too high

resulting in lowered groundwater levels. The conductance values were reduced

manually until a better calibration was achieved.

D

KWLC

*)*(= Equation 2

where:

C = conductance (m2/day)

(L*W) = is the surface area of the grid cell face exchanging flow with the external

source/sink (m2)

K = average hydraulic conductivity of the aquifer material separating the external

source/sink from the model grid (m/day)

D = is the distance from the external source/sink to the model grid (m)

Drain cells were assigned along the central swale within Layer 1. Drainage was set

at 750m2/day, with a drainage depth of 1m below ground level. This was designed

to mimic loss of water from the model domain via evapotranspiration by vegetation

and evaporative processes along the swale.


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The loss of groundwater from the model domain via direct seepage from the canals

was simulated by assigning drain cells in the canal estates within Layer 1. Drainage

of 1000m2/day was initially set, with a drainage depth of 1m below the land surface.

Initial attempts to assign these cells as fixed head cells failed due to the proposed

fixed head elevation (0.3m) lying below the bottom elevations of some cells in this

area, which the computer program would not accept.

Figure 13. Boundary conditions for a) Layer 1 and b) Layers 2, 3 and 4

for central Bribie Island using model layers shown in Figure 11.


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Monitoring Bores

DNRMW, HLA and QUT monitoring bores are represented in the model as

observation points. The locations of the monitoring bores in the upper, perched

aquifer are shown in Figure 14 and the bores in the basal aquifer are shown in

Figure 15. Within the model, Layer 1 (the shallow, unconfined aquifer) contained 25

monitoring bores and Layer 4 (the basal, semi-confined aquifer) contained 20

monitoring bores. The bores were used as model calibration points to achieve

calibration in steady-state.

Initial Hydraulic Heads

Initial hydraulic heads for the model were subset from the whole island steady-state

model completed by Spring (2005). The head data for Layers 1 and 4 was

contoured in Surfer and then imported into Visual MODFLOW.

Figure 14. Location of 25 shallow monitoring bores used in the model (Layer 1)


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Figure 15. Location of 20 deep monitoring bores used in the model (Layer 4)

Hydraulic Conductivities

Initial steady-state hydraulic conductivities were spatially invariant and based on

field test results conducted by Armstrong (2006) and HLA (2002). This method

resulted in a poor calibration between the field and the simulated water levels.

Zones were established as shown in Figure 16 and the parameter optimisation

software WinPEST was used to estimate the distribution of hydraulic conductivities.

Observed groundwater levels were matched to hydrologic inputs through the

process of inverse parameter estimation. Inverse modelling helps with

determination of parameter values that produce the best possible fit to the available

observations (Hill, 1998). This was a valuable time-saving tool which enhanced the

model calibration. However caution should be exercised when using inverse

problem solving otherwise the program can generate unrealistic values for aquifer


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parameters. Therefore the calibrated hydraulic conductivity values in the model

were restricted to between 1 and 110m/day.

Figure 16. Zones of hydraulic conductivities showing observation bores

Recharge and Evapotranspiration

Recharge was applied across the model domain as a percentage of the annual

rainfall and it was assumed that it did not vary spatially within the model. The initial

aquifer recharge rate of 95mm/year (7% of the average annual rainfall) (Harbison,

1998) was applied to the model domain. Factors including evapotranspiration,

surface water runoff and interception by vegetation are expected to account for the

remainder of the rainfall (around 93%). Recharge of the aquifer was increased to

218mm/year (16% of the average annual rainfall) when potential evapotranspiration

was included into the model.

Evapotranspiration (ET) is expected to make up a large portion of the total

groundwater discharge for Bribie Island. Estimations of ET rates from water

balance models range from 60% (1003mm/year, Williams, 1998) to 77%

(1293mm/year, Bubb and Croton, 2000) of the pan evaporation (1679mm/year).

The bulk of rainfall removal occurs before recharge of the groundwater system.


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The ET parameters were split into 3 zones which are displayed in Figure 17.

Divisions were based on the dominant vegetation types on the island: pine

plantation, swale and National Park. ET rates range from 180 to 270mm/year

depending on the vegetation type. The rooting depth of mature aged pines in

unsaturated soil profiles could range from 3 to 5 metres (K. Bubb, pers comm.,

2005), so the extinction depth in the pine plantation areas was set at 3m. Extinction

depth in the swale and National Park areas was set at 2.5m.

Figure 17. Evapotranspiration zones split according to dominant vegetation type

as shown in example photographs

Model Calibration and Sensitivity Assessment

Model calibration is undertaken to refine a models representation of the

hydrogeologic framework, hydraulic properties, and boundary conditions to achieve


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a desired degree of correspondence between the model simulations and

observations of the groundwater flow system (ASTM, 1996).

Visual MODFLOW supports the PEST package (Doherty, 1994) and can optimise

hydraulic conductivity, storage, and recharge (Waterloo Hydrogeologic, 1995).

PEST was used to optimise hydraulic conductivity in the model to minimize the

calibration residuals from the water level observations. The calibrated horizontal

hydraulic conductivity values were restricted to between 1 and 110m/day. Due to

the variability in time and period of water level records, an average water level per

monitoring bore was used for the calibration of the steady-state model.

The conductance values for the general head boundaries at the northern and

southern boundaries of the model were reduced manually until a better calibration

was achieved. The conductance values ranged from 0.012 to 0.5m2/day.

Sensitivity analysis is defined as the quantitative evaluation of the impact or

uncertainty in model inputs on the degree of calibration of a model and on its results

or conclusions (ASTM, 1994). When user-defined parameters within the model are

varied, it is possible to determine how sensitive the model is to these changes.

There are four types of sensitivity which are illustrated in Figure 18. Sensitivity type

is characterised by whether the changes to the calibration residuals and model

conclusions are significant or insignificant.

Sensitivity assessment was conducted on the following model inputs:

evapotranspiration and drain parameters and general head boundary conductance.

Figure 18. Summary of the four types of sensitivity

(modified from ASTM, 1994)


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4. RESULTS

4.1 HYDRAULIC MONITORING DATA

4.1.1 Climate

Bribie Island has a sub-tropical climate and experiences a wet summer and a dry

winter. Figure 19 reveals that the maximum temperatures in the Moreton Bay area

range from 19C in winter and 28C in summer.

Caloundra

0

5

10

15

20

25

30

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

TempoC

Redcliffe

0

5

10

15

20

25

30


TempoC

Cape Moreton

0

5

10

15

20

25

30


TempoC

Mean daily maximum temperature (oC)

Mean daily minimum temperature (oC)

Figure 19. Mean daily temperatures for Caloundra, Cape Moreton and Redcliffe

Rainfall records shown in Figure 20 reveal a seasonal trend in the data with a peak

period for rainfall occurring over summer and early autumn (December through

March). The mean annual rainfall from the Bongaree station, which operated for


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nearly 59 years, is 1358mm/year. The mean monthly pan evaporation values from

the University of Queensland Bribie Island weather station (1970 1995) were

compared to mean monthly rainfall from the nearby Bongaree station in Figure 21.

Pan evaporation values exhibit seasonal fluctuations and usually exceed rainfall

from July through January. The mean annual pan evaporation was measured as

1679mm/year.

0

50

100

150

200

250


MeanMonthlyRainfall(mm)

Bore 14100090

Bongaree Station

University of Qld

Figure 20. Average monthly rainfall on southern Bribie Island

Figure 21. Mean monthly rainfall compared to mean monthly pan evaporation


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4.1.2 Monitoring Bore Network

Data from all monitoring bores was used to develop a geological framework for the

central catchment of Bribie Island. The monitoring bores located within the central

area of Bribie Island are summarised in Appendix C and locations of all bores are

illustrated in Figure 22.

Figure 22. Location of monitoring bores used to build the geological framework


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Lithological data from different sources (DNRMW, HLA and QUT) was collated and

interpreted to unify the data. Different naming conventions were used for lithology

in the various drilling programs conducted over many years. For example, material

in the upper profile that was described variably as sandstone, indurated sand or

Coffee Rock in previous drill hole logs were grouped into an indurated sand

assemblage.

Data was plotted in 3-dimensional space and interpolations of lithological data

between monitoring bores were made using the HydroGeo Analyst computer

package. Figure 23 shows the results of the process and graphically displays the

heterogeneous nature of the Bribie Island sandmass.

Standing water levels were recorded between May and November 2003 to obtain

site specific information to assist with understanding the groundwater flow

processes in the central catchment of Bribie Island (Appendix D).

Hydrograph analysis is an important method of presenting periodic measurements

(time series) of groundwater levels as the graphs display baseline trends in the

data. When recharge and discharge within an aquifer system are in balance,

hydrographs show that water level data can vary significantly from year to year, but

will remain relatively stable over the long term. When rainfall is inadequate to

compensate for discharges from the aquifer, such as during droughts or due to

excessive pumping, the water level will fall over time.

Figure 24 shows a hydrograph of water levels recorded from a selection of

representative monitoring bores with long-term data. Groundwater levels are

plotted with a residual rainfall mass curve (RRMC) calculated for the site 14100090

(automatic tipping bucket rainfall gauge). The RRMC shows the cumulative

difference between the rainfall recorded for a month and the average rainfall for

each month. This curve is used to illustrate trends in rainfall to assist with the

detection of seasonal and longer-term climatic variations. An increase in the RRMC

indicates periods of above average rainfall and decreases indicate

joanne m. jackson thesis

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