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UNIVERSITY OF WESTERN AUSTRALIA SCHOOL OF ENVIRONMENTAL SYSTEMS ENGINEERING
Changes to water balance due to urbanisation
A study of Alkimos, Western Australia
ALISTAIR TREMAYNE
FINAL YEAR PROJECT DISSERTATION
3/11/2010
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Abstract The Gnangara groundwater system is Perth’s primary water resource. The system’s coastal
superficial aquifer is the Gnangara mound and supplies over 60% of Perth’s scheme water while
supporting $100 million worth of agriculture. It also supports regionally-significant wetlands and
ecosystems that depend on groundwater. Perth’s rainfall has declined significantly over the last
century, whilst abstraction of water from the mound has increased. These two factors, coupled with
altered land uses are driving a reduction in groundwater levels in the system.
An increasing population will require continued expansion of the Perth area. New developments
must take in to account the effect that urbanisation will have on the local water balance. The
Alkimos Eglinton area is approximately 40km North of the Perth CBD in Western Australia. It is the
first completely new development in Perth for some time. This development will create a significant
new demand on the underlying Gnangara Mound.
The ongoing effects of climate change will include significant alterations to the hydrosphere. In
particular, sea level rise and precipitation changes will affect the hydrological cycles of both
urbanised and undeveloped areas. With no intervention the Gnangara’s groundwater levels will
continue to decline as rainfall decreases. Current abstraction trends are unsustainable from an
environmental, social and economic perspective. Perth’s future water resource sustainability will
rely on managing the Gnangara groundwater system effectively to deal with declining recharge
levels. In particular, integrated water management strategies should be implemented within the
Alkimos urban design, particularly the use of alternative water sources for non-potable uses.
The likelihood of sea-level rise, coupled with the potential for increased abstraction of groundwater
from the Gnangara mound combine to make saltwater intrusion a very real threat. However, proper
planning and suitable abstraction management should render this unlikely. Managed aquifer
recharge is a strategy that could be utilised to mitigate saltwater intrusion issues, which would also
provide a more sustainable resource from a water balance perspective. It is recommended that
further research be undertaken regarding the implementation of managed aquifer recharge
schemes, with particular emphasis on utilising treated wastewater.
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Table of Contents Abstract ................................................................................................................................................... 3
Table of Contents .................................................................................................................................... 5
Table of Figures ....................................................................................................................................... 7
1 Introduction .................................................................................................................................... 8
2 Literature Review .......................................................................................................................... 10
2.1 Hydrology .............................................................................................................................. 10
2.2 Saltwater Intrusion................................................................................................................ 12
2.3 Climate Change ..................................................................................................................... 15
2.3.1 Climate Change - Rainfall .............................................................................................. 16
2.3.2 Climate Change - Sea level ............................................................................................ 17
2.4 Urbanisation .......................................................................................................................... 18
2.5 Gnangara Groundwater System............................................................................................ 20
2.6 Alkimos .................................................................................................................................. 23
2.7 Management Options ........................................................................................................... 26
3 Approach ....................................................................................................................................... 27
3.1 Data Collection and Analysis ................................................................................................. 27
3.1.1 Groundwater ................................................................................................................. 27
3.1.2 Rainfall .......................................................................................................................... 28
3.1.3 Climate Change ............................................................................................................. 29
3.1.4 Land Use ........................................................................................................................ 29
3.2 Saltwater Interface Approach ............................................................................................... 30
3.3 Water Balance Approach ...................................................................................................... 30
4 Results ........................................................................................................................................... 31
5 Discussion ...................................................................................................................................... 34
5.1 Water Balance ....................................................................................................................... 34
5.2 Saltwater Intrusion................................................................................................................ 35
5.3 Management Options .............................................................. Error! Bookmark not defined.
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5.4 Complications, errors and assumptions ................................................................................ 36
6 Conclusions & Recommendations ................................................................................................ 37
7 Appendices .................................................................................................................................... 38
7.1 Appendix A – Project area landscape ................................................................................... 38
7.2 Appendix B - District Structure Plan ...................................................................................... 39
7.3 Appendix C - Groundwater Sites (full image) ........................................................................ 40
8 References .................................................................................................................................... 41
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Table of Figures Figure 2-1: The hydrologic cycle of the Gnangara groundwater system .............................................. 11
Figure 2-2: Simplified schematic of a saltwater interface .................................................................... 13
Figure 2-3: A simplified schematic of lateral seawater intrusion) ........................................................ 14
Figure 2-4: Perth annual rainfall: 9 year moving average ..................................................................... 16
Figure 2-5: Best estimate of precipitation change (%) of Australia by 2030, 2050 and 2070. ............. 17
Figure 2-6: Land use on the Gnangara system ...................................................................................... 21
Figure 2-7: Groundwater decline in the Gnangara mound since 1979................................................. 22
Figure 2-8: Project area landscape summary ....................................................................................... 24
Figure 2-9: Current district structure plan map. ................................................................................... 25
Figure 3-1: Map of groundwater sites in the Alkimos Eglinton area. ................................................... 27
Figure 3-2: Groundwater contour map of Alkimos. .............................................................................. 28
Figure 3-3: Monthly rainfall data for Two Rocks................................................................................... 28
Figure 4-1: Saltwater interface contour map of Alkimos.. .................................................................... 31
Figure 4-2: Saltwater interface cross-sections ...................................................................................... 32
Figure 4-3: Location of saltwater interface cross-sections from Figure 4-2 ......................................... 32
Figure 4-4: Saltwater interface contour map for sea-level rise of 0.3m ............................................... 33
Figure 4-5: Saltwater interface contour map for sea-level rise of 0.9m ............................................... 33
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1 Introduction Australia relies on groundwater as its primary source of potable water, and as a necessity for
agricultural and horticultural industries. As the Australian population continues to grow, increasing
pressure is being placed on our water systems and the need for urbanisation increases. The
Gnangara groundwater system is Perth’s primary water resource. The system’s coastal superficial
aquifer is the Gnangara mound and supplies over 60% of Perth’s scheme water while supporting
$100 million worth of agriculture. It also supports regionally-significant wetlands and ecosystems
that depend on groundwater.
Perth’s rainfall has declined significantly over the last century, whilst abstraction of water from the
mound has increased. These two factors, coupled with altered land uses are driving a reduction in
groundwater levels in the system. Current trends are unsustainable from an environmental, social
and economic perspective. With no intervention and abstraction remaining at current levels, water
will continue to decline. Wetlands end ecological zones will be severely affected, future water
resource availability will be reduced and industries that rely on groundwater will face economic
challenges in finding new water sources.
Pressures on groundwater systems will be further exacerbated by a changing climate. It is generally
accepted that climate change will continue to occur, and have a large impact on the hydrosphere.
Rainfall in Perth has already declined by almost a third in the last century, and this is predicted to
continue. Precipitation is the primary input for the system, so any reduction in this will greatly affect
the water balance.
A saltwater/freshwater interface exists where the freshwater of the aquifer meets the seawater.
This saltwater interface will change depending on groundwater levels in the aquifer. Declining
groundwater levels will allow the saltwater wedge to proceed further inland and could lead to
potential salinity issues. Abstraction causes a drop in the vertical head around the immediate bore
area. If this is situated near the coast, contamination of abstraction wells could occur. The potential
for saltwater intrusion from increased abstraction or sea-level changes due to climate change needs
to be established.
Urbanisation affects the hydrological cycle in many ways. An increase in impervious surfaces and
clearing of vegetation will affect the local water balance, while sewerage and water supply systems
create new water pathways. The diversion and collection of stormwater will also affect recharge
rates. Different environments behave differently under urbanised scenarios, and there exists a lack
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of knowledge concerning how each situation will be affected. Of particular interest is how
groundwater recharge is affected, as it is this component that should balance groundwater
abstraction.
The Alkimos Eglinton development is located 40km North of the Perth CBD. The 2,600 hectare area
will provide housing for over 50,000 people. The area is currently mostly native vegetation,
consisting of low lying banksia woodland on gradual dune systems. The development plans include a
water treatment plant, which will abstract water from the Gnangara mound to supply the area.
The culmination of changing climate conditions, increasing populations and a historical over-reliance
on groundwater systems places our water resources under compounding pressure. Perth’s future
water resource sustainability will rely on managing the Gnangara groundwater system effectively to
deal with declining recharge levels. Understanding the effect that urbanisation will have on water
balance in the project area is essential to be able to properly plan with sustainable strategies.
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2 Literature Review
2.1 Hydrology Understanding the hydrologic cycle is an integral component in planning for the sustainability of our
water resources throughout the future. The circulation of water through the environment is a
complex system that significantly affects our consumable water resources. The majority of Australian
cities, and indeed many cities globally rely on groundwater as the primary potable water source.
Groundwater that is directly linked to the surface through soil is an unconfined, or superficial aquifer.
This study focuses primarily on the superficial aquifer that supplies Perth, the Gnangara Mound.
Water enters a system through precipitation. When rainfall occurs, the path it travels is governed by
environmental factors. Overland runoff is generated by steep slopes and soils with low infiltration
rates. Runoff can also be higher during the first rains of the wet season due to a build up of
hydrophobic organic material. Loose, sandy soils and flat terrain will encourage infiltration, allowing
the water to flow through the ground to the superficial aquifer. The presence of vegetation can also
promote infiltration, dependant on vegetation type and coverage. Water will leave the system to the
ocean or to streams that occur at valleys, and through evaporation and transpiration processes. A
small percentage of water may also continue to percolate down and recharge confined aquifers
below (Tindall & Kunkell 1999).
Evapotranspiration is the term used to describe losses from the system through direct evaporation
and transpiration by vegetation. It is influenced by many environmental factors, including
temperature, sunlight radiance factors, vegetation type, vegetation cover, foliage amounts (leaf area
index), and atmospheric conditions such as wind speed and humidity. Therefore, any changes to
vegetation or climate conditions will alter the balance and force evapotranspiration values to adjust
accordingly (Finch 1998).
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Figure 2-1: The hydrologic cycle of the Gnangara groundwater system (Department of Water 2010)
Recharge is the defining factor when examining water balances involving superficial aquifers. It is the
amount of water an aquifer receives when losses through drainage and evapotranspiration have
been removed. It is this component that offsets the amount withdrawn through abstraction.
Recharge is affected by many environmental conditions, and can be intentionally increased through
appropriate management techniques (Dillon 2005). Any changes to environmental factors will alter
the components of the cycle, and the system will adjust until it is balanced.
Groundwater is constantly flowing from areas of high pressure to low pressure, or from higher
groundwater levels to lower. Once again, the composition of the soil will affect how quickly
groundwater flows are. Sandy soils, such as those experienced in the Perth region will exhibit faster
flows (Salama, Silberstein & Pollock 2005).
Calculating recharge, or other factors, is based on balancing the inputs and outputs of a defined
water system. A wide variety of water balance models are used, with varying degrees of complexity
and accuracy. They are generally based on some variation of the following equation:
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This equation is often simplified by ignoring minor components, or grouping factors together. For
example, upward capillary flow is rare, and normally quite a low component. If there is also no
irrigation taking place, then these factors can be ignored, simplifying the inputs to just precipitation.
Conversely, runoff and lateral drainage are often combined to simply ‘drainage’, and in an aquifer
with a confining base, there will be minimal vertical drainage. For most of the Gnangara system,
there is considered to be no runoff or drainage due to the high infiltration rates of the soil, so the
equation can be simplified to:
This is not completely accurate, but allows a quick estimation of recharge values in different
situations (Tindall & Kunkell 1999).
2.2 Saltwater Intrusion Where the saltwater of the ocean meets the freshwater of an aquifer, the differing densities cause a
saltwater wedge to form. The lighter freshwater will sit atop the denser seawater, with the gradual
change in pressure creating a wedge-shaped formation. A transitional zone wherein the salt and
fresh waters mix due to diffusion and dispersion processes exists, exhibiting a concentration gradient
(Barlow 2003; Werner, Habermehl & Laity 2005). It is largely developed by the shifting of the
interface with each tidal cycle. Tidal cycles affect groundwater levels near the coast and also apply a
large influence upon the characteristics of the transitional zone (Shrivastava 1998). The horizontal
thickness of the transition zone can differ dramatically in different environments, however the
vertical range is generally quite thin (Barlow 2003). When examining saltwater intrusion, it is
generally assumed that there is a sharp boundary between the salt and freshwater, known as the
saltwater/freshwater interface, demonstrated in Figure 2-2 below.
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Figure 2-2: Simplified schematic of a saltwater interface (Barlow 2003)
The position of this saltwater/freshwater interface will change depending on sea level and the
hydrostatic pressure, or groundwater level of the aquifer (Mahaad 2008). The approximate depth of
the interface can easily be calculated using the Ghyben-Herzberg relationship, while there exists
more detailed models to deal with substantial vertical head gradients (Izuka & Gingerich 1998). The
Ghyben-Herzberg relationshipnis a simple ratio based on the differing densities and pressure of the
fresh and salt waters.
In the absence of actual data, seawater is assumed to have a density of 1025 kg/m3, and freshwater
1000 kg/m3. This simplifies the Ghyben-Herzberg relationship to d=40h. That is, where the
groundwater level sits one metre above sea level, the saltwater interface will be 40m below sea level.
Furthermore, a change to sea-level or groundwater levels of 1m will alter the depth to saltwater by
40m. Due to this ratio, small changes to groundwater or sea levels can have a significant impact on
the location of the interface (Werner, Habermehl & Laity 2005).
Hydrostatic pressure within the aquifer is dependent upon aquifer recharge and abstraction levels.
Where there is a general decline in groundwater levels, the reduced pressure will allow the saltwater
interface to propagate further inland. This can cause contamination of entire groundwater sources
(Barlow 2003). The movement of the interface inland is known as lateral seawater intrusion, and is
demonstrated in slide (b) of Figure 2-3.
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Where a well is abstracting water, a reduction in pressure propagates out radially from the
abstraction point. Known as the ‘cone of depression’, the depth and extent of this is determined by
the size and volume of the pumping well, as well as the composition of the aquifer. If this is situated
near the saltwater interface, ‘up-coning’ will occur, wherein saltwater will be drawn towards the well
and potentially cause saline contamination (Werner, Habermehl & Laity 2005). This process is
displayed in slide (c) of Figure 2-3, below.
Figure 2-3: A simplified schematic of lateral seawater intrusion (b) and up-coning (c) (Werner, Habermehl & Laity 2005)
Increased or poorly managed abstraction or changes to recharge can cause localised or regional
drops in groundwater levels, which will alter the seawater interface position as described. However,
changes to sea-level will drive the same processes. Therefore, a combination of increased abstraction
and sea-level rise due to climate change could increase the risk of saltwater intrusion.
Over-extraction of groundwater has been the cause of many coastal aquifers experiencing seawater
intrusion. Areas of the United States have experienced severe salinisation, in particular in Los Angeles’
coastal aquifers (reference). It is also a serious and pervasive issue along the Mediterranean coast.
Specifically, about 60% of Spain’s coastal aquifers have been contaminated by seawater intrusion
(Universidad de Granada 2007). In the vast majority of these cases, the cause has been excess
abstraction.
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There exists scope for the use of managed aquifer recharge to maintain a desirable saltwater
interface position (Mahaad 2008). Injecting water in strategic areas can increase aquifer pressure to
combat the intrusion of saltwater. This is reviewed in more detail in Error! Reference source not
found.: Managed Aquifer Recharge.
2.3 Climate Change The International Panel for Climate Change (IPCC) is the intergovernmental organisation for climate
change research founded by the United Nations Environment Program. All UN countries may
contribute, and its international nature allows it to be authoritative, rigorous yet balanced. As such,
the IPCC’s reports are considered extremely important to future policy making. The most recently
completed report, Climate Change 2007: IPCC Fourth Assessment Report (Parry et al. 2007) contains
detailed analysis of the earth’s changing climate and future predictions.
To expand on these reports and enhance the Australian knowledge-base, the Climate Change in
Australia (CSIRO 2007) report was commissioned by the Australian Greenhouse Office. This report,
prepared mostly by CSIRO and the Bureau of Meteorology compliments the IPCC’s conclusions with
more relevant Australian research. It addresses all aspects of climate change, providing detailed
analysis of previous changes and predicting future impacts in different regions of Australia. It has
been developed using all relevant data across the nation, and provides projections for nationwide
responses under differing scenarios.
Globally and within Australia, climates have already been observed to have changed significantly
over the last century. Climate systems are interrelated, complex processes, and each individual
change will alter the balance and affect other components. Of particular interest to this study are
changes to precipitation and sea-level rise.
Sea level rise and precipitation changes will affect the hydrological cycles of both urbanised and
undeveloped areas. Sea level changes will influence the salt-water interface, while precipitation
changes will directly alter inputs in to the system, upsetting established water balances (Holman
2006). Temperature changes and atmospheric conditions will have an effect on evapotranspiration,
however it is the direct rainfall input that dominates any changes to groundwater recharge.
In addition to affecting the overall water balance or interface positions, any climate change will affect
the distribution of groundwater in the system, and sea-level rises will change discharge patterns to
the ocean. This will, in turn, further affect the saltwater interface position, highlighting the complex
balance that these systems adhere to (Timms, Anderson & Carly 2008).
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2.3.1 Climate Change - Rainfall
Perth’s rainfall has already declined significantly during the previous century. Southwest Western
Australia has experienced one of the most severe reductions in precipitation in the country (CSIRO
2007). Today’s average annual rainfall stands significantly lower than those recorded for the early
1900s. This is easily demonstrated when examining a nine year moving average, as displayed in
Figure 2-4. There is a break circa 1970 that is observed, at which point rainfall significantly declines.
Today’s nine year average is approximately 250mm lower than that recorded in 1920 (Bureau of
Meteorology 2010).
Figure 2-4: Perth annual rainfall: 9 year moving average
There were 18 primary scenarios for future climate conditions that were modelled as part of the
Climate Change in Australia (CSIRO 2007) report. While outcomes varied between scenarios for the
majority of climate factors and locations, southwest Western Australia exhibited a high level of
consistency for all precipitation projections. The southwest was the only region that recorded a high
probability of reduced rainfall in each scenario modelled. There was even a high level of consistency
across different probabilities. The extreme percentiles, describing scenarios that are the best
projection but least likely to occur, predicted rainfall increase in many areas of Australia, but never
did for the southwest. Predicted rainfall decrease for the best estimate (50th percentile) ranged from
5 to 20% across all models. This was verified by increased resolution modelling in the region.
Figure 2-5 demonstrates the consistency of the modelled results for the best estimate under each
scenario by the years 2030, 2050 and 2070. The southwest experiences the greatest decline across
Australia in each model.
500
550
600
650
700
750
800
850
900
950
1000
1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010
An
nu
al r
ain
fall
(mm
)
Year
Perth rainfall: 9 yr moving average
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Figure 2-5: Best estimate (50
th percentile) of precipitation change (%) of Australia by 2030, 2050 and 2070 (CSIRO 2007).
2.3.2 Climate Change - Sea level
Globally, sea levels have risen by approximately 17 cm during the twentieth century (Parry et al.
2007). The average rate of global sea level rise is approximately 2 mm per year (Timms, Anderson &
Carly 2008). Measurements from Bureau of Meteorology tide gauges indicate that the average sea-
level rise around Australia is currently 1.2 mm each year (Bureau of Meteorology 2010).
The IPCC project global sea levels to rise by 18 to 59 cm by 2100 (Parry et al. 2007). In addition, a
further 10 to 20 cm of sea level rise caused by melting ice sheets is possible. They also note that
further unquantifiable ice sheet contributions could increase the upper limit of this projection
significantly. Currently, the best estimates provided by the CSIRO (2007) indicate sea-level increase of
0.2 to 0.5 m by 2050 and 0.5 to 0.9 m by 2100.
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In addition to these natural rises in sea-level, melting of the Arctic, Antarctic and Greenland icesheets
that may occur due to climate change could cause further increases in sea-level. According to the
IPCC (2007) total melting of the Greenland ice sheet over a period spanning thousands of years
would eventually elevate global sea levels by an estimated 7m.
Changes to wind speeds and more intense weather systems combined with higher sea levels create a
potential for significantly increased storm surges. The increased inundation could further affect the
saltwater interface, or cause seawater inundation, wherein seawater flows over the surface into low-
lying areas (Timms, Anderson & Carly 2008).
2.4 Urbanisation Urbanisation affects environmental factors of the area in many ways. Vegetation types and coverage
are changed dramatically as land is cleared, while an increase in impervious surfaces occurs from the
construction of roads and buildings . Landforms may be altered, as areas are flattened or built up to
accommodate different constructions, and new water pathways are introduced. The inclusion of
sewerage systems, water resource systems, irrigation and stormwater drains all provide new
pathways for water to travel (Lerner 1990).
Urbanisation has a significant effect on local water balances. In particular, the increase in impervious
surfaces and change in soil quality affect runoff characteristics (Haase 2009). The reduction in
established vegetation will alter evapotranspiration processes, whilst changes in topography and
landform will modify water pathways (Tindall & Kunkell 1999). While all these factors may be
significantly altered, it is the culmination of these impacting aquifer recharge that is of main interest.
Typically, urbanisation has been expected to cause immediate aquifer recharge to be decreased, due
to impervious surfaces causing increased runoff (Lerner 1990). In Haase’s (2009) investigation of the
effects of long term urbanisation on water balance, direct runoff was seen to increase dramatically
for any land with an urbanised impervious surface area of greater than 20%. For areas with 80-100%
of impervious surfaces, runoff was increased by more than 300% compared to natural landscapes.
However, evapotranspiration under urbanised conditions decreased significantly: 150mm/a for 20-40%
impervious surface, up to 450 mm/a for 80-100%. The combination of these factors within the water
balance model meant that recharge itself only dropped by a small amount. Over the full spatial and
temporal scale of the 130 year study period, evapotranspiration dropped by 25%, runoff increased by
182%, but recharge exhibited a reduction of only 4%.
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Urbanising an area has a much greater impact than just increasing impervious surfaces and
decreasing vegetation. There are more complex processes involved in these systems and total
recharge can actually be increased due to changing water pathways (Lerner 1990). The addition of
water supply and sewerage systems creates an opportunity for water leakage to occur and
stormwater collection systems divert water from its natural course, potentially increasing local
recharge. This is particularly relevant with Western Australia’s coastal sandy aquifers, where high
infiltration rates may contribute to the potential for an overall increase in recharge after urbanisation
has occurred.
In addition to overall water balance and recharge issues, there are several additional impacts related
to urbanisation that are important, particularly from a social perspective. Evapotranspiration is
considered an important process for maintaining air temperature and humidity levels in urban
landscapes. Areas of vegetation in open space provide a significant heat loss in dense urban
environments (Pauleit, Ennos & Golding 2005). These are significant social issues associated with
water resource management in urban areas.
Water quality is another significant concern where urbanisation is concerned. The major source of
groundwater contamination in developed countries involves saline intrusion, however rapidly
developing cities are encountering more severe health issues (Rygaard, Binning & Albrechtsen 2011).
Where resources and expertise are lacking, urban water supply systems are constructed with
minimal long term design goals. Regulations concerning pollution and waste management may not
be sufficient, and the culmination of these can lead to contamination resulting from placing
abstraction bores too shallow or near sources of pollution. This has led to significant health issues in
many areas. It is unlikely to happen with the expertise and resources of Australia, however it
increases the considerations when developing water resource management plans.
New urban projects in modern developed countries will generally now acknowledge the need for an
understanding of water and land management techniques to improve water system efficiency and
sustainability. This could include managed aquifer recharge, from large scale water reuse to
stormwater infiltration areas. Water reuse on open spaces is considered beneficial, or more simple
solutions such as education and efficient system design (Durham, Rinck-Pfeiffer & Guendert 2003).
This is discussed in further detail in section 2.7 : Management Options.
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2.5 Gnangara Groundwater System The Gnangara groundwater system is Perth’s single largest source of water, accounting for over 60%
of scheme water and servicing approximately 1.8 million people (Gnangara Coordinating Commitee
2009). It stretches from the Indian Ocean coastline, to Ellen Brook in the East and is bordered by
Gingin Brook in the North and the Swan River in the South. The system stretches over 2200 square
kilometres, and is host to a wide variety of land uses (Department of Water 2010). The Gnangara
Mound itself is the superficial aquifer of the system. The mound overlies two confined aquifers, the
first being the Leederville and the deeper being the expansive Yarragadee aquifer.
Based on climate data from 1997-2004, total recharge for the mound is 242 GL/a, which is equal to
18 percent of rainfall. This is a 26 percent decrease compared to recharge based on climate data
from 1976-2003, which is 342 GL/a or 20% of rainfall. This 26% decrease in recharge correlates to
only a 13% reduction in rainfall, indicating how sensitive recharge of the system is to rainfall (Xu 2008)
The Gnangara groundwater system is already highly modified from its natural state, as demonstrated
in the land use map of Figure 2-6 below. A large proportion of the native banksia woodland
vegetation has been cleared, not only for urban expansion but for agriculture and horticulture uses.
Native vegetation currently accounts for 60 000 hectares of the system (Xu 2008). Urbanised areas
providing residence for over 600 000 people currently exist on the system, a component that will
inevitably continue to increase. There are several extensive pine plantations that cover a large part of
the system. Pine has a significantly higher transpiration rate than the native banksia woodland. As
these plantations have matured, they have increasingly become responsible for a significant water
loss from the system (Gnangara Coordinating Commitee 2009).
Abstraction for scheme water and the use of land and water to support employment, timber, food
production and other resources alter natural processes significantly, while making it an integral
component of Perth’s economy and society. The Gnangara Coordinating Committee (2009) also
argue that it is water from the Gnangara system that supports the many open green spaces that
contribute significantly to Perth’s lifestyle and provide a valuable heat sink in a warming climate.
Under current conditions, these social and economic values cannot be maintained alongside desired
environmental outcomes.
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Figure 2-6: Land use on the Gnangara system (Department of Water 2010)
In addition to the large amount of scheme water abstracted from the mound, there are numerous
private bores. These include residential and horticultural bores along with those used for agricultural
purposes. This heavy reliance upon the Gnangara Mound as Perth’s primary water resource has
placed severe pressure on water levels in the system. Coupled with the declining rainfall that the
area has experienced over the previous 50 years and the changing land use such as pine plantations,
levels in the system have declined significantly over the last 30 years. Figure 2-7 indicates the
progressive decline of groundwater levels in the superficial aquifer. The decline of water levels in the
Gnangara mound are likely to continue. Wetlands have already been affected, and at the crest of the
mound more are expected to dry out under any circumstance (Gnangara Coordinating Commitee
2009).
22
Figure 2-7: Groundwater decline in the Gnangara mound since 1979
The need for improved management techniques with regard to this valuable resource has culminated
in the establishment of the Gnangara Sustainability Strategy (2009). The strategy incorporates input
from several Western Australian Government departments. It includes an analysis of the impacts of
climate, water abstraction and land use on the water balance of the entire Gnangara groundwater
system, developing models for the period between 2008 and 2030.
The Gnangara Coordinating Committee (2009) notes that even with recommended land and water
management options implemented, climate change will ensure that water levels in the Gnangara
Mound are still likely to decline. They reason that with changing the land use of pine plantations,
increasing recharge from proper urban planning and the eventual use of recycled water that
environmental values of the system will still be able to be preserved. These integrated options will
create a more sustainable interaction with the system, while allowing it to continue supporting the
land and water uses that currently rely so heavily on it.
The Gnangara Sustainability Strategy (2009) comments on the near-obsolete regulations for public
water supply systems. Environmental management regulations and Ministerial conditions for public
supply borefields do not take into consideration our continually drying climate. The impact that
climate change will have on groundwater levels must be incorporated in to future amendments to
the conditions imposed in these areas. Management needs to be adaptive, where continuous
monitoring can influence actions that are required to ensure the sustainability of this resource.
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2.6 Alkimos
The Alkimos Eglinton project area is approximately 40km North of the Perth CBD, Western Australia.
The 2,600 hectare region stretches over 7.5 km of coastline and is located within the City of
Wanneroo. The surrounding area, termed the Northwest Corridor, has been identified as a major
growth region for the Perth Metropolitan area for some time, and development is likely to continue
with an ever increasing population. Once constructed, the project will effectively connect once-
regional Yanchep to the metropolitan Perth area (Landcorp et al. 2008).
The project area is predominantly natural landscape, with minimal human development to date.
Approximately 80% of the area is covered by natural vegetation, consisting mostly of banksia and
eucalyptus woodland (Landcorp et al. 2008). Most of this vegetation is in good to excellent condition,
but with some degradation existing due to unrestricted vehicular access (Alkimos Water Alliance
2008). There are four main landscape characters identified in the Structure Plan. As detailed in
Figure 2-8, most of the land consists of broad undulating coastal heath land. This section includes a
ridge running parallel with the coast, with a peak of approximately 55m AHD. A 1 kilometre belt of
defined parabolic dunes fronts the coast, which exhibit steep sides and elevations up to 25m. The
foreshore consists of unstable dunes with low vegetation and occasional blowouts (Landcorp et al.
2008).
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Figure 2-8: Project area landscape summary (Landcorp et al. 2008) (fullsize image included as Appendix)
The soil is largely a highly permeable mix of medium to fine grained quartz, overlying calcareous sand
and limestone (Alkimos Water Alliance 2008). Groundwater levels beneath the project area vary
from 0m along the coast to a maximum of 3m about 3 kilometres from the coast. The uneven
topography of the area results in highly variable depth to groundwater values. The confining layer of
the superficial aquifer exists between 30 and 35 metres below the Australian Height Datum (AHD)
(Landcorp et al. 2008)
25
Of the development area, approximately 500 hectares will be preserved, with 1300 set aside for
urban, 150 each for commercial and regional activity centres, 160 hectares worth of roads, with the
remaining divided between other activity centres, a marina, modified open space, rail reserve and
other minor uses. The current district structure plan is displayed in Figure 2-9 below. The
development is bordered on the east by the Mitchell Freeway, with Marmion Avenue running
through the centre of it.
Figure 2-9: Current district structure plan map. Fullsize image included as an appendix.
The development plans include a new groundwater treatment plant on the Eastern side of the
development neighbouring the Mitchell Freeway. A series of groundwater bores dispersed
throughout the project area will be connected via collector mains to this treatment facility. It is also
expected that private land owners will install groundwater bores for domestic use. A wastewater
treatment plant will be located in the southern section of the project area that will service the new
26
development, Yanchep and possibly other Northern suburbs. It will discharge treated water via an
ocean outfall pipe.
2.7 Management Options
As the pressures of climate change and increased development increase, there is a need for more
efficient management of our water resources. In the past, water supply systems have focused on
‘end-pipe’ solutions, trying to get the water to the user as cheaply as possible (Rygaard, Binning &
Albrechtsen 2011). Integrated Water Resource Management is the act of managing water and water
resources in connected, sustainable methods (Durham, Rinck-Pfeiffer & Guendert 2003). There are
several methods to achieve this, from simple education to intentionally increasing aquifer recharge.
Managed aquifer recharge refers to intentionally increasing aquifer recharge for the purpose of
storage and/or treatment of water. Also referred to as enhanced recharge, water banking,
underground storage or artificial recharge, the latter term has generally been retired due to public
connotations of the word artificial (Dillon 2005). Using treated wastewater to recharge an aquifer
could be employed to mitigate saltwater intrusion, by increasing groundwater levels. It is also a
sustainable solution for ensuring minimal effects on the natural water balance. It is a method that
has been possible for some time, however public perceptions have largely prevented its
implementation. Public perception is increasingly important as water systems become more
community based and managed recharge using treated wastewater becomes an increasingly
desirable, yet contentious solution (Po, Kaercher & Nancarrow 2003).
An Integrated Water Management study has been undertaken for the development. This study
identifies the potential for managing water balance through several techniques, including water
efficiency, water sensitive urban design and community based solutions. There is no confirmation of
the amount of these options that will be implemented.
Water efficiency options include the use of water efficient fittings in buildings, water efficient
irrigation systems and plants that require minimal water. Water sensitive urban design involves
alternative uses or management of water to achieve sustainability goals, such as incorporating
stormwater drainage in to the urban design to create recharge or reuse potential. Also, there exists a
large potential for water savings through the utilisation of alternative water for non-drinking uses.
According to the district structure plan, this could represent a 70-80% saving of scheme water
compared to projected demand patterns (Landcorp et al. 2008).
27
3 Approach
3.1 Data Collection and Analysis
3.1.1 Groundwater
Groundwater data was supplied by the Department of Water. Data was collected between 1989 and
2009, but there was no structured sampling regime. Figure 3-1 (full-sized figure in appendix) details
the groundwater bore locations. The project area is entirely contained within the Southern end of
the black box. The cluster of purple bores at the top of the box is the Yanchep town centre, past the
limits of the project area.
Figure 3-1: Map of groundwater sites in the Alkimos Eglinton area. (Department of Water 2009)
Considerable time was spent organising, collating and separating the data between bore locations,
dates, and measurements. All water levels were converted to metres above the Australian Height
Datum (AHD). The 17 bore locations that were relevant to the project location and had sufficient
data were identified. Data for each was analysed and a median value with correlating dates was
assigned to each location. These were collated in to a single data file and Matlab was utilised to
interpolate and create a full groundwater contour map (Figure 3-2).
28
Figure 3-2: Groundwater contour map of Alkimos. Black lines represent approximate project area.
3.1.2 Rainfall
Rainfall data was obtained from the Bureau of Meteorology’s publicly available archive. Data was
obtained from the major Perth sampling stations (Midland and Perth Airport) for a general overview
of rainfall trends. The closest station to the project area is Tamala Bay, however there is only very
recent data available. Yanchep and Two Rocks are also within a 15 km radius, with Yanchep
containing data from 1930 to 1990 and Two rocks from 1975 to 2010. An overview of monthly
rainfall data for Two Rocks is presented in Figure 3-3 below. Total average rainfall for Two Rocks was
676 mm annually. This is slightly lower than the annual average for Perth over the same period,
which is approximately 721 mm annually.
Figure 3-3: Monthly rainfall data for Two Rocks
0
50
100
150
200
250
300
1 2 3 4 5 6 7 8 9 10 11 12
Rai
nfa
ll (m
m)
Month
Rainfall at Two Rocks 1975-2010
Min
Mean
Max
29
3.1.3 Climate Change
Using the projected figures established by the IPCC (Parry et al. 2007) and CSIRO (2007) as a guide,
the effect of climate change upon saltwater intrusion was incorporated through simple incremental
increases in the sea-level. The range for projected sea-level rise varied depending on factors such as
ice-melt which are difficult to predict. Therefore, values at the lower and upper range of projection
for the next 50 to 100 years were selected as 0.3 and 0.9m.
Rainfall was projected to decline in all the CSIRO’s (2007) projections, but by varying amounts
depending on each scenario. Table 3-1 below indicates the range of rainfall reduction over each
scenario by the years 2030, 2050 and 2070.
Year 2030 2050 2070
Range of rainfall
reduction
(percentage)
5-10 5-20 10-20
Table 3-1: Projected rainfall reduction in percentage for the years 2030, 2050 and 2070 (CSIRO 2007)
3.1.4 Land Use
Urbanised land use was obtained from the District Structure Plan. Land use allocation is displayed in
Table 3-2 below.
Land Use Allocation Area (ha)
Major land uses
Urban 1285.8
Future potential urban 28
Service Commercial 152.7
Regional Activity Centre 154.8
District Activity Centre 65.9
Coastal Village Activity Centres 36.3
Regional Open Space 406.4
Public Purposes 222.7
Sub TOTAL 2352.6
Minor Land Uses
Primary Regional Roads 108
Secondary Regional Roads 60.2
Foreshore 58.4
Rail Reserve 22.2
Marina 17.1
Pipidinny Rd Reserve 6.3
Regional Open Space (other 2.1
30
ownership)
Sub TOTAL 274.3
TOTAL 2626.9
Table 3-2: Land Use Allocation by Area (adapted from (Landcorp et al. 2008)
3.2 Saltwater Interface Approach The interpolated groundwater map was used to calculate the saltwater interface position with the
Ghyben-Herzberg relationship. This was compared to the location of the confining layer via the
construction of interface cross-sections through three transects.
The process was repeated with the additional influence of sea-level rise. To simulate sea-level rise,
data was incrementally altered based on the IPCC and CSIRO projections. Scenarios of 0.3m and
0.9m were modelled. These correspond to the lower and upper estimates of sea level change by
2050 and 2100 respectively (CSIRO 2007). The coastline was modified to account for sea level rise,
and groundwater heights above sea-level were reduced.
Finally, to simulate abstraction processes, several random drops in pressure were inserted in to the
interpolated data. It was intended to identify the effect these localised ‘abstraction points’ had on
the interface position, however an error in interpolation and calculation processes manipulated the
original data incorrectly, and the results were discarded. The process was not repeated due to time
constraints.
Saltwater interface contour plots were created for all relevant scenarios.
3.3 Water Balance Approach It was originally intended to establish a fully integrated numerical model using the Perth Regional
Aquifer Modelling System (PRAMS), which is a module developed to cater for the Gnangara system
within MODFLOW. Recharge can be calculated in such a model using the Vertical Flux Model (VFM).
However, due to time constraints and technical difficulties, the model wasn’t completed in time.
While it is unfortunate that the model could not be completed, there is still knowledge to be gained
from a comprehensive analysis of the data and relevant literature.
31
4 Results
The saltwater interface was calculated to extend to a depth of 60 metres at the Eastern edge of the
project area.
Figure 4-1: Saltwater interface contour map of Alkimos. Black lines represent approximate project area.
As identified previously, the confining layer is only 30-35 metres below sea-level. To further examine
this, cross sections of the saltwater interface were constructed. Figure 4-2 displays the saltwater
interface of three cross-sections, running through transects directly inland displayed in Figure 4-3.
32
Figure 4-2: Saltwater interface cross-sections
Figure 4-3: Location of saltwater interface cross-sections from Figure 4-2
For much of the project area, the interface will intercept the confining layer approximately 1km from
the coast. There is one particular area, indicated by the Northern cross-section wherein the initial
gradient is much more gradual, and the confining layer is not intersected until the 2km mark.
Impacts of sea-level change are displayed via further saltwater interface contour maps below. The
first, Figure 4-4, shows the predicted saltwater interface location for a sea-level rise or 0.3m, which
corresponds to a likely increase by 2050. Figure 4-5 demonstrates the effect of a sea-level rise of
0.9m, which corresponds to the upper limits of the IPCC’s projections for the year 2100.
0
10
20
30
40
50
60
70
80
0 0.5 1 1.5 2 2.5 3 3.5
De
pth
be
low
se
a le
vel (
m)
Distance from Coast (km)
Saltwater Interface Cross-sections
Northern
Middle
Southern
Confining layer
33
Figure 4-4: Saltwater interface contour map for sea-level rise of 0.3m
Figure 4-5: Saltwater interface contour map for sea-level rise of 0.9m
34
5 Discussion
5.1 Water Balance Climate change modelling is a complex science based on many variables that are difficult to project.
There is a reasonable level of variation between different scenarios and different probabilities.
However, all the scenarios identified by the IPCC and CSIRO predict that rainfall will decline in
southwest Western Australia. Add this to the extreme decline that has already been experienced,
and the Gnangara groundwater system is already unbalanced and experiencing a naturally reduced
recharge. The impact of increased abstraction and land use alteration has further upset the natural
balance of the system and with continued development of urban areas, this is likely to increase.
A basic water balance equation was introduced in section 2.1: Hydrology. There is no irrigation in the
area, and no upward flow, which results in the only input for the system being precipitation. Due to
vegetation, topographical and soil conditions, it is assumed that drainage does not occur on this area
of the Gnangara mound. Therefore, the simplified water balance suggests that recharge is equal to
precipitation less evapotranspiration.
The removal of vegetation to facilitate urbanisation will reduce the evapotranspiration value.
However, any increase in recharge that occurs will likely be offset by the increased abstraction. It has
already been established that precipitation will continue to decrease, by as much as 20% by 2070. At
the same time, increasing evapotranspiration due to increased temperatures could occur. A negative
or minimal recharge value will be further exacerbated by these climate changes.
The combination of decreasing rainfall, increasing abstraction and changing land use can ensure that
groundwater levels will not remain steady in the future. A shift in land and water management
techniques is essential to reverse the effect of urbanisation, and ensure that the resource is used
sustainably.
Haase (2009) argues that from an environmental and water balance point of view, a compact, high
density city would be more desirable as it allows the preservation of large areas of natural landscape.
However, this would cause a drop in evapotranspiration within the urban area, which is considered a
crucial factor in temperature control. The desire for open spaces within urban areas in Perth to
control temperatures and enhance amenity and lifestyle has already been established (Gnangara
Coordinating Commitee 2009). Large enough urban areas will start to affect the local climate, and
develop their own complex microclimates. Over a long period, water balance could be further
affected by the impact of these unnatural climate systems on the surrounding area (Arthur-Hartranft,
Carlson & Clarke 2003). In a region that is already under pressure from changing climate conditions,
35
the argument for smaller, denser urban development is counterintuitive to the goals of
environmental and social benefit.
There have been several integrated water resource management opportunities identified in the
structure plan. It is important that as planning continues, they implement some, if not all of these
suggestions. In particular, the availability of alternative water for non-drinking water uses could
provide a significant reduction on scheme water reliance. This would severely reduce pressure on
local groundwater.
5.2 Saltwater Intrusion The current scenario saltwater interface contour map indicates that most of the project area is
reasonably risk free from the threat of saltwater intrusion. Over the majority of the project area, the
saltwater/freshwater interface will intercept the confining layer within 1km. There are at least two
areas where this will be extended, with one transect in particular (Northern cross-section in
Figure 4-2) hovering between 10 and 20m below sea level for the first kilometre, and not
intercepting the confining layer until 2 km. If significant abstraction were to occur within this area,
there is potential for seawater intrusion to occur. However, it is expected that abstraction bores
would be positioned so as to avoid this. The fact that water will be abstracted through a series of
dispersed bores, rather than one main one withdrawing a large volume also ensures that smaller
pressure drops are spread over the project area, minimising the possibility for saltwater intrusion at
each location.
With sea-level rise incorporated, the saltwater interface shifts inland. Using a figure of 0.3m (lower
end of estimated projections), the aforementioned Northern transect could experience serious
lateral seawater intrusion issues very close to the surface. Modifying the sealevel by 0.9m (higher
range of estimated sea-level change by 2100) indicates that saltwater intrusion would be a serious
issue over a reasonable amount of the project area. There is a large area wherein saltwater intrusion
has occurred, signified by the zero values for the saltwater interface. This is not necessarily a truthful
representative, for reasons outlined below, but it does relay the extreme impact that sea-level rise
could have on groundwater quality.
It is important to note that the method used to simulate sealevel rise here involves many inherent
assumptions. While the changing coastline that would accompany sea-level rise is accounted for, it
changing morphology of the coast that would occur is not. The dunes that would prevent nundation
could be altered during this time period, vastly changing the landscape. In addition, sea-level rise is a
gradual process that occurs over a number of years. Groundwater would adjust as the sea rose and
the balance changes, and discharge patterns and the groundwater shape would be altered. The
36
modelling completed could be compared somewhat to an overnight rise of sea-level, and is
therefore not truly indicative. However, it does relay the potentially large impact that could be
experienced by sea rise of less than a metre.
5.3 Complications, errors and assumptions
The groundwater bore locations provided by the department of Water do not cover the entire
project area. There is also some question as to the dates matching up. Only having 17 data points to
interpolate a map of 2600 hectares will inherently contain some errors. In an area of that size, just
one additional data point at a critical location could alter a large section of the map. This is
particularly relevant with regard to the quick groundwater flows experienced in the region.
The process used to create sea-level rise scenarios has some inherent flaws, that have been
previously identified in the saltwater interface discussion.
37
6 Conclusions & Recommendations Perth relies on the Gnangara groundwater system for to drive economies, provide potable water for
a major sector of the public and support many ecologically significant wetland areas. The
combination of decreasing rainfall, increasing abstraction and changing land use can ensure that
groundwater levels will not remain steady in the future. Under current conditions, social and
economic values cannot be maintained alongside desired environmental outcomes. A shift in land
and water management techniques is essential to reverse the effect of urbanisation, and ensure that
the resource is used sustainably.
There have been several integrated water resource management opportunities identified in the
structure plan. It is important that as planning continues, they implement some, if not all of these
suggestions. In particular, the availability of alternative water for non-drinking water uses could
provide a significant reduction on scheme water reliance. This would severely reduce pressure on
local groundwater.
The likelihood of sea-level rise, coupled with the potential for increased abstraction of groundwater
from the Gnangara mound combine to make saltwater intrusion a very real threat. While proper
planning and suitable abstraction management should render this unlikely the examples set by
global scenarios provide a warning for ignoring this threat. Managed aquifer recharge is a strategy
that could be utilised to mitigate saltwater intrusion issues, which would also provide a more
sustainable resource from a water balance perspective.
As was intended for this study, the completion of an appropriate groundwater model, such as
PRAMS would provide a much more detailed analysis of the impact of this development. It is also
recommended that more research be undertaken regarding the implementation of managed aquifer
recharge schemes, with particular emphasis on utilising treated wastewater.
38
7 Appendices
7.1 Appendix A – Project area landscape
39
7.2 Appendix B - District Structure Plan
40
7.3 Appendix C - Groundwater Sites (full image)
41
8 References
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