Estimation of submarine groundwater discharge into Geographe Bay, Bunbury, Western Australia

Journal of Geochemical Exploration 106 (2010) 197–210

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Journal of Geochemical Exploration j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j g e o ex p

Estimation of submarine groundwater discharge into Geographe Bay, Bunbury, Western Australia Sunil Varma a,⁎, Jeffrey Turner b, Jim Underschultz a a b

CSIRO Petroleum Resources, 26 Dick Perry Avenue, Kensington WA 6151, Australia CSIRO Land and Water, Private Bag 5 Wembley WA 6913, Australia

a r t i c l e

i n f o

Article history: Received 11 May 2009 Accepted 12 February 2010 Available online 21 February 2010 Keywords: Submarine groundwater discharge Numerical modeling Satellite imagery Southern Perth Basin Australia

a b s t r a c t A study of the submarine groundwater discharge (SGD) into Geographe Bay between Bunbury and Dunsborough in the south west of Western Australia has been carried out under the CSIRO Wealth from Oceans Flagship program. The study focused on establishing a conceptual understanding of SGD and its indirect quantification using hydrogeological (hydraulic) and modeling techniques. Satellite infrared images and near shore bathymetry were used to obtain evidence of the possible geographic distribution of significant SGD. The study area forms an offshore part of the Southern Perth Basin. Past studies have shown that there is substantial groundwater flow within the Superficial, Leederville and South West Yarragadee aquifers of the Southern Perth Basin in the offshore direction. The groundwater from these aquifers must ultimately discharge offshore at the seabed into Geographe Bay in the north and the Southern Ocean in the south. Some discharge also takes place onshore via rivers and drains. The results of the study show that groundwater discharge into Geographe Bay from the Superficial Aquifer between Bunbury and Busselton is 240–284 m³/day/km of the 67 km long coastline. The offshore groundwater flow from the Leederville Aquifer along the 55 km length of the coast where the aquifer occurs is estimated at 270 m3/d/km of the coastline. The offshore flow from the South West Yarragadee Aquifer is at the rate of 3360 m3/d/km of the Geographe Bay coastline. The large flow rates in the South West Yarragadee Aquifer are due to substantial downward leakage from the overlying Leederville Aquifer as well as significant recharge from rainfall in the outcropping areas. The total offshore groundwater discharge to Geographe Bay from all aquifers is estimated to be around 80 million m3/year on the basis of hydraulic assessment and the modeling results. Remotely sensed infrared images were analysed to detect evidence of any submarine groundwater discharge. The seasonal temperature contrast between the groundwater discharge at the seabed and the seawater makes it theoretically possible to detect areas of significant offshore groundwater seepage using suitable thermal images. ASTER thermal-infrared satellite images were acquired and analysed for this study. The ASTER images showed a low-temperature plume (21 °C) in the southwest of the study area near Dunsborough Fault, possibly groundwater discharge, extending in an easterly direction and being surrounded by seawater of about 23 °C. Similar cooler zones were observed along the coastline indicating groundwater discharge from the Superficial Aquifer. Following a hypothesis that former onshore coastal groundwater discharge features are likely to continue to be SGD focus areas following marine transgression, the near shore bathymetry for Geographe Bay revealed a series of ridges and troughs running sub-parallel to the present-day beach. The hillshade rendition shows a sub-parallel trough feature located about 4–5 km offshore. The bathymetric features identified offshore and the low-temperature plume as observed in the satellite images provide indicators and target areas for more detailed investigation and actual measurement of SGD fluxes, for example by CTD (Conductivity, Temperature and Depth) and SSS (side-scan sonar) probing around target bathymetric features. In the interim, this study has provided valuable information about the rate of potential groundwater seepage from the Southern Perth Basin aquifers into Geographe Bay indicating the scale of possible offshore groundwater resource availability based on SGD recovery and management. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.

⁎ Corresponding author. Tel.: + 61 8 64368731; fax: +61 8 64368555. E-mail address: [email protected] (S. Varma). 0375-6742/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2010.02.003

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1. Introduction 1.1. Background Submarine groundwater discharge (SGD) is defined broadly as all water flowing from onshore coastal aquifer systems via seabed sediments into the near shore marine environment. The term includes meteoric water derived from sub-surface terrestrial drainage from deep and shallow coastal aquifers, and seawater circulation through the near shore seabed. At the continental scale, discharge of surface water runoff at the coast is conspicuous and comparatively well documented. However groundwater discharge into the marine environment along vast stretches of coastline goes mostly unrecognised as one of the three major discharge routes of water from the Australian continent: the other two being evapotranspiration and surface runoff. To date there has been no national-scale effort to assess SGD as a potentially major sink for the national water resources and the extent to which this is a recoverable and useable water resource for coastal zones. Work done to quantify SGD from shallow aquifers at “point” coastal locations indicates that SGD over only several tens of kilometres of coastline, is equivalent to regional surface water discharges to the ocean from rivers and estuaries (Smith et al., 2003). Until recently, common understanding in the hydrology and oceanographic sciences was that the majority of nutrients and pollutants introduced to the ocean were via surface discharge of rivers and that other sources were insignificant. An article in Nature by Capone and Bautista (1985) caused a paradigm shift in thinking when they discovered that in their study area (Great South Bay, New York) about 60% of the influx to the ocean was via SGD. This result has since been confirmed by other follow-up studies from around the world. In the Australian context of arid climate and low river flows the proportion of ocean discharge of groundwater is likely to be even larger. Hence, the contaminant and nutrient influx to the ocean that impact on the health of benthic and marine aquatic communities may be largely dependent on submarine groundwater discharge. 1.2. Purpose and scope The objective of this study was to make an assessment of the submarine groundwater discharge (SGD) into Geographe Bay in the south west of Western Australia using information from previous hydrogeological and modeling studies. The offshore discharge of groundwater from the aquifers of the Southern Perth Basin has not been well investigated in spite of an increased pressure on the development of the South West Yarragadee Aquifer for horticultural and public drinking water supply. In the past, very detailed studies of SGD have been conducted in Cockburn Sound in Western Australia (Smith et al., 2003) including quantitative hydrogeologically based SGD estimates (Smith and Nield, 2003). Internationally, several sites have been well described by Burnett et al. (2006). This study has focused on establishing a conceptual understanding of SGD and its indirect quantification using hydrogeological and modeling techniques in the Geographe Bay. It was also envisaged that this project will result in a list of recommendations for field application of a suite of methodologies for identification and quantification of SGD in a future stage of the project.

The present study involves mapping areas of SGD in Geographe Bay by satellite thermal IR imagery that detects significant groundwater discharge by identifying the thermal contrast between the groundwater discharge and the seawater. A quantification of the potential offshore groundwater flux using Darcy's equation is included, as are the water balances derived from the SWAMS (South West Aquifers Modeling System) numerical model. This assumes that the offshore flux estimated along the coastline will eventually discharge into the Bay. This study contributes to the verification of the SWAMS model. 2. Description of the study area Geographe Bay is located in the south west of Western Australia about 220 km southwest of Perth (Fig. 1). The Bay extends from Bunbury in the north to Cape Naturaliste in the west and is about 100 km wide. 2.1. Onshore study area The onshore coastal part of the study area comprises the 10–15 km wide Swan Coastal Plain which slopes gently from Whicher and Darling Scarps at about 60 m elevation with reference to the Australian Height Datum (equivalent to the mean sea level plus 0.026 m) down to the coast. The plain is covered by sediments of Pliocene–Pleistocene and Holocene age known as the superficial formations (Allen, 1976). Dunes of the Bassendean, Spearwood and Quindalup System occur along the coastal belt. Basalt outcrops at Bunbury and it also occurs near the surface (below the Spearwood Dunes) to the southeast of Bunbury. The Swan Coastal Plain is drained by several rivers and streams that flow into the ocean at Geographe Bay. There are a suite of environmentally significant wetlands such as the Vasse–Wonnerup wetlands close to the coastal margin that are dependent on the shallow groundwater of the Swan Coastal Plain. The Swan Coastal Plain has been cleared extensively for agriculture. Cattle and sheep farming and potato and lucerne cropping, are the predominant agricultural practices. The agricultural practices have relied extensively on the use of fertilisers. The Whicher Scarp lies to the south of the Swan Coastal Plain which rises to about 190 m elevation Australian Height datum (AHD) at the Blackwood Plateau. The plateau is mostly planar, with a gentle southwards slope. Its surface is mainly covered with laterite (Water Corporation, 2005). 2.2. Climate The south-west of Western Australia has a Mediterranean type climate with cool wet winters and warm dry summers. The average annual rainfall for Bunbury from 1995 to 2008 was 736 mm. The average rainfall for the same period for Busselton is 812 mm. Average pan evaporation at Bunbury is about 1500 mm. About 70–80% of the rainfall in the region takes place in the five months from May to September. Bunbury has a long-term annual mean minimum air temperature of 11 °C and a mean maximum of 22.9 °C. The July temperatures (peak winter) range from 7.3 to 17.3 °C, whilst in February (peak summer) the temperature varies from 15.6 to 29.8 °C (Source: Bureau of Meteorology, Australia).

1.3. Methodology

2.3. Physical oceanography

In the past many studies have focused on methods to quantify SGD either through point measurements, hydrogeological models, or by using geochemical tracers. Each of these approaches provides estimates that are applicable at a range of both temporal and spatial scales. Application and comparison of results from different techniques used to estimate SGD at the same time and place is vital for identifying reliable field methods for a particular geological setting.

The physical oceanography of Geographe Bay was studied in detail by Fahrner and Pattiaratchi (1995) for the Water Corporation of Western Australia, and later discussed by McMahon et al. (1997) as part of a study into the status of the shallow seagrass system in the Geographe Bay. The water depth increases to 3–4 m within the first several hundred metres of the shore, however, in general the Geographe Bay is relatively protected (not affected by waves) and has a gently

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Fig. 1. Location of the study area.

sloping bathymetry of 2 m/km up to about 15 km from the shore. The seabed contains extensive meadows of seagrass in the deeper (2–14 m) regions (McMahon et al., 1997). The near shore zone is characterised by submarine sandbars which rise up to 2 m above the surrounding seafloor. From around 15 to 75 km from the shore the slope is very gentle at b0.5 m/km in an area known as ‘The Inner Shelf’. Beyond this the seafloor slopes rapidly to the edge of the continental shelf over an area known as ‘The Outer Shelf’ (Fahrner and Pattiaratchi, 1995). The tidal range is small and is typically less than 1 m, hence, water movement is mainly wind driven (McMahon et al., 1997). The circulation pattern in the Geographe Bay area is due to a combination of influences including the oceanic currents such as the Leeuwin Current and the Capes Current (Fahrner and Pattiaratchi, 1995). McMahon et al. (1997) measured the water temperature at several sites in Geographe Bay between Bunbury and Dunsborough from January 1994 to January 1995. The temperature peaked at the end of summer (February) with a mean of 21.6 ± 0.4 °C. The mean minimum temperature was recorded in July 1994 at 14.8 ± 0.4 °C. There was a gradient of temperatures along the coast in summer with 1 °C cooler temperatures in the south-west of the bay in comparison to the northeasterly sites. SKM (2003) found that water temperature at selected Geographe Bay sites ranged between 14 and 25 °C between March 2001 and November 2002. The maximum seawater temperature was recorded in March 2001, however, during other months the temperature remained below 22.5 °C including the peak summer months of January–February 2002. Seawater at all sites was generally well-mixed and vertical temperature stratification was evident only in November 2001 when the surface 0.6 m was warmer. The salinity of seawater ranged between 33,100 and 37,200 mg/L TDS. SKM (2003) found no evidence of salinity stratification. 3. Geological and hydrogeological overview The study area is a part of the Southern Perth Basin. The geology of the basin has been described in Crostella and Backhouse (2000) and

Iasky and Lockwood (2004). Most of the study area lies within the Bunbury Trough subdivision of the Southern Perth Basin (Fig. 1). The Bunbury Trough is bounded to the west by the Busselton Fault and to the east by the Darling Fault. The Bunbury Trough contains about 11,000 m depth of sediments of Quaternary to Permian age overlying a Precambrian granitic basement. The Vasse Shelf is bounded to the east by the Busselton Fault and the west by the Dunsborough Fault and contains about 3000 m of Permian to Mesozoic sediments. The stratigraphic sequence of the Southern Perth Basin comprises Permian to Quaternary sediments as shown in Table 1. Several previous hydrogeological investigations by GSWA, Department of Water and the Water Corporation, including exploratory drillings, have contributed to the current state of knowledge of the hydrogeology of the Southern Perth Basin such as Wharton (1980, 1981a,b, 1982), Commander (1984), Appleyard (1991), Baddock (1994, 1995), Panasiewicz (1996) and Water Corporation (2005). The superficial formations on the Swan Coastal Plain form an unconfined system known as the Superficial Aquifer. The Leederville Aquifer is comprised of the Vasse Member of the Leederville Formation which underlies the Mowen Aquitard (consisting of the Mowen and Quindalup Members). However for most of the study area the Mowen Aquitard is absent (Fig. 2) and the Leederville Aquifer directly underlies the Superficial Aquifer. Near Bunbury the Leederville Aquifer is also absent due to erosion. The South West Yarragadee Aquifer comprises the Yarragadee Formation, the Lesueur Sandstone and the Cockleshell Gully Formation in the southern part of the Southern Perth Basin where these occur at relatively shallow depths. The South West Yarragadee Aquifer is not present in the northern Vasse Shelf. In the study area the aquifer comprises only of the Yarragadee Formation and generally underlies the Leederville Aquifer. The Sue Coal Measures is not considered to have significant groundwater flow due to low permeability. Groundwater at depths more than 1700 m in the Southern Perth Basin have high salinity and considered to have insignificant flow (Water Corporation, 2005).

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Table 1 Generalised stratigraphy of the Southern Perth Basin (from Water Corporation, 2005). Age

Formation

Maximum Intersected Thickness

Lithology

Tertiary to Quaternary unconformity Cretaceous

Superficial formations Leederville Formation Quindalup Member Mowen Member Vasse Member South Perth shale Bunbury basalt Parmelia Formation Yarragadee Formation Cockleshell Gully Formation Lesueur Sandstone Sabina Sandstone Sue Coal Measures Basement

30 m

Alluvial, swamp, estuarine deposits Sandstone, shale, conglomerate

Unconformity Jurassic Triassic Permian Precambrian

65 m 96 m 288 m 110 m 106 m 264 m 1252 m 1275 m 2556 m 518 m 1839 m

4. HydrogeologIcal assessment of submarine groundwater discharge 4.1. Superficial Aquifer Conceptually, groundwater discharge from the Superficial Aquifer occurs into Geographe Bay along the coastline above a saltwater wedge. The watertable contours in the Superficial Aquifer for September 2006 (winter maxima) and April 2007 (summer minima) are shown in Figs. 3 and 4. The watertable contours range from 0 at the coastline to 40 m AHD near the Whicher Scarp. Seasonal fluctuation of the watertable is from 0.5 m near the coast to 2 m near the Whicher Scarp. Horizontal hydraulic gradients vary from 0.0013 in the west to 0.0026 in the east of the study area. The average hydraulic gradient close to the coastline has been estimated as 0.0016 in summer and 0.0018 in winter. Hydraulic conductivity of the Superficial Aquifer ranges from 0.001 to 30 m/d based on the lithology; however the hydraulic conductivities used to calibrate SWAMS groundwater model range mostly between 10 and 20 m/d (Sun, 2005). The saturated thickness of the Superficial Aquifer varies from 5 to 20 m, however, near the coastal area of Geographe Bay it is around 10 m (Fig. 5). The groundwater discharge along the coastline between Bunbury and Busselton is estimated using the Darcy's equation below. QS = kbiL

ð1Þ

In the above equation, ‘QS’ is the groundwater discharge into the Geographe Bay from the Superficial Aquifer. k is the average hydraulic conductivity estimated to be 15 m/d, b is the saturated thickness near the coast (10–10.5 m), L is the length of the coastline (67 km) along which the discharge takes place and i is the hydraulic gradient (0.0016 in summer and 0.0018 in winter). Using Darcy's equation the groundwater discharge from the Superficial Aquifer is calculated as 240 and 284 m3/d/km of the coastline in summer and winter, respectively. This equates to an aggregate discharge of 6.5 × 106 m3/year. 4.2. Leederville Aquifer The Leederville Aquifer is typically about 100 m thick, reaching in excess of 200 m in places. Near the coastline the average thickness of the Leederville Aquifer is about 120 m (Water Corporation, 2005). Groundwater flow in the Leederville Aquifer is to the northwest in the Bunbury Trough and to the north in the Vasse Shelf (Fig. 6). Water level contours are highest on the Blackwood Plateau where most of

Shale, sandstone, siltstone Porphyritic basalt Sand overlying shale Sandstone, shale, siltstone, minor conglomerate, coal Sandstone, shale, siltstone, coal Sandstone, gravel, siltstone Sandstone, coal, shale Sandstone, siltstone and coal

the recharge to the aquifer takes place and seasonal fluctuations of the water levels in the aquifer are minor. Hydraulic gradients in the Leederville Aquifer vary from 0.0012 in the Bunbury Trough to 0.0024 on the Vasse Shelf, with an average of about 0.0015. Hydraulic conductivities derived from several pumping tests, range from 0.2 to 3 m/d with an average of 1.5 m/d across the study area. Offshore groundwater flux along the 55 km length of the coast where the Leederville Aquifer is present is estimated using Darcy's Law (Eq. (1)) to be 5.4 × 106 m3/year. This equates to a daily offshore throughflow of 270 m3/d/km of coastline. 4.3. South West Yarragadee Aquifer Groundwater within the South West Yarragadee Aquifer flows predominantly to the north and south from the main recharge areas on the Blackwood Plateau where the Yarragadee Formation outcrops. The aquifer is likely to discharge to overlying formations adjacent to the coast and beneath Geographe Bay in the north or to the Southern Ocean in the south (Water Corporation, 2005). A potentially major area for groundwater discharge was identified offshore from Bunbury where the South West Yarragadee Aquifer outcrops at the sea floor (Fig. 7). The groundwater flow patterns in the South West Yarragadee Aquifer are shown in Fig. 7. In the study area groundwater flow in the South West Yarragadee Aquifer is northwards discharging offshore beneath the Geographe Bay. Potentiometric heads vary from 5 m AHD near the coastline to 40 m AHD at the Blackwood Plateau where the aquifer is recharged by rainfall. The aquifer also receives significant groundwater recharge as leakage from the overlying Leederville and Superficial Aquifers (Varma, 2009). Hydraulic gradients in the South West Yarragadee Aquifer are around 0.0007. Hydraulic conductivities derived from pumping tests as part of previous studies indicate an average of around 20 m/d in the eastern part of the Southern Perth Basin and 7 m/d in the western part of the basin. On the regional scale the hydraulic conductivity is expected to be lower than those values obtained from pumping test data because usually the tests are carried out in production bores that are screened in the best quality section of the aquifer. The regional hydraulic conductivity interpreted from groundwater flow rates derived from carbon-14 analysis of groundwater and the measured hydraulic gradient is 8 m/d. The South West Yarragadee Aquifer reaches up to about 1700 m in thickness, however, along the Geographe Bay coastline the aquifer thickness is typically 600 m. Offshore groundwater flux beneath the Geographe Bay along 55 km of the coastline where the aquifer occurs is estimated using the Darcy's Law (Eq. (1)) as 67.5 × 106 m3/year in total or 3360 m3/d/km of the coastline.

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Fig. 2. Areas where the Mowen Aquitard is absent is shaded.

5. Submarine groundwater discharge derived from the SWAMS model The SWAMS (South West Aquifers Modeling System) groundwater model (Sun, 2005) was developed to predict the impact of various groundwater extraction scenarios from the different aquifers on

groundwater levels near GDEs (groundwater-dependent ecosystems) and the groundwater discharge to rivers and the ocean. The model was later modified by CSIRO by applying a coupled VFM (Vertical Flux Model) based recharge (CSIRO, 2009). The model was calibrated in a transient mode for the period 1990 to 2008. The model was subsequently used to determine the impacts of potential climate

Fig. 3. Watertable contours (m AHD) for September 2006. Groundwater flow is in a direction perpendicular to the contours.

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Fig. 4. Watertable contours (m AHD) for April 2007. Groundwater flow is in a direction perpendicular to the contours.

change and increased groundwater demands on the groundwater levels and flows. The results from the SWAMS model simulations were used in this study to estimate SGD by analysing the cellular water budgets along the coastline (the constant head boundary) for

the Superficial Aquifer, and the cells representing the offshore discharge zones of the Leederville and the Yarragadee aquifers. The SWAMS model domain extends approximately 190 km in the north–south direction and 70 km in the east–west direction, covering an

Fig. 5. Saturated thickness of the Superficial Aquifer in m AHD.

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Fig. 6. Leederville Aquifer water levels for September 2006 in m AHD. The shaded areas are where the aquifer is absent and the South West Yarragadee Aquifer directly underlies the Superficial Aquifer onshore and outcrops on the sea floor offshore.

area of about 8500 km2 that includes both offshore and onshore parts of the Southern Perth Basin. The model grid has 363 rows and 193 columns with grid size ranging from 250 m × 250 m to 1000 m × 1000 m. The

part of the model domain covering the study area is shown in Fig. 8. Vertically, the model has eight layers to represent the major geological formations of the Southern Perth Basin.

Fig. 7. South West Yarragadee Aquifer water levels for September 2006 in m AHD.

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Groundwater discharge into the ocean has been simulated in the SWAMS model using the ‘constant head’ boundary conditions applied to each aquifer that has a potential offshore flux of groundwater. In the Superficial Aquifer that occurs only on the Swan Coastal Plain in the study area, the northern boundary is the coastline. The model cells at this boundary have been given constant heads equal to 0 m AHD representing the sea level. Usually in numerical groundwater flow modeling, most components of the water balance for the modeled area, such as recharge and pumping, are estimated prior to modeling and are provided as input to the model. Groundwater discharge components are unknown but can be derived from the modeling assuming the model is well calibrated and other components of the water balance are known within reasonable accuracy. Aquifers below the Superficial Aquifer extend offshore and eventually discharge into the ocean either directly or through the overlying strata. The model boundary conditions offshore consist of constant heads covering the submerged areas. The constant heads for these layers were equated to environmental heads, i.e. heads greater than 0 m AHD to represent a higher density seawater column. Using the Ghyben–Herzberg principle a 2.5 m constant head value was defined for every 100 m depth of sea water (Sun, 2005). Offshore groundwater discharge estimates were obtained from the model for each aquifer from the average water balances for the 2000– 2007 period. 5.1. Superficial Aquifer The water balance of the Superficial Aquifer on the Swan Coastal Plain, estimated from the SWAMS model, consists of 32 × 106 m3/year of net rainfall recharge that contributes to the groundwater flow in this area of which the groundwater discharge to the ocean is 14.5 × 106 m3/year. In the model the offshore groundwater discharge from the Superficial Aquifer takes place along a longer coastal boundary which extends about 87 km in total from the Dunsborough Fault in the east to the north of Lake Leschenault about 20 km north of Bunbury. The average groundwater discharge along this length of the coastline from the Superficial Aquifer is 457 m3/d/km. The offshore groundwater discharge estimated from the model is about 60% more

than that calculated using the Darcy's Law suggesting that the recharge to the Superficial Aquifer in SWAMS may be an overestimate. 5.2. Leederville Aquifer The Leederville Aquifer receives a net rainfall recharge of 151 ×106 m3/year. It also receives 38 ×106 m3/year as leakage from the overlying Superficial Aquifer. There is a net leakage of about 98 × 106 m3/ year from this aquifer into the underlying South West Yarragadee Aquifer in the onshore part of the model. The Leederville Aquifer has several significant rivers incised into it that receive a total of about 106 × 106 m3/ year of groundwater discharge as baseflow from the aquifer. Abstraction from this aquifer is about 18× 106 m3/year. The total offshore flow of groundwater from the Leederville Aquifer is estimated from the model water balance as 14 ×106 m3/year. Of this the offshore groundwater flow over approximately 62 km of the northern coastline in the model is 7 ×106 m3/year, equating to an average flow of 309 m3/d/km into Geographe Bay. 5.3. South West Yarragadee Aquifer The South West Yarragadee Aquifer receives a net rainfall recharge of 19 × 106 m3/year. The aquifer receives an additional recharge of 114 × 106 m3/year as net leakage from the overlying Leederville and Superficial Aquifers. About 37 × 106 m3 of groundwater is extracted each year from this aquifer and its contribution to the Blackwood River baseflow is 13× 106 m3/year. It is estimated from the model water balance that the offshore groundwater flow from the South West Yarragadee Aquifer is 130 × 106 m3/year, of which 67× 106 m3/year flows offshore in the Geographe Bay area. The average flux per kilometre of the Geographe Bay coastline is estimated at 3000 m3/d. As the Leederville and South West Yarragadee aquifers extend offshore beneath Geographe Bay, and conceptually the head gradient between the two aquifers offshore is upwards (Water Corporation, 2005), the actual groundwater discharging into Geographe Bay from the Leederville Aquifer as derived from the model is 23 × 106 m3/year which is more than that at the coastline. Accordingly, the ocean

Fig. 8. SWAMS model grid layout for the study area.

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discharge from the South West Yarragadee Aquifer is estimated as 52 × 106 m3/year which is less than that at the coastline. This is because some groundwater flow from the South West Yarragadee Aquifer will take place upwards into the Leederville Aquifer prior to discharging at the seabed. However, the total groundwater discharge into the Geographe Bay from the Leederville and South West Yarragadee aquifers estimated from modeling is 74 × 106 m3/year. The total offshore discharge of groundwater from the three major aquifers derived from modeling and hydraulic assessment are similar and around 80 × 106 m3/year. The differences in the groundwater discharge obtained from hydraulic and modeling methods for the individual aquifers is likely due to a simplified representation of the complex hydrogeology such as application of local estimates of hydraulic conductivity at a regional scale, and potential errors in the model boundary conditions such as recharge.

6. Satellite thermal infrared images of Geographe Bay It was anticipated that the temperature difference between the discharging groundwater and the surrounding sea water would make it possible for detection of any groundwater seepage at the ocean floor using thermal images. There have been several studies conducted for detection of SGD using thermal-infrared sensors mounted on aircrafts and satellites. Roseen et al. (2001) conducted a series of thermalinfrared aerial surveys over the Great Bay Estuary in coastal New Hampshire. This study delineated the large-scale groundwater flux into the estuary. This flux was then used to estimate the nutrient loading to the estuarine ecosystem. Groundwater discharge locations were mapped in Rehoboth and Indian River bays in coastal Delaware using ground-, aerial- and satellite-based thermal-infrared imagery (McKenna et al., 2001). Warmer groundwater (~15 °C) discharging into cooler bay water (~5 °C) was identified in images from aerial and ground surveys and in a LANDSAT 7 image. Akawwi (2006) used aerial thermal-infrared imagery to detect SGD in the Dead Sea based on temperature contrast between the warmer groundwater and the cooler Dead Sea water. Mulligan and Charette (2005) were able to detect SGD in the Waquoit Bay, Massachusetts based on contrasting temperatures of the groundwater (13.5–14.5 °C) and the Bay (19–20 °C). ASTER thermal image has been used to detect groundwater discharge in the Mediterranean Sea in northern Lebanon (UNESCO, 2004). Satellite images in the visible and thermal infrared portions of the electromagnetic spectrum were sourced from Geoscience Australia and from the Australian Bureau of Meteorology. Images from three different satellites, NOAA, LANDSAT and ASTER, were obtained for this study. Images were selected for the period when there was a maximum contrast expected between the groundwater and the ocean temperatures to increase the chance of detection of any offshore groundwater seepage. Groundwater monitoring bores in the coastal areas between Bunbury and Busselton show groundwater temperatures to be 19– 21 °C. Turner and Dighton (2008) report groundwater temperatures at bore-heads in a transect of the Leederville Aquifer between Cowaramup and Busselton in November 2006 of 19.8 °C (one standard deviation: 1.4 °C, n = 24). McMahon et al. (1997) measured seawater temperatures in Geographe Bay of between 14.8 °C in winter and 21.6 °C in summer. There was also a gradient of temperatures along the coast in summer with 1 °C cooler temperatures in the south-west of the bay in comparison to the northeasterly sites. SKM (2003) measured seawater temperatures ranging between 14 °C and 25 °C (mostly b22.5 °C) at six Geographe Bay sites with maxima in early autumn. With this thermal information it was believed that satellite imagery during either peak winter or peak summer would offer the maximum thermal contrast between the seawater and the discharging groundwater.

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The NOAA satellites are equipped with the Advanced Very High Resolution Radiometer (AVHRR) sensor. NOAA images were obtained in the 10.5–11.5 micron band which is ideal for mapping sea surface temperatures. Images were obtained for the months of February and June 2007 when a maximum temperature contrast between the seawater and the groundwater seepage was expected due temperatures of the sea water being warmer and cooler than the groundwater in these months respectively. It was found that the NOAA infrared images were unable to detect any submarine groundwater seepage into the ocean possibly due to the poor spatial resolution of the images (1 km×1 km) which is unable to detect localised seepages. It is also possible that the seepage rate may be insufficient to affect the seawater temperature at this larger scale. A comparison between the sea surface temperatures from the AVHRR sensor on the NOAA satellites and the locally measured temperature at Busselton by Pearce et al. (2008) showed that whilst the main temperature variations were generally reproduced by the satellite data, there were occasions when the satellite data differed from the in situ measurements by up to 2 °C. This indicates some limitations of the AVHRR to resolve near shore localised temperatures variations. The LANDSAT 7 images were obtained from the Australian Centre for Remote Sensing (ACRES) located within Geoscience Australia. The LANDSAT 7 belongs to a series of LANDSAT remote sensing satellites operated by the United States Geological Survey (USGS). It has the Enhanced Thematic Mapper Plus (ETM+) sensor which has 7 spectral bands and a panchromatic band with 15-metre resolution. The thermal band has a high resolution of 60 m. A fault with the LANDSAT 7 Scan Line Corrector (SLC) occurred on 31 May 2003 resulting in small gaps in the processed products. As recent images of the Geographe Bay were affected by the SLC error, therefore, only images prior to 31 May 2003 were scanned for their suitability in this study. LANDSAT 7 image was obtained for thermal band 6 for summer of 2003 as shown in Fig. 9. The thermal band image has been digitally enhanced to highlight features where the thermal emissions are low. Much of the Geographe Bay area was covered by cloud. While there are some local variations in the thermal band in the offshore area to the north represented by localised warm zones in generally cooler ocean water, it not clear whether this is an indication of SGD. The groundwater is normally expected to be cooler that the seawater during February. It is difficult to interpret the image for any submarine groundwater discharge in a daytime image as the warmer land conditions are likely to affect the near shore water temperature and mask the effect of any cooler groundwater seepage. The LANDSAT thermal-infrared image has also not been converted to sea surface temperature and hence may have inherent inaccuracies related to atmospheric effects and cloud contamination (Fisher and Mustard, 2004). ASTER is the Advanced Spaceborne Thermal Emission and Reflection Radiometer, a multi-spectral sensor onboard one of NASA's Earth Observing System satellites, Terra. ASTER sensors measure reflected and emitted electromagnetic radiation from Earth's surface and atmosphere on 14 channels (or bands) (Kalinowski and Oliver, 2004). The thermal infrared radiation (TIR) images have a resolution of 90 m. ASTER data are used for a range of applications including land-use studies, mapping, geology, water resources, coastal resources, environment and generation of digital elevation models (DEM). Unlike LANDSAT, repeat coverage by the ASTER sensor is more infrequent. A search of the GA product catalogue located an image of the study area for 25 January 2008 (Fig. 10). The image however was for daytime that resulted in warm land conditions affecting the near shore water temperature and probably masked any evidence of SGD. Night time infrared (NTIR) image was requested through Geoscience Australia GPR to avoid any masking of SGD from the daytime heating of the land. Accordingly, an ASTER NTIR image for 20 Feb 2008 was identified and obtained for this study (Fig. 11). The night time image shows a cooler temperature plume (20–22 °C) in the southwest part of Geographe Bay extending in a northeasterly direction. The surrounding

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Fig. 9. LANDSAT 7 image in thermal band on 21/2/2003 at 9:54 AM local time.

Fig. 10. ASTER image for 25 Jan 08 11:29 AM local time.

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seawater temperature is 22–24 °C. The wind speed was 19 km/h and the wind direction was south-southeasterly on the date of the image and is reflected in the northerly concave orientation of the plume. The plume represents potential groundwater discharge into a slightly warmer ocean. The plume appears to be concentrated near the Dunsborough Fault which may be the source of a focused groundwater discharge from the Southern Perth Basin aquifers abutting against the impermeable basement rocks of the Leeuwin Ridge to the west of the fault. Fig. 12 shows the temperature variation along a transect (T1 as shown in Fig. 11). The cooler water temperature between 1 and 6 km from the coast is indicates an area of possible groundwater discharge. The cooler seawater temperatures in the southwest of the Bay as shown the ASTER NTIR image (Fig. 11) are consistent with the temperature gradient in the southwesterly direction as observed by McMahon et al. (1997). Zone of cooler water can also be observed along the coastline and could represent groundwater discharge from the Superficial Aquifer. A cooler anomaly is also observed in the Leschenault Inlet indicating groundwater discharge in the inlet. There does not appear to be an indication of any groundwater discharge in the area of the South West Yarragadee outcrop on the seabed to the west of Bunbury. This could be due to very low vertical hydraulic gradients in the South West Yarragadee Aquifer in the onshore Bunbury area and perhaps a more diffused discharge over a larger aquifer outcrop area such that the contrast with the seawater temperature may be insignificant. It may also be possible that the groundwater discharge from the South West Yarragadee Aquifer takes

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Fig. 12. Temperature variation with distance from coast along transect T1.

place further offshore or by upward leakage through the overlying Leederville Aquifer which in turn discharges into the ocean. 7. Seabed bathymetric analysis Given the timescale of groundwater flow in the deep aquifer systems (about 1 m/yr) and marine transgression due to sea level rise of about 90 m in the past ten thousand years, we can expect that

Fig. 11. ASTER NTIR image for 21 February 2008 00:09 AM Local time.

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marine transgression has drowned former onshore topographic features of the landscape. Thus offshore seabed topography and bathymetry is likely to be an indicator for the preferential discharge of groundwater from either the superficial formations or via vertically upward fluxes from deeper formations. A present-day analogue is the eastern shoreline of Lake Clifton in the Yalgorup National Park, about 25 km north of the study area, which discharges fresh groundwater from the superficial formations along this margin. Our contention is that analogous offshore structural features, drowned by recent marine transgression, are likely to be preferred locations for SGD in the present day. The back-beach lagoons at Wonnerup and Busselton in the study area are examples of present-day systems. Figs. 13–15 show three unscaled renditions of the near shore bathymetry for Geographe Bay. Inspection of the figures reveals a series of ridges and troughs running sub-parallel to the present-day beach. Deeper troughs are particularly noticeable at the discharge of the Capel River and at Leschenault Estuary. The hillshade rendition (Fig. 15) shows a sub-parallel trough feature located about 4 to 5 km offshore. These bathymetric features identified offshore provide indicators and target areas for more detailed investigation and possibly actual measurement of SGD fluxes, for example by CTD probing around target bathymetric features. 8. Discussions An assessment of the offshore groundwater discharge in Geographe Bay has been carried out using hydrogeological and modeling techniques as well as remote sensing. The groundwater discharge from the Superficial, Leederville and South West Yarragadee aquifers was estimated using hydraulic assessment (Darcy's Law) to be 240–284 m3/d, 270 m3/d and 3360 m3/d/km of the coastline, respectively. Groundwater discharge into the ocean has also been estimated using the SWAMS model which gives 417 m3/d, 309 m3/d and 3000 m3/d/km of the coastline respectively for superficial, Leederville and South West Yarragadee aquifers. The discharge values derived from the model are of a similar order to those obtained using the Darcy's equation. The discrepancies in the results may be due to simplification of a complex geology and

hydrogeology in the model, and the assumptions in the model boundary conditions. Overall the offshore groundwater discharge to the Geographe Bay is estimated to be approximately 80× 106 m3/year on the basis of hydraulic assessment and numerical modeling. Satellite infrared images were analysed to detect the occurrence of any submarine groundwater discharge based on a temperature contrast between discharging groundwater and the ambient seawater. The NOAA and LANDSAT infrared images were unable to detect any submarine groundwater seepage into the Bay possibly due to the poor spatial resolution of the NOAA images and the masking of any cooler groundwater seepage by coastal heating in the daytime LANDSAT image. The LANDSAT thermal-infrared image had not been processed to detect sea surface temperature and hence would be affected by atmospheric and cloud conditions. The ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) night time image for February 2008 showed a cooler temperature water (~21 °C) originating in the southwest of the Geographe Bay near the Dunsborough Fault and extending in a general easterly direction. The cool anomaly is an indication of groundwater discharge which is surrounded by seawater of about 23 °C (i.e. a 2 °C contrast). This suggests that the Dunsborough Fault may be an area of focused groundwater discharge for the Southern Perth Basin aquifers. Similar cooler zones were observed along the coastline indicating possible groundwater discharge. The offshore seabed depressions are also likely to be indicators for the preferential discharge of groundwater. These provide the target areas for more detailed investigation and actual measurement of SGD fluxes, for example by CTD (Conductivity, Temperature and depth) and SSS (sidescan sonar) probing around bathymetric features. There was no indication of groundwater discharge in the area of the South West Yarragadee outcrop on the seabed immediately to the west of Bunbury. One explanation is that there is a low vertical hydraulic gradient in the South West Yarragadee Aquifer in the onshore Bunbury area which does not allow for a detectable flux of groundwater and another reason may be that the flux is of a diffused nature over a larger aquifer area. It is also possible that the groundwater discharge from the South West Yarragadee Aquifer takes place farther offshore from deeper stratigraphic unit or near shore by upward leakage through the overlying Leederville Aquifer.

Fig. 13. Geographe Bay — unscaled shoreline stretch rendition.

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Fig. 14. Geographe Bay — unscaled offshore stretch rendition.

9. Conclusions and recommendations This study has provided valuable information about the rate of groundwater seepage from the Southern Perth Basin aquifers in Geographe Bay from Darcy's Law and helped in verification of the SWAMS model. Satellite images have been analysed to detect

signatures of SGD in the Geographe Bay with night time image from the summer month (February) showing the best potential for detection of offshore SGD. Significant further work is required to identify suitable methodologies for accurate identification of areas of submarine groundwater discharge and its quantification. It is recommended that

Fig. 15. Geographe Bay bathymetry — hillshade stretch rendition.

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follow-up studies of SGD in Geographe Bay include the following methods for detection and quantification of SGD. • Further research on the application of ASTER images to SGD detection. • Electromagnetic (EM) surveys for SGD identification. • Seabed bathymetry mapping using side-scan sonar equipment. • Conductivity, Temperature and Depth (CTD) studies at potential SGD sites including seawater sampling and seabed probing. • Isotope analysis of the near shore groundwater and seawater for SGD quantification, and • Correlation of SGD results with occurrences of seagrass meadows. Acknowledgments The authors acknowledge the hydrogeological advice received from Philip Commander of the WA Department of Water. The authors acknowledge the assistance and advice of Dr Dirk Slawinski (CSIRO Marine and Atmospheric Research) and Geoff Hodgson (CSIRO Land and Water) for their assistance with bathymetric imagery of Geographe Bay. The Western Australian Department of Water made available the groundwater monitoring and bore data required for this study. References Akawwi, E., 2006. Locating zones and quantifying the submarine groundwater discharge into the eastern shores of the Dead Sea, Jordan. Dissertation in fulfilment of the requirements for a doctoral degree at the Faculty of Mathematical-Natural Sciences of the Georg-August University, Goettingen. Allen, A.D., 1976. Outline of the hydrogeology of the superficial formations of the Swan Coastal Plain. Western Australia Geological Survey, Annual Report for 1975, pp. 31–42. Appleyard, S.J., 1991. The geology and hydrogeology of the Cowaramup borehole line, Perth Basin, Western Australia. Western Australia Geological Survey, Professional Papers, Report, 30, pp. 1–12. Baddock, L.J., 1994. Geology and hydrogeology of the Karridale Borehole Line, Perth Basin. Western Australia Geological Survey Professional Papers Report, 37, pp. 1–18. Baddock, L.J., 1995. Geology and hydrogeology of the Scott Coastal Plain, Perth Basin. Western Australia Geological Survey, Record 1995/7. Burnett, W.C., Aggarwal, P.K., Aureli, A., Bokuniewicz, H., Cable, J.E., Charette, M.A., Kontar, E., Krupa, S., Kulkarni, K.M., Loveless, A., Moore, W.S., Oberdorfer, J.A., Oliveira, J., Ozyurt, N., Povinec, P., Privitera, A.M.G., Rajar, R., Ramessur, R.T., Scholten, J., Stieglitz, T., Taniguchi, M., Turner, J.V., 2006. Review — Quantifying Submarine Groundwater Discharge in the Coastal Zone via Multiple Methods. : Science of the Total Environment, 367. Elsevier, pp. 498–543. Capone, D.G., Bautista, M.F., 1985. A groundwater source of nitrate in nearshore marine sediments. Nature 313, 214–216. Commander, D.P., 1984. Bunbury shallow-drilling groundwater investigation. Western Australia Geological Survey, Professional Papers for 1982, Report, 12, pp. 32–52. Crostella, A., Backhouse, J., 2000. Geology and Petroleum Exploration of the Central and Southern Perth Basin, Western Australia. CSIRO, 2009. Groundwater yields in south-west Western Australia. A report to the Australian Government from the CSIRO South-West Western Australia Sustainable Yields Project. CSIRO Water for a Healthy Country Flagship, Australia, 330 pp.

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