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Submarine groundwater discharge into the near-shore zone of the Great Barrier Reef, Australia
Marine Pollution Bulletin 51 (2005) 51–59 www.elsevier.com/locate/marpolbul
Submarine groundwater discharge into the near-shore zone of the Great Barrier Reef, Australia Thomas Stieglitz a
a,b,*
Marine Geophysical Laboratory, School of Mathematical and Physical Sciences, James Cook University, Townsville QLD 4811, Australia b Australian Institute of Marine Science, Townsville QLD 4810, Australia
Abstract Along the tropical coastline of the Great Barrier Reef (GBR) region, little is known to date about submarine groundwater discharge (SGD) into the near-shore ocean. In an oceanographic sense, SGD consists of freshwater flow from land as well as seawater circulated through sediments. Recent radiochemical and geophysical studies, using the tracer 222Rn and apparent ground conductivity respectively, provide evidence for SGD to occur in a variety of hydrogeological settings. In this paper, a non-quantitative overview of different settings of SGD in the region is presented: (1) recirculation of seawater through animal burrows in mangrove forests, (2) freshwater SGD from unconfined aquifers as a narrow coastal fringe of freshwater along Wet Tropics beaches, (3) SGD from coastal dune systems in form of localised freshwater springs in the intertidal zone, (4) inner-shelf SGD from confined submarine aquifer systems comprised of riverine paleochannels incised into the shelf. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Submarine groundwater discharge; Radon; Ground conductivity; Hydrogeology; Great Barrier Reef
1. Introduction Rivers are obvious and visible pathways for terrestrial runoff. Hidden from view, the direct discharge of groundwater into the sea, termed submarine groundwater discharge (SGD) is a poorly understood flow path and often overlooked. SGD as a source of freshwater and dissolved constituents is the least studied element of the water, salt and nutrient budgets of the coastal oceans (Hussain et al., 1999; Rutkowski et al., 1999; Zektser, 2000), particularly along Australian shores. According to Johannes (1980), ‘‘SGD should occur anywhere that an aquifer is hydraulically connected with the sea through permeable rocks or bottom sediments and where the [density-corrected] head is above sea level’’. * Address: Marine Geophysical Laboratory, School of Mathematical and Physical Sciences, James Cook University, Townsville QLD 4811, Australia. Tel.: +61 7 4781 5432. E-mail address: [email protected]
0025-326X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpolbul.2004.10.055
These conditions are met in many, if not most coastal areas (Corbett et al., 1999; Zektser, 2000). Within the coastal plain and seafloor sediments, a complex mixing zone of freshwater and seawater exists. This interface has been termed a ‘‘subterranean estuary’’ (Moore, 1999), in recognition of the similarities of the physical and chemical processes occurring in such interfaces and surface estuaries. In addition to fresh groundwater discharge from terrestrial aquifers, the flow of recirculated seawater through the sediment constitutes SGD in an oceanographic sense with often similar ecological importance (Burnett et al., 2003). SGD is potentially important to the coastal zone because of the likely high concentrations of nutrients in groundwater compared to seawater. Even a small net flux of submarine groundwater can deliver a comparatively large flux of nutrients to the sea. It has been suggested that SGD may be an important natural source of nutrients for coral reefs (Johannes, 1980), seagrass beds (Rutkowski et al., 1999) and other coastal communities
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(Simmons, 1992). In some areas, estimated fluxes of nitrogen, phosphorus and silica from SGD exceed that from local rivers (Simmons, loc cit; Johannes, 1980), sometimes being two orders of magnitude higher in concentration than the receiving coastal waters (Johannes and Hearn, 1985). In some cases, coastal eutrophication and an increase of algal blooms have been directly connected to SGD (Corbett et al., 1999; Hussain et al., 1999). In addition, SGD has been found to support the release of trace gases from the seafloor (Bussmann and Suess, 1998) and to support habitat of benthic microorganisms and fauna (Hovland and Judd, 1988). In the GBR region, very little experimental data exists on SGD. However, some estimations are available from models spanning spatial scales from the catchment of the whole GBR (2000 km coastline) to single river catchments (20 km coastline). Using hypsometric data Zektser et al. (1983) estimated SGD along the East Australian coastline to be 8% of the total freshwater input per annum. The associated dissolved solids discharge was estimated to be 59% of the discharge of dissolved solids from surface (i.e. riverine/estuarine) waters into the sea. Because SGD constitutes a loss term of the Ôresource groundwaterÕ in the coastal plain, it is also a vital component of models developed for groundwater management. For example, one such model estimates SGD from the Lower Burdekin region (50 km coastline) to range from 3000 to 68,000 ML/year (Arunakumaren et al., 2000), depending on hydrogeological approach and assumptions. In order to evaluate the potential significance of SGD, an understanding of the hydrogeological conditions controlling discharge is required. This paper takes a first step by presenting an overview of recent observations made on incidence and origin of SGD in the GBR region. A quantification of fluxes of groundwater to the ocean will require detailed further studies of the processes described herein.
2. Methods Observations were made along 500 km of coastline of the central GBR from Townsville at latitude 19.2°S to Cape Flattery at 15°S (Fig. 1). Radiochemical and geoelectric investigations were carried out as well visual observations of some discharge sites. Generally, aquifers in the coastal plain of the GBR are recharged in the austral summer months from January to April. Average annual rainfall is 1 m or less in dry topical regions, and up to 4 m in the Wet Tropics region (15.5°S to 19°S) respectively. Data presented here was collected in 2002 and 2003. Rainfall in both years was significantly below average in the region, and no significant recharge of aquifers in the coastal plain did occur (Australian Bureau of Meteorology, www.bom.gov.au; Queensland Department of Natural Resources and Mines, pers.
10oS
Elim Beach Australia
40oS 120oE
16oS
150oE
Activity (Bqm -3 ) 0
17oS
20
40
60
80
Cairns
Ella Bay
Dunk Is
18oS Hinchinbrook Channel
Halifax Bay
19oS 50 km
Tow nsville o
146 E
147oE
148oE
Fig. 1. Map of the central Great Barrier Reef region with SGD study sites. 222Rn activity along a 250 km long shore-parallel transect recorded in April 2003 (shipÕs track 1 nm offshore from the coastline shown). Each bar represents a one-hour-average of 222Rn activity continuously measured whilst steaming, thereby averaging over 5–7 nautical miles of coastline. Note that the width of the bars was chosen for clarity, and does not represent the distance over which the 222Rn activity was averaged.
comm.). In the GBR, only during floods river water pushes seawater out of the estuaries (Furnas, 2003). In 2003, no such riverine flood events occurred before or during April, and therefore during the time of radiochemical data collection in April 2003 it must be assumed that the net inflow of waters from riverine sources into the lagoon was negligible (Australian Bureau of Meteorology, www.bom.gov.au). This was confirmed by a lack of visible flood plumes at the time of data collection. 2.1. Radon-tracing 222
Rn is recognised as a very reliable tracer to identify groundwater input into surface water bodies (e.g. Cable et al., 1996; Burnett and Dulaiova, 2003). 222Rn is a conservative noble gas, and alpha-decays to 218Po with a half-life of 3.8 days, which in turn alpha-decays to 214 Pb with a half-life of 3 min. The 222Rn activity in groundwater is typically 2 to 3 orders of magnitude
T. Stieglitz / Marine Pollution Bulletin 51 (2005) 51–59
greater than in surface waters like seawater, providing a large, measurable activity contrast. In order to establish the incidence of SGD on a scale of 100Õs of km an automated system to sample 222Rn in situ, using a commercial Radon-in-air monitor has been modified from an existing device (Burnett et al., 2001) for sampling from a moving vessel, and thereby recording the spatial distribution of 222Rn activity. Whilst steaming with a vessel, ocean surface water is sampled by pumping water with a non-cavitating positive displacement pump through a heavily weighted pipe that is pulled along just under the water surface. With this setup, loss of the noble gas Radon into air bubbles in the system is effectively eliminated. The water is pushed through a jet in an air-water exchanger in which Radon de-gases from water to a closed air loop. After 20 to 25 min, 222Rn equilibrium between gas and water phase is established (Lane-Smith et al., 2002). The air stream is fed via a desiccant to a continuously recording 222Rn counting system. In this ÔtowingÕ setup, two continuous Radonin-air counters of the make Durridge Rad-7 are used in parallel in order to provide adequate counting efficiency. In the RAD-7, an electric field above a silicon semiconductor detector at ground potential attracts the positively charged daughter of Radon 218Po+ which is counted as a measure of the 222Rn activity in air. The 222 Rn activity in water is then calculated from the measured in-air activity with a known temperature correction (Weigel, 1978). A similar experimental setup has previously been used for stationary sampling (i.e. not moving with the vessel), and is described in detail by Burnett et al. (2001). The Rad-7Õs are factory-calibrated. 222 Rn activities are given in the SI unit Bqm 3 (decays per second per m3), and one-sigma uncertainties are reported for 222Rn measurements throughout this paper. The shipÕs track from Townsville to Cape Tribulation shown in Fig. 1 is a composite of two voyages: Dunk Island to Townsville (southward), and Dunk Island to Cape Tribulation (northward), both in early April 2003. 222Rn was continuously sampled along the track at a distance of one nautical mile from the coast (Fig. 1), and through the centre of Hinchinbrook Channel which is 0.8 to 2.5 nautical miles wide. Data was continuously recorded with both counters in 1/4 h intervals, and subsequently averaged over one hour to obtain adequate counting statistics. At a vessel speed of between 5 and 7 knots, a spatial resolution of Radon activity of between 5 and 7 nautical miles of coastline was achieved. In addition, point samples of 222Rn activity of SGD at Ella Bay were recorded with the same instrumentation, however in a stationary deployment. The data is not corrected for potential supply by the parent nuclide 226Ra in the water column, which is expected to be small, typically less than 1 Bqm 3 (Hancock, CSIRO Land and Water Canberra, unpublished data; Brunskill, Australian Institute of Marine Science, unpublished data).
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2.2. Geoelectric transects Transects of apparent ground conductivity of sediments were recorded at Ella Bay in May 2003, at Elim Beach in April 2002, and near Wonky Holes located in Halifax Bay in January 2000 (Fig. 2). Freshwater wetlands are located immediately behind the comparatively steep beach at Ella Bay. At Elim Beach, an extensive coastal sand dune system is located behind a shallowsloping beach and intertidal zone. Apparent ground conductivity was recorded with a purpose-built conductivity sensor consisting of a Wenner-array of four ring-electrodes, mounted on a fibreglass rod with an equal spacing of ca 1 cm. The two inner electrodes serve as current electrodes to generate an electrical field in the soil, and the two outer electrodes are voltage electrodes. The electrical field is generated using a simple AC-generator. The induced voltage is amplified, and both current and voltage are measured with conventional multimeters. The ratio of current to induced voltage is proportional to the bulk ground conductivity in a sphere around the electrodes with a diameter of 4 cm. The design of a similar sensor is described in more detail by Stieglitz et al. (2000a). Relative errors are less than 5% (Stieglitz et al., 2000a). The fibreglass rod with the electrodes is glued into a heavy steel rod, which can be pushed into the sediment to depths up to 1.5 m. A transect recorded with this probe consisted of 8–10 profiles. At the beach sites, each profile was taken by inserting the probe into the ground, taking a reading at a particular depth, and then gradually pushing the probe further into the ground. In the deeper-water seafloor sediments, the probe was dropped into the seafloor, measuring the conductivity at a depth of 1.5 m below the sediment surface. Depth was measured with an echosounder with a 9° beam angle. Apparent ground conductivity data was contoured applying standard methods. The ÔapparentÕ ground conductivity measured with this probe is a function of (a) porewater fraction and (b) porewater conductivity, which in turn is a function of porewater salinity and temperature. Therefore, it is not possible to derive porewater salinity from this measurement alone, however some limited information on porewater salinity can be obtained from the following considerations. Commonly, marine sediments saturated with water with a conductivity C have an apparent conductivity reduced by a factor of 2–3 compared to C, resulting from the presence of non-conducting particles (Kermabon et al., 1969). Hence at a typical conductivity of 55 mScm 1 of seawater, sediments with an apparent bulk ground conductivity of greater than 18 mScm 1 can be assumed to be saturated with seawater. Sediments of the same type with a bulk ground conductivity of significantly less than 18 mScm 1 can be assumed to
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T. Stieglitz / Marine Pollution Bulletin 51 (2005) 51–59
Fig. 2. Apparent ground conductivity transects (a) at Ella Bay in May 2003, (b) at Elim Beach in April 2002 and (c) through a Wonky Hole in Halifax Bay in January 2000, together with photographs of discharge sites at (a) and (b) and acoustic multibeam image of the investigated Wonky Hole at (c) respectively. Note the different scales on the axes. The same conductivity colour scheme applies. The cross in (a) marks the highest location of visible seeps, and the grid in the multibeam image in (c) has 10 m spacings in both directions.
be saturated with brackish water, or––at values close to zero––with fresh water.
of the beach in Ella Bay were of salinity 27.1 psu and had a 222Rn activity of 161 ± 6 Bqm 3. 3.2. Geoelectric transects and visual observations
3. Results 3.1. Radionuclide tracing A shore-parallel transect of the 222Rn activity recorded one nautical mile offshore shows localised elevations of 222Rn activity along parts of the Wet Tropics coastline, around 17.3°S and 18.5°S latitude respectively (Fig. 1). It is important to note that the rainfall in the year of recording was significantly below average for the region (Australian Bureau of Meteorology, www. bom.gov.au). Nevertheless, two statistically significant peaks can be observed, with a maximum of 33 ± 3 Bqm 3 and 71 ± 4 Bqm 3 against a background of less than 20 Bqm 3. Point samples of seawater 10 m offshore
At Ella Bay, a sharp interface persists between low and high bulk sediment conductivity at the sediment surface, whereas at Elim Beach bulk ground conductivity at the surface (as well as at depth) is non-uniform and shows no distinct, single boundary. In 20 m of water depth, apparent conductivity of the seafloor in a ÔWonky HoleÕ is significantly reduced at the bottom of the depression compared to the surrounding seafloor. The later finding is discussed in more detail in Stieglitz and Ridd (2000). Visually, the physical discharge sites also differ significantly: at Ella Bay, discharge occurs in the form of seeps along the beach face (Fig. 2a), immediately behind which freshwater wetlands are located. Discharge at
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Elim Beach occurs from discrete, circular springs in the flat intertidal zone (Fig. 2b). Likewise, Wonky Holes are discrete, pockmark-like depressions, the morphology of which is described in detail in Stieglitz and Ridd (2000) and Stieglitz and Ridd (in press).
4. Discussion 4.1. Utility of a shore-parallel SGD
222
Rn transect to detect
The purpose of recording the 222Rn transect was to identify the broad distribution of SGD into the ocean. Significantly elevated 222Rn activities along parts of the Wet Tropics coastline, around 17.3°S and 18.5°S latitude, were detected in the late wet season in 2003 (Fig. 1). 222Rn is commonly sampled relatively close to the source, i.e. at distances less than 100 m. Therefore, a priori it was not clear if 222Rn would be detectable with the methods applied here as far offshore as one nautical mile. The measured variable activity of 222Rn however indicates that the sampling strategy is successful in identifying locations or zones of 222Rn input into the ocean. At the given averaging time and vessel speed, a spatial resolution of between 5 and 7 nautical miles was achieved, which is considered adequate for investigations on the scale of hundreds of kilometres. To obtain a more detailed spatial distribution of SGD, the vessel speed could be reduced at the expense of spatial coverage. To trace back to the source, some systematic effects need to be taken into account, i.e. (1) the local oceanography, (2) a systematic shift of observed 222Rn in steaming direction due to a non-negligible response time of 15 min of the detection system. Firstly, in the GBR lagoon coastal waters are deflected northwards by the influence of the Southeast trade winds for most of the year (Burrage et al., 2002). Therefore, an observed offshore 222Rn elevation is expected to originate southwards of the signal. The elevated 222Rn activities at latitude 17.3°S occur just north of Ella Bay, a location of known SGD (see below). Anecdotal evidence from locals and personal observation by the author suggest that also Cowley Beach to the South of Ella Bay and Bramston Beach to its North discharge groundwater. The spatial resolution of the data does not allow for discrimination between these sources, however visual observations at all three sites suggest that at the time of data collection Ella Bay was the only place where significant discharge occurred at the time. The elevated 222Rn at 18.5° latitude occurred in Hinchinbrook Channel between the mainland and the 30 km long Hinchinbrook Island. Mixing of channel water with ocean water occurs at a much slower rate than at an open coast. Hence the residual 222Rn activity of coastal waters is greater than would be expected at an
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open coastline at similar input rate. From the presented data, it is not possible to derive the relative magnitude of the 222Rn input at 17.3° and 18.5° latitude respectively without quantification of lateral mixing. Secondly, a systematic shift of data in steaming direction must be taken into account due to the intrinsic response time of the counting system of 15 min. The 222 Rn activity is determined by counting the decay of the daughter 218Po to 214Pb, which is in equilibrium with the 222Rn to 218Po decay after 5 half-lives, i.e. 5 by 3 min. Therefore a change in 222Rn activity will be detected 15 min later than it occurred. The elevated 222 Rn activity at latitude 17.3°S was recorded when travelling northwards, and hence this effect works in the same geographic direction as the oceanographic northward deflection of coastal waters. Other factors need to be taken into account in order to derive SGD fluxes from 222Rn activities (e.g. Burnett and Dulaiova, 2003). For instance, the loss of the noble gas Radon to the atmosphere needs to be known. It depends on factors such as temperature and wind speed, both of which did not significantly vary whilst recording the transect. The evasion of 222Rn to the atmosphere is here assumed constant, and systematic changes in 222Rn are therefore considered meaningful. The two regions of elevated 222Rn are discussed separately below.
5. Conceptual hydrogeological models of SGD in the GBR 5.1. Seawater recirculation through crustacean burrows––Hinchinbrook Channel Hinchinbrook Channel is a renowned important habitat for extensive mangrove forests (e.g. Bunt and Bunt, 1999). A large portion of these forests consists of the Red Mangrove Rhizophora stylosa, which is one of the most common mangrove species in the region. Crustacean burrows in the forest floor between the root system of these trees have been shown to be flushed rapidly and efficiently by tidal circulation (Stieglitz et al., 2000b). Per tidal cycle, 100–150 l m 2 were found to be exchanged across the forest floor, or, equivalently, 10–40% of tidal water was flowing through burrows at an inundation level of 1 m and 0.25 m respectively (Stieglitz et al., 2000b). This flux of ÔgroundwaterÕ across the sedimentwater interface is likely accompanied by a flux of 222 Rn, which the water acquires when in contact with sediments (Corbett et al., 1999). It is reasonable to conclude that at least some of the observed elevation of 222 Rn in Hinchinbrook Channel can be attributed to this recirculation of seawater (Fig. 3a). The Herbert River, which debouches into Hinchinbrook Channel was not discharging freshwater into the channel at the time, and is unlikely to have contributed to the observed 222 Rn distribution. In the catchment of the GBR,
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Fig. 3. Conceptual hydrogeological models for SGD in the GBR region; (a) Seawater recirculation through crustacean burrows, (b) Discharge from unconfined coastal aquifers, (c) Discharge from confined or semi-confined coastal aquifers in form of discrete springs, (d) Discharge from confined submarine aquifers through ÔWonky Holes. The spatial scale of each groundwater system is indicated (cf. Table 1).
8000 km2 or 38% of the total area of wetlands (including floodplains and riparian vegetation) are mangrove forests (Furnas, 2003). This type of groundwater flux, accompanied by a flux of 222Rn as well as ecologically relevant solutes, should persist along all these mangrove coastlines, and is itself a process worthy of more detailed investigation. 5.2. Discharge from unconfined coastal aquifers––Ella Bay The coastal transect shows elevated 222Rn at latitude 17.3°S (Fig. 1). On the beach face of Ella Bay just to the south, SGD was prominently visible in form of seeps (Fig. 2a). Observations of inshore 222Rn activity, salinity and beach sediment conductivity suggest a source of 222 Rn and SGD in the intertidal zone at Ella Bay. The seeps discharged undiluted freshwater with salinity 0 psu, and seawater just offshore from the beach was of salinity 27.1 psu and had a 222Rn activity of 161 ± 6 Bqm 3, i.e. significantly higher than 33 ± 3 Bqm 3 measured along the transect one nautical mile offshore, and also significantly higher than activities found of outershelf seawater of typically less than 5 Bqm 3 (Stieglitz, unpublished data). At low tide, a shore-normal ground conductivity transect shows a well-defined interface between low and high ground conductivity at 14 m from the high tide mark, with discharge of fresh groundwater occurring between high and low tide mark from 5 m to 14 m respectively along the transect (Fig. 2a). Based on these observations, it is reasonable to infer that SGD into Ella Bay is a major contributing source of the observed elevation of 222Rn at latitude 17.3°S. Discharge in the intertidal zone from coastal wetlands is the most commonly observed hydrogeological setting of SGD
(Fig. 3b). Such a connection of unconfined coastal aquifers and the ocean is expected to occur along most of the GBR coastline, and to be accompanied by SGD flux where an aquifer water level above sealevel are present (Johannes, 1980). How much and for how long discharge occurs depends on the hydraulic head in the aquifer systems. Hydrogeologically similar SGD has been observed e.g. in Southwest Australia (Johannes and Hearn, 1985), Florida (Corbett et al., 1999) and Long Island, New York (Bokuniewicz, 1980). 5.3. Discharge from large dune systems––‘Buiur Bindi’ at Elim Beach For thousands of years, indigenous people inhabiting the coastlines of Eastern Cape York Peninsula have sourced freshwater from springs in the intertidal zone. In the local language of the Guggu Yimithirr people, ÔbuiurÕ means freshwater and Ôbuiur bindiÕ refers to a spring in the ocean. In some areas where these springs are found, such as Elim Beach, surface discharge through creeks and rivers are largely absent. In these areas, the discharge of groundwater can be expected to provide a major pathway for land-derived water and solutes, and may play an important role in the near-shore ecology of the adjacent coastal embayments. For instance, Melaleuca spp. trees can be found growing in the intertidal zone at Elim Beach, indicating the presence of freshwater all year round: such trees can not tolerate full seawater salt concentrations for extended periods of time. Freshwater wetlands perched in extensive coastal dune systems inland of the beaches have water levels above sea level all year round. These wetlands are a likely source of the fresh groundwater discharged along the coastline (Fig. 3c).
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The apparent ground conductivity in Elim Beach sediments is significantly more variable than at Ella Bay (Fig. 2). Here, low conductivity water is found at the surface up to 10 m from the high tide mark, and in discrete locations at 3 m and 70–80 m respectively. Digging revealed the shallow beach stratigraphy: the top 10–20 cm of sediment consist of relatively porous sand, below which a layer of dense clay with mangrove roots embedded was found, from 20 cm to 50 cm depth below the modern sediment surface. Below 50 cm of depth, porous sand was found again. It is likely that this clay layer provides at least partial confinement of the shallow aquifer, resulting in the observed non-uniform pattern of ground conductivity and the discrete nature of the spring sites (Fig. 3c). There is anecdotal evidence, that SGD occurs in this form also in the Cape Flattery and Shelbourne Bay regions, where similar extensive dune systems with perched wetlands are situated. Some of theses spring sites are marked on Admiralty charts. 5.4. Discharge from confined submarine aquifers–– ‘Wonky Holes’ A further––geologically curious––process that delivers SGD into the ocean is the discharge through ÔWonky HolesÕ from confined submarine aquifers comprised of riverine paleochannels (Stieglitz and Ridd, 2000). Paleochannels were incised into the shelf during low sealevel stand, in-filled with coarse sediments, and subsequently capped by relatively fine, impermeable terrestrially-derived sediments during Holocene sealevel rise (Johnson and Searle, 1984). The paleochannels appear to be continuously connected to the shore landwards from Wonky Holes, and are likely to provide a pathway for groundwater derived from the coastal plain to be discharged onto the inner/mid shelf of the GBR (Fig. 3c; Stieglitz and Ridd, 2000). A conductivity transect across a Wonky Holes shows significantly reduced conductivity in the depression (Fig. 2c), and a hydrographic transect just above a Wonky Hole showed reduced bottom water salinity over a Wonky Hole compared to the surrounding seafloor (Stieglitz and Ridd, 2000). Wonky Holes were first described in Halifax Bay, and are therefore marked in Fig. 1 in Halifax Bay. On-going research indicates that Wonky Holes are present offshore from most rivers in the central and northern GBR lagoon (Stieglitz, unpublished data).
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5.5. Potential ecological significance of SGD in the region Australia is the driest continent on earth, and the discharge of groundwater––as well as surface water––into its coastal ocean might be expected to be small compared to other continents. However, on small temporal and spatial scales, the processes documented here may be ecologically significant. The coastal hydrology in the GBR region differs from many other regions in the world in two important aspects. On one hand, the coastal plain is comparatively narrow (<70 km), and hence coastal aquifer systems that can potentially drain into the ocean are relatively small. On the other hand, the region experiences a pronounced wet/dry season cycle with the bulk of the average annual rainfall of 1–4 m falling in the austral summer from January to April. The combination of the two effects suggests that SGD is reasonably localised and highly seasonal in the region (as is surface discharge). It is likely though that due to the storage effect in coastal aquifers––although small––SGD persists longer into the dry season than surface discharge does, possibly even throughout the year in some cases, as indicated in Table 1. At Elim Beach for instance, such an extended input of freshwater into the ocean may have a significant impact on local coastal ecosystems. Hydraulic connections from the land to the ocean (and vice versa) span spatial scales from a few meters to kilometres (Table 1), extending underneath the modern coastline and coastal plain. Within the catchment of the GBR region, strong relationships exist between intensive agricultural land use and the quality of shallow groundwater (Anonymous, Department of Natural Resources). SGD is a source of terrestrially derived groundwater, which may be affected by agricultural land use. Recently introduced water quality guidelines for the GBR region imposed by the Great Barrier Reef Marine Park Authority, a federal protection agency, target surface runoff only, and do not account for other potential pathways such as SGD for nutrients, trace elements and potentially contaminants to enter the near-shore zone. Sites and processes described here are meant to serve as examples only; there is significant anecdotal evidence that these processes occur elsewhere along the GBR coastline. Little experimental knowledge exists on the connectivity of coastal aquifers to the ocean in the region. From the presented qualitative evidence for SGD, no reliable
Table 1 Likely temporal and spatial scales of four different hydrogeological settings of SGD in the GBR Hydrogeological model
Spatial scale
Timing of discharge
Seawater recirculation through animal burrows (3a) Discharge from unconfined coastal aquifers connected to freshwater wetlands (3 b) Discharge from (semi)confined coastal aquifers in extensive coastal dune systems (3c) Discharge from confined submarine aquifers through ÔWonky HolesÕ (3d)
1–10 m 100–1000 m 1000–10,000 m 10,000 m
All year round Wet and early dry season All year round? Wet season
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estimate of the magnitude of the SGD flux can be made. Further steps to identify the ecological significance of SGD, in particular as pathways for ecologically significant solutes, will require the quantification of the discharge from the various geological settings. This is is the subject of on-going research. 6. Conclusion 222
Rn input into the coastal ocean has been located along the central GBR coastline, and likely sources of elevated 222Rn activities have been identified. The results suggest that continuous sampling of 222Rn along a coastal transect is a useful, non-quantitative tool to reveal input zones of groundwater along a coastline on large scales. From geophysical and radiochemical investigations, SGD is suggested to occur in a variety of hydrogeological settings in the GBR region, originating from both unconfined and confined coastal aquifer systems and operating on different spatial and likely also temporal scales. Four hydrogeological settings of SGD in the region were identified: (1) recirculation of seawater through animal burrows in mangrove forests, (2) fresh SGD from unconfined aquifers as a narrow coastal fringe of freshwater along Wet Tropics beaches, (3) SGD from extensive coastal dune systems in form of localised freshwater springs in the intertidal zone, (4) inner-shelf SGD from confined submarine aquifer systems comprised of riverine paleochannels incised into the shelf. Settings (1) and (2) are suggested to occur on large scales in the GBR region, given the spatial extent of mangrove forests and coastal aquifer systems. Acknowledgments P Ridd (JCU), G Brunskill (AIMS), G Hancock, PG Cook (CSIRO Land & Water) and P Doherty (AIMS) provided inspirational and hands-on support of this work. Thanks for technical support are due to S Thomas, S Choukroun, M OÕLeary, K Hooper, J Cavanagh, D Lane-Smith and masters & crew of the RV Lady Basten. B Burnett, E Kontar, B Moore, S Krupa and other members of the SCOR/LOICZ/IOC working group 112 have provided advice on SGD issues, in particular on methods. The permission to work at Elim Beach by the traditional owners is greatly appreciated. This research is jointly funded by the Australian Research Council and the Australian Institute of Marine Science. References Anon., 1999. Nutrient balances and transport from agricultural and rainforest lands: a case study in the Johnstone River Catchment. Project No DAQ3S Final Report, Queensland Department of Natural Resources, Brisbane, 85 pp.
Arunakumaren, N., McMahon, G., Bajracharya, K., 2000. Water management in the Lower Burdekin. Queensland Department of Natural Resources, Brisbane,120 pp. Bokuniewicz, H., 1980. Groundwater seepage into great south bay, New York. Estuarine, Coastal and Shelf Science 10, 437–444. Bunt, J.S., Bunt, E.D., 1999. Complexity and variety of zonal pattern in the mangroves of the hinchinbrook area, northeastern Australia. Mangroves and Salt Marshes 3, 165–176. Burnett, W.C., Bokuniewicz, H., Huettel, M., Moore, W.S., Taniguchi, M., 2003. Groundwater and pore water inputs to the coastal zone. Biogeochemistry 66, 3–33. Burnett, W.C., Dulaiova, H., 2003. Estimating the dynamics of groundwater input into the coastal zone via continuous radon-222 measurements. Journal of Environmental Radioactivity 69, 21–35. Burnett, W.C., Kim, G., Lane-Smith, D.R., 2001. A continuous radon monitor for assessment of radon in coastal ocean waters. Journal of Radioanalytical and Nuclear Chemistry 249, 167–172. Burrage, D.M., Heron, M.L., Hacker, J.M., Stieglitz, T.C., Steinberg, C.R., Prytz, A., 2002. Evolution and dynamics of tropical river plumes in the Great Barrier Reef: An integrated remote sensing and in situ study. Journal of Geophysical Research Special Issue on Salinity 10.1029/2001JC001024. Bussmann, I., Suess, E., 1998. Groundwater seepage in eckernfoerde bay (western baltic sea): effect on methane and salinity distribution of the water column. Continental Shelf Research 18, 1795–1806. Cable, J.E., Bugna, G.C., Burnett, W.C., Chanton, J.P., 1996. Application of Rn-222 and CH4 for assessment of groundwater discharge to the coastal ocean. Limnology and Oceanography 41, 1347–1353. Corbett, D.R., Chanton, J., Burnett, W., Dillon, K., Rutkowski, C., Fourqurean, J.W., 1999. Patterns of groundwater discharge into Florida Bay. Limnology and Oceanography 44, 1045–1055. Furnas, M., 2003. Catchment and Corals. Australian Institute of Marine Science, Townsville, 334 pp. Hovland, M., Judd, A.G., 1988. Seabed pockmarks and seepages. Graham & Trotman, London, 293 pp. Hussain, N., Church, T.M., Kim, G., 1999. Use of 222Rn and 226Ra to trace groundwater discharge into the Chesapeake Bay. Marine Chemistry 65, 127–134. Johannes, R.E., 1980. The ecological significance of submarine discharge of groundwater. Marine Ecology Progress Series 3, 365–373. Johannes, R.E., Hearn, C.J., 1985. The effect of submarine groundwater discharge on nutrient and salinity regimes in a coastal lagoon off perth, western Australia. Estuarine, Coastal and Shelf Science 21, 789–800. Johnson, D., Searle, D., 1984. Post-glacial seismic stratigraphy, central GBR, Australia. Sedimentology 31, 335–352. Kermabon, A., Gehin, C., Blavier, P., 1969. A deep-sea electrical resistivity probe for measuring porosity and density of unconsolidated sediments. Geophysics 34, 554–571. Lane-Smith, D.R., Burnett, W.C., Dulaiova, H., 2002. Continuous radon-222 measurements in the coastal zone. Sea Technology 43, 37–45. Moore, W.S., 1999. The subterranean estuary: a reaction zone of ground water and sea water. Marine Chemistry 65, 111–125. Rutkowski Jr., C.M., Burnett, W.C., Iverson, R.L., Chanton, J.P., 1999. The effect of groundwater seepage on nutrient delivery and seagrass distribution in the northeastern Gulf of Mexico. Estuaries 22, 1033–1040. Simmons, G.M., 1992. Importance of submarine groundwater discharge (SGWD) and seawater cycling to material flux across sediment/water interfaces in marine environments. Marine Ecology Progress Series 84, 173–184. Stieglitz, T.C., Ridd, P.V., 2000. Submarine Groundwater Discharge from Paleochannels? Wonky Holes on the Inner Shelf of the Great Barrier Reef, Australia. In: Proceedings HYDRO 2000 (26th
T. Stieglitz / Marine Pollution Bulletin 51 (2005) 51–59 National and 3rd International Hydrology and Water Resources Symposium of The Institution of Engineers, Australia), Perth, pp. 1265–1271. Stieglitz, T.C., Ridd, P.V., Hollins, S., 2000a. A small sensor for detecting animal burrows and monitoring burrow water conductivity. Wetlands Ecology and Management 8, 1–7. Stieglitz, T.C., Ridd, P.V., Mu¨ller, P., 2000b. Passive irrigation and functional morphology of crustacean burrows in a tropical mangrove swamp. Hydrobiologia 421, 69–76.
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Weigel, F., 1978. Radon. Chemiker Zeitung 102, 287. Zektser, I.S., 2000. Groundwater and the Environment. Lewis, Boca Raton, 150 pp. Zektser, I.S., Dzhamalov, R.G., Safronova, T.I., 1983. The role of submarine groundwater discharge in the water balance of Australia. In: Ground water in water resources planning. Proceedings Koblenz Symposium, 1983, vol. 1, Koblenz, pp. 209–219.
Submarine groundwater discharge into the near-shore zone of the Great Barrier Reef, Australia Thomas Stieglitz a
a,b,*
Marine Geophysical Laboratory, School of Mathematical and Physical Sciences, James Cook University, Townsville QLD 4811, Australia b Australian Institute of Marine Science, Townsville QLD 4810, Australia
Abstract Along the tropical coastline of the Great Barrier Reef (GBR) region, little is known to date about submarine groundwater discharge (SGD) into the near-shore ocean. In an oceanographic sense, SGD consists of freshwater flow from land as well as seawater circulated through sediments. Recent radiochemical and geophysical studies, using the tracer 222Rn and apparent ground conductivity respectively, provide evidence for SGD to occur in a variety of hydrogeological settings. In this paper, a non-quantitative overview of different settings of SGD in the region is presented: (1) recirculation of seawater through animal burrows in mangrove forests, (2) freshwater SGD from unconfined aquifers as a narrow coastal fringe of freshwater along Wet Tropics beaches, (3) SGD from coastal dune systems in form of localised freshwater springs in the intertidal zone, (4) inner-shelf SGD from confined submarine aquifer systems comprised of riverine paleochannels incised into the shelf. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Submarine groundwater discharge; Radon; Ground conductivity; Hydrogeology; Great Barrier Reef
1. Introduction Rivers are obvious and visible pathways for terrestrial runoff. Hidden from view, the direct discharge of groundwater into the sea, termed submarine groundwater discharge (SGD) is a poorly understood flow path and often overlooked. SGD as a source of freshwater and dissolved constituents is the least studied element of the water, salt and nutrient budgets of the coastal oceans (Hussain et al., 1999; Rutkowski et al., 1999; Zektser, 2000), particularly along Australian shores. According to Johannes (1980), ‘‘SGD should occur anywhere that an aquifer is hydraulically connected with the sea through permeable rocks or bottom sediments and where the [density-corrected] head is above sea level’’. * Address: Marine Geophysical Laboratory, School of Mathematical and Physical Sciences, James Cook University, Townsville QLD 4811, Australia. Tel.: +61 7 4781 5432. E-mail address: [email protected]
0025-326X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpolbul.2004.10.055
These conditions are met in many, if not most coastal areas (Corbett et al., 1999; Zektser, 2000). Within the coastal plain and seafloor sediments, a complex mixing zone of freshwater and seawater exists. This interface has been termed a ‘‘subterranean estuary’’ (Moore, 1999), in recognition of the similarities of the physical and chemical processes occurring in such interfaces and surface estuaries. In addition to fresh groundwater discharge from terrestrial aquifers, the flow of recirculated seawater through the sediment constitutes SGD in an oceanographic sense with often similar ecological importance (Burnett et al., 2003). SGD is potentially important to the coastal zone because of the likely high concentrations of nutrients in groundwater compared to seawater. Even a small net flux of submarine groundwater can deliver a comparatively large flux of nutrients to the sea. It has been suggested that SGD may be an important natural source of nutrients for coral reefs (Johannes, 1980), seagrass beds (Rutkowski et al., 1999) and other coastal communities
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T. Stieglitz / Marine Pollution Bulletin 51 (2005) 51–59
(Simmons, 1992). In some areas, estimated fluxes of nitrogen, phosphorus and silica from SGD exceed that from local rivers (Simmons, loc cit; Johannes, 1980), sometimes being two orders of magnitude higher in concentration than the receiving coastal waters (Johannes and Hearn, 1985). In some cases, coastal eutrophication and an increase of algal blooms have been directly connected to SGD (Corbett et al., 1999; Hussain et al., 1999). In addition, SGD has been found to support the release of trace gases from the seafloor (Bussmann and Suess, 1998) and to support habitat of benthic microorganisms and fauna (Hovland and Judd, 1988). In the GBR region, very little experimental data exists on SGD. However, some estimations are available from models spanning spatial scales from the catchment of the whole GBR (2000 km coastline) to single river catchments (20 km coastline). Using hypsometric data Zektser et al. (1983) estimated SGD along the East Australian coastline to be 8% of the total freshwater input per annum. The associated dissolved solids discharge was estimated to be 59% of the discharge of dissolved solids from surface (i.e. riverine/estuarine) waters into the sea. Because SGD constitutes a loss term of the Ôresource groundwaterÕ in the coastal plain, it is also a vital component of models developed for groundwater management. For example, one such model estimates SGD from the Lower Burdekin region (50 km coastline) to range from 3000 to 68,000 ML/year (Arunakumaren et al., 2000), depending on hydrogeological approach and assumptions. In order to evaluate the potential significance of SGD, an understanding of the hydrogeological conditions controlling discharge is required. This paper takes a first step by presenting an overview of recent observations made on incidence and origin of SGD in the GBR region. A quantification of fluxes of groundwater to the ocean will require detailed further studies of the processes described herein.
2. Methods Observations were made along 500 km of coastline of the central GBR from Townsville at latitude 19.2°S to Cape Flattery at 15°S (Fig. 1). Radiochemical and geoelectric investigations were carried out as well visual observations of some discharge sites. Generally, aquifers in the coastal plain of the GBR are recharged in the austral summer months from January to April. Average annual rainfall is 1 m or less in dry topical regions, and up to 4 m in the Wet Tropics region (15.5°S to 19°S) respectively. Data presented here was collected in 2002 and 2003. Rainfall in both years was significantly below average in the region, and no significant recharge of aquifers in the coastal plain did occur (Australian Bureau of Meteorology, www.bom.gov.au; Queensland Department of Natural Resources and Mines, pers.
10oS
Elim Beach Australia
40oS 120oE
16oS
150oE
Activity (Bqm -3 ) 0
17oS
20
40
60
80
Cairns
Ella Bay
Dunk Is
18oS Hinchinbrook Channel
Halifax Bay
19oS 50 km
Tow nsville o
146 E
147oE
148oE
Fig. 1. Map of the central Great Barrier Reef region with SGD study sites. 222Rn activity along a 250 km long shore-parallel transect recorded in April 2003 (shipÕs track 1 nm offshore from the coastline shown). Each bar represents a one-hour-average of 222Rn activity continuously measured whilst steaming, thereby averaging over 5–7 nautical miles of coastline. Note that the width of the bars was chosen for clarity, and does not represent the distance over which the 222Rn activity was averaged.
comm.). In the GBR, only during floods river water pushes seawater out of the estuaries (Furnas, 2003). In 2003, no such riverine flood events occurred before or during April, and therefore during the time of radiochemical data collection in April 2003 it must be assumed that the net inflow of waters from riverine sources into the lagoon was negligible (Australian Bureau of Meteorology, www.bom.gov.au). This was confirmed by a lack of visible flood plumes at the time of data collection. 2.1. Radon-tracing 222
Rn is recognised as a very reliable tracer to identify groundwater input into surface water bodies (e.g. Cable et al., 1996; Burnett and Dulaiova, 2003). 222Rn is a conservative noble gas, and alpha-decays to 218Po with a half-life of 3.8 days, which in turn alpha-decays to 214 Pb with a half-life of 3 min. The 222Rn activity in groundwater is typically 2 to 3 orders of magnitude
T. Stieglitz / Marine Pollution Bulletin 51 (2005) 51–59
greater than in surface waters like seawater, providing a large, measurable activity contrast. In order to establish the incidence of SGD on a scale of 100Õs of km an automated system to sample 222Rn in situ, using a commercial Radon-in-air monitor has been modified from an existing device (Burnett et al., 2001) for sampling from a moving vessel, and thereby recording the spatial distribution of 222Rn activity. Whilst steaming with a vessel, ocean surface water is sampled by pumping water with a non-cavitating positive displacement pump through a heavily weighted pipe that is pulled along just under the water surface. With this setup, loss of the noble gas Radon into air bubbles in the system is effectively eliminated. The water is pushed through a jet in an air-water exchanger in which Radon de-gases from water to a closed air loop. After 20 to 25 min, 222Rn equilibrium between gas and water phase is established (Lane-Smith et al., 2002). The air stream is fed via a desiccant to a continuously recording 222Rn counting system. In this ÔtowingÕ setup, two continuous Radonin-air counters of the make Durridge Rad-7 are used in parallel in order to provide adequate counting efficiency. In the RAD-7, an electric field above a silicon semiconductor detector at ground potential attracts the positively charged daughter of Radon 218Po+ which is counted as a measure of the 222Rn activity in air. The 222 Rn activity in water is then calculated from the measured in-air activity with a known temperature correction (Weigel, 1978). A similar experimental setup has previously been used for stationary sampling (i.e. not moving with the vessel), and is described in detail by Burnett et al. (2001). The Rad-7Õs are factory-calibrated. 222 Rn activities are given in the SI unit Bqm 3 (decays per second per m3), and one-sigma uncertainties are reported for 222Rn measurements throughout this paper. The shipÕs track from Townsville to Cape Tribulation shown in Fig. 1 is a composite of two voyages: Dunk Island to Townsville (southward), and Dunk Island to Cape Tribulation (northward), both in early April 2003. 222Rn was continuously sampled along the track at a distance of one nautical mile from the coast (Fig. 1), and through the centre of Hinchinbrook Channel which is 0.8 to 2.5 nautical miles wide. Data was continuously recorded with both counters in 1/4 h intervals, and subsequently averaged over one hour to obtain adequate counting statistics. At a vessel speed of between 5 and 7 knots, a spatial resolution of Radon activity of between 5 and 7 nautical miles of coastline was achieved. In addition, point samples of 222Rn activity of SGD at Ella Bay were recorded with the same instrumentation, however in a stationary deployment. The data is not corrected for potential supply by the parent nuclide 226Ra in the water column, which is expected to be small, typically less than 1 Bqm 3 (Hancock, CSIRO Land and Water Canberra, unpublished data; Brunskill, Australian Institute of Marine Science, unpublished data).
53
2.2. Geoelectric transects Transects of apparent ground conductivity of sediments were recorded at Ella Bay in May 2003, at Elim Beach in April 2002, and near Wonky Holes located in Halifax Bay in January 2000 (Fig. 2). Freshwater wetlands are located immediately behind the comparatively steep beach at Ella Bay. At Elim Beach, an extensive coastal sand dune system is located behind a shallowsloping beach and intertidal zone. Apparent ground conductivity was recorded with a purpose-built conductivity sensor consisting of a Wenner-array of four ring-electrodes, mounted on a fibreglass rod with an equal spacing of ca 1 cm. The two inner electrodes serve as current electrodes to generate an electrical field in the soil, and the two outer electrodes are voltage electrodes. The electrical field is generated using a simple AC-generator. The induced voltage is amplified, and both current and voltage are measured with conventional multimeters. The ratio of current to induced voltage is proportional to the bulk ground conductivity in a sphere around the electrodes with a diameter of 4 cm. The design of a similar sensor is described in more detail by Stieglitz et al. (2000a). Relative errors are less than 5% (Stieglitz et al., 2000a). The fibreglass rod with the electrodes is glued into a heavy steel rod, which can be pushed into the sediment to depths up to 1.5 m. A transect recorded with this probe consisted of 8–10 profiles. At the beach sites, each profile was taken by inserting the probe into the ground, taking a reading at a particular depth, and then gradually pushing the probe further into the ground. In the deeper-water seafloor sediments, the probe was dropped into the seafloor, measuring the conductivity at a depth of 1.5 m below the sediment surface. Depth was measured with an echosounder with a 9° beam angle. Apparent ground conductivity data was contoured applying standard methods. The ÔapparentÕ ground conductivity measured with this probe is a function of (a) porewater fraction and (b) porewater conductivity, which in turn is a function of porewater salinity and temperature. Therefore, it is not possible to derive porewater salinity from this measurement alone, however some limited information on porewater salinity can be obtained from the following considerations. Commonly, marine sediments saturated with water with a conductivity C have an apparent conductivity reduced by a factor of 2–3 compared to C, resulting from the presence of non-conducting particles (Kermabon et al., 1969). Hence at a typical conductivity of 55 mScm 1 of seawater, sediments with an apparent bulk ground conductivity of greater than 18 mScm 1 can be assumed to be saturated with seawater. Sediments of the same type with a bulk ground conductivity of significantly less than 18 mScm 1 can be assumed to
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T. Stieglitz / Marine Pollution Bulletin 51 (2005) 51–59
Fig. 2. Apparent ground conductivity transects (a) at Ella Bay in May 2003, (b) at Elim Beach in April 2002 and (c) through a Wonky Hole in Halifax Bay in January 2000, together with photographs of discharge sites at (a) and (b) and acoustic multibeam image of the investigated Wonky Hole at (c) respectively. Note the different scales on the axes. The same conductivity colour scheme applies. The cross in (a) marks the highest location of visible seeps, and the grid in the multibeam image in (c) has 10 m spacings in both directions.
be saturated with brackish water, or––at values close to zero––with fresh water.
of the beach in Ella Bay were of salinity 27.1 psu and had a 222Rn activity of 161 ± 6 Bqm 3. 3.2. Geoelectric transects and visual observations
3. Results 3.1. Radionuclide tracing A shore-parallel transect of the 222Rn activity recorded one nautical mile offshore shows localised elevations of 222Rn activity along parts of the Wet Tropics coastline, around 17.3°S and 18.5°S latitude respectively (Fig. 1). It is important to note that the rainfall in the year of recording was significantly below average for the region (Australian Bureau of Meteorology, www. bom.gov.au). Nevertheless, two statistically significant peaks can be observed, with a maximum of 33 ± 3 Bqm 3 and 71 ± 4 Bqm 3 against a background of less than 20 Bqm 3. Point samples of seawater 10 m offshore
At Ella Bay, a sharp interface persists between low and high bulk sediment conductivity at the sediment surface, whereas at Elim Beach bulk ground conductivity at the surface (as well as at depth) is non-uniform and shows no distinct, single boundary. In 20 m of water depth, apparent conductivity of the seafloor in a ÔWonky HoleÕ is significantly reduced at the bottom of the depression compared to the surrounding seafloor. The later finding is discussed in more detail in Stieglitz and Ridd (2000). Visually, the physical discharge sites also differ significantly: at Ella Bay, discharge occurs in the form of seeps along the beach face (Fig. 2a), immediately behind which freshwater wetlands are located. Discharge at
T. Stieglitz / Marine Pollution Bulletin 51 (2005) 51–59
Elim Beach occurs from discrete, circular springs in the flat intertidal zone (Fig. 2b). Likewise, Wonky Holes are discrete, pockmark-like depressions, the morphology of which is described in detail in Stieglitz and Ridd (2000) and Stieglitz and Ridd (in press).
4. Discussion 4.1. Utility of a shore-parallel SGD
222
Rn transect to detect
The purpose of recording the 222Rn transect was to identify the broad distribution of SGD into the ocean. Significantly elevated 222Rn activities along parts of the Wet Tropics coastline, around 17.3°S and 18.5°S latitude, were detected in the late wet season in 2003 (Fig. 1). 222Rn is commonly sampled relatively close to the source, i.e. at distances less than 100 m. Therefore, a priori it was not clear if 222Rn would be detectable with the methods applied here as far offshore as one nautical mile. The measured variable activity of 222Rn however indicates that the sampling strategy is successful in identifying locations or zones of 222Rn input into the ocean. At the given averaging time and vessel speed, a spatial resolution of between 5 and 7 nautical miles was achieved, which is considered adequate for investigations on the scale of hundreds of kilometres. To obtain a more detailed spatial distribution of SGD, the vessel speed could be reduced at the expense of spatial coverage. To trace back to the source, some systematic effects need to be taken into account, i.e. (1) the local oceanography, (2) a systematic shift of observed 222Rn in steaming direction due to a non-negligible response time of 15 min of the detection system. Firstly, in the GBR lagoon coastal waters are deflected northwards by the influence of the Southeast trade winds for most of the year (Burrage et al., 2002). Therefore, an observed offshore 222Rn elevation is expected to originate southwards of the signal. The elevated 222Rn activities at latitude 17.3°S occur just north of Ella Bay, a location of known SGD (see below). Anecdotal evidence from locals and personal observation by the author suggest that also Cowley Beach to the South of Ella Bay and Bramston Beach to its North discharge groundwater. The spatial resolution of the data does not allow for discrimination between these sources, however visual observations at all three sites suggest that at the time of data collection Ella Bay was the only place where significant discharge occurred at the time. The elevated 222Rn at 18.5° latitude occurred in Hinchinbrook Channel between the mainland and the 30 km long Hinchinbrook Island. Mixing of channel water with ocean water occurs at a much slower rate than at an open coast. Hence the residual 222Rn activity of coastal waters is greater than would be expected at an
55
open coastline at similar input rate. From the presented data, it is not possible to derive the relative magnitude of the 222Rn input at 17.3° and 18.5° latitude respectively without quantification of lateral mixing. Secondly, a systematic shift of data in steaming direction must be taken into account due to the intrinsic response time of the counting system of 15 min. The 222 Rn activity is determined by counting the decay of the daughter 218Po to 214Pb, which is in equilibrium with the 222Rn to 218Po decay after 5 half-lives, i.e. 5 by 3 min. Therefore a change in 222Rn activity will be detected 15 min later than it occurred. The elevated 222 Rn activity at latitude 17.3°S was recorded when travelling northwards, and hence this effect works in the same geographic direction as the oceanographic northward deflection of coastal waters. Other factors need to be taken into account in order to derive SGD fluxes from 222Rn activities (e.g. Burnett and Dulaiova, 2003). For instance, the loss of the noble gas Radon to the atmosphere needs to be known. It depends on factors such as temperature and wind speed, both of which did not significantly vary whilst recording the transect. The evasion of 222Rn to the atmosphere is here assumed constant, and systematic changes in 222Rn are therefore considered meaningful. The two regions of elevated 222Rn are discussed separately below.
5. Conceptual hydrogeological models of SGD in the GBR 5.1. Seawater recirculation through crustacean burrows––Hinchinbrook Channel Hinchinbrook Channel is a renowned important habitat for extensive mangrove forests (e.g. Bunt and Bunt, 1999). A large portion of these forests consists of the Red Mangrove Rhizophora stylosa, which is one of the most common mangrove species in the region. Crustacean burrows in the forest floor between the root system of these trees have been shown to be flushed rapidly and efficiently by tidal circulation (Stieglitz et al., 2000b). Per tidal cycle, 100–150 l m 2 were found to be exchanged across the forest floor, or, equivalently, 10–40% of tidal water was flowing through burrows at an inundation level of 1 m and 0.25 m respectively (Stieglitz et al., 2000b). This flux of ÔgroundwaterÕ across the sedimentwater interface is likely accompanied by a flux of 222 Rn, which the water acquires when in contact with sediments (Corbett et al., 1999). It is reasonable to conclude that at least some of the observed elevation of 222 Rn in Hinchinbrook Channel can be attributed to this recirculation of seawater (Fig. 3a). The Herbert River, which debouches into Hinchinbrook Channel was not discharging freshwater into the channel at the time, and is unlikely to have contributed to the observed 222 Rn distribution. In the catchment of the GBR,
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Fig. 3. Conceptual hydrogeological models for SGD in the GBR region; (a) Seawater recirculation through crustacean burrows, (b) Discharge from unconfined coastal aquifers, (c) Discharge from confined or semi-confined coastal aquifers in form of discrete springs, (d) Discharge from confined submarine aquifers through ÔWonky Holes. The spatial scale of each groundwater system is indicated (cf. Table 1).
8000 km2 or 38% of the total area of wetlands (including floodplains and riparian vegetation) are mangrove forests (Furnas, 2003). This type of groundwater flux, accompanied by a flux of 222Rn as well as ecologically relevant solutes, should persist along all these mangrove coastlines, and is itself a process worthy of more detailed investigation. 5.2. Discharge from unconfined coastal aquifers––Ella Bay The coastal transect shows elevated 222Rn at latitude 17.3°S (Fig. 1). On the beach face of Ella Bay just to the south, SGD was prominently visible in form of seeps (Fig. 2a). Observations of inshore 222Rn activity, salinity and beach sediment conductivity suggest a source of 222 Rn and SGD in the intertidal zone at Ella Bay. The seeps discharged undiluted freshwater with salinity 0 psu, and seawater just offshore from the beach was of salinity 27.1 psu and had a 222Rn activity of 161 ± 6 Bqm 3, i.e. significantly higher than 33 ± 3 Bqm 3 measured along the transect one nautical mile offshore, and also significantly higher than activities found of outershelf seawater of typically less than 5 Bqm 3 (Stieglitz, unpublished data). At low tide, a shore-normal ground conductivity transect shows a well-defined interface between low and high ground conductivity at 14 m from the high tide mark, with discharge of fresh groundwater occurring between high and low tide mark from 5 m to 14 m respectively along the transect (Fig. 2a). Based on these observations, it is reasonable to infer that SGD into Ella Bay is a major contributing source of the observed elevation of 222Rn at latitude 17.3°S. Discharge in the intertidal zone from coastal wetlands is the most commonly observed hydrogeological setting of SGD
(Fig. 3b). Such a connection of unconfined coastal aquifers and the ocean is expected to occur along most of the GBR coastline, and to be accompanied by SGD flux where an aquifer water level above sealevel are present (Johannes, 1980). How much and for how long discharge occurs depends on the hydraulic head in the aquifer systems. Hydrogeologically similar SGD has been observed e.g. in Southwest Australia (Johannes and Hearn, 1985), Florida (Corbett et al., 1999) and Long Island, New York (Bokuniewicz, 1980). 5.3. Discharge from large dune systems––‘Buiur Bindi’ at Elim Beach For thousands of years, indigenous people inhabiting the coastlines of Eastern Cape York Peninsula have sourced freshwater from springs in the intertidal zone. In the local language of the Guggu Yimithirr people, ÔbuiurÕ means freshwater and Ôbuiur bindiÕ refers to a spring in the ocean. In some areas where these springs are found, such as Elim Beach, surface discharge through creeks and rivers are largely absent. In these areas, the discharge of groundwater can be expected to provide a major pathway for land-derived water and solutes, and may play an important role in the near-shore ecology of the adjacent coastal embayments. For instance, Melaleuca spp. trees can be found growing in the intertidal zone at Elim Beach, indicating the presence of freshwater all year round: such trees can not tolerate full seawater salt concentrations for extended periods of time. Freshwater wetlands perched in extensive coastal dune systems inland of the beaches have water levels above sea level all year round. These wetlands are a likely source of the fresh groundwater discharged along the coastline (Fig. 3c).
T. Stieglitz / Marine Pollution Bulletin 51 (2005) 51–59
The apparent ground conductivity in Elim Beach sediments is significantly more variable than at Ella Bay (Fig. 2). Here, low conductivity water is found at the surface up to 10 m from the high tide mark, and in discrete locations at 3 m and 70–80 m respectively. Digging revealed the shallow beach stratigraphy: the top 10–20 cm of sediment consist of relatively porous sand, below which a layer of dense clay with mangrove roots embedded was found, from 20 cm to 50 cm depth below the modern sediment surface. Below 50 cm of depth, porous sand was found again. It is likely that this clay layer provides at least partial confinement of the shallow aquifer, resulting in the observed non-uniform pattern of ground conductivity and the discrete nature of the spring sites (Fig. 3c). There is anecdotal evidence, that SGD occurs in this form also in the Cape Flattery and Shelbourne Bay regions, where similar extensive dune systems with perched wetlands are situated. Some of theses spring sites are marked on Admiralty charts. 5.4. Discharge from confined submarine aquifers–– ‘Wonky Holes’ A further––geologically curious––process that delivers SGD into the ocean is the discharge through ÔWonky HolesÕ from confined submarine aquifers comprised of riverine paleochannels (Stieglitz and Ridd, 2000). Paleochannels were incised into the shelf during low sealevel stand, in-filled with coarse sediments, and subsequently capped by relatively fine, impermeable terrestrially-derived sediments during Holocene sealevel rise (Johnson and Searle, 1984). The paleochannels appear to be continuously connected to the shore landwards from Wonky Holes, and are likely to provide a pathway for groundwater derived from the coastal plain to be discharged onto the inner/mid shelf of the GBR (Fig. 3c; Stieglitz and Ridd, 2000). A conductivity transect across a Wonky Holes shows significantly reduced conductivity in the depression (Fig. 2c), and a hydrographic transect just above a Wonky Hole showed reduced bottom water salinity over a Wonky Hole compared to the surrounding seafloor (Stieglitz and Ridd, 2000). Wonky Holes were first described in Halifax Bay, and are therefore marked in Fig. 1 in Halifax Bay. On-going research indicates that Wonky Holes are present offshore from most rivers in the central and northern GBR lagoon (Stieglitz, unpublished data).
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5.5. Potential ecological significance of SGD in the region Australia is the driest continent on earth, and the discharge of groundwater––as well as surface water––into its coastal ocean might be expected to be small compared to other continents. However, on small temporal and spatial scales, the processes documented here may be ecologically significant. The coastal hydrology in the GBR region differs from many other regions in the world in two important aspects. On one hand, the coastal plain is comparatively narrow (<70 km), and hence coastal aquifer systems that can potentially drain into the ocean are relatively small. On the other hand, the region experiences a pronounced wet/dry season cycle with the bulk of the average annual rainfall of 1–4 m falling in the austral summer from January to April. The combination of the two effects suggests that SGD is reasonably localised and highly seasonal in the region (as is surface discharge). It is likely though that due to the storage effect in coastal aquifers––although small––SGD persists longer into the dry season than surface discharge does, possibly even throughout the year in some cases, as indicated in Table 1. At Elim Beach for instance, such an extended input of freshwater into the ocean may have a significant impact on local coastal ecosystems. Hydraulic connections from the land to the ocean (and vice versa) span spatial scales from a few meters to kilometres (Table 1), extending underneath the modern coastline and coastal plain. Within the catchment of the GBR region, strong relationships exist between intensive agricultural land use and the quality of shallow groundwater (Anonymous, Department of Natural Resources). SGD is a source of terrestrially derived groundwater, which may be affected by agricultural land use. Recently introduced water quality guidelines for the GBR region imposed by the Great Barrier Reef Marine Park Authority, a federal protection agency, target surface runoff only, and do not account for other potential pathways such as SGD for nutrients, trace elements and potentially contaminants to enter the near-shore zone. Sites and processes described here are meant to serve as examples only; there is significant anecdotal evidence that these processes occur elsewhere along the GBR coastline. Little experimental knowledge exists on the connectivity of coastal aquifers to the ocean in the region. From the presented qualitative evidence for SGD, no reliable
Table 1 Likely temporal and spatial scales of four different hydrogeological settings of SGD in the GBR Hydrogeological model
Spatial scale
Timing of discharge
Seawater recirculation through animal burrows (3a) Discharge from unconfined coastal aquifers connected to freshwater wetlands (3 b) Discharge from (semi)confined coastal aquifers in extensive coastal dune systems (3c) Discharge from confined submarine aquifers through ÔWonky HolesÕ (3d)
1–10 m 100–1000 m 1000–10,000 m 10,000 m
All year round Wet and early dry season All year round? Wet season
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estimate of the magnitude of the SGD flux can be made. Further steps to identify the ecological significance of SGD, in particular as pathways for ecologically significant solutes, will require the quantification of the discharge from the various geological settings. This is is the subject of on-going research. 6. Conclusion 222
Rn input into the coastal ocean has been located along the central GBR coastline, and likely sources of elevated 222Rn activities have been identified. The results suggest that continuous sampling of 222Rn along a coastal transect is a useful, non-quantitative tool to reveal input zones of groundwater along a coastline on large scales. From geophysical and radiochemical investigations, SGD is suggested to occur in a variety of hydrogeological settings in the GBR region, originating from both unconfined and confined coastal aquifer systems and operating on different spatial and likely also temporal scales. Four hydrogeological settings of SGD in the region were identified: (1) recirculation of seawater through animal burrows in mangrove forests, (2) fresh SGD from unconfined aquifers as a narrow coastal fringe of freshwater along Wet Tropics beaches, (3) SGD from extensive coastal dune systems in form of localised freshwater springs in the intertidal zone, (4) inner-shelf SGD from confined submarine aquifer systems comprised of riverine paleochannels incised into the shelf. Settings (1) and (2) are suggested to occur on large scales in the GBR region, given the spatial extent of mangrove forests and coastal aquifer systems. Acknowledgments P Ridd (JCU), G Brunskill (AIMS), G Hancock, PG Cook (CSIRO Land & Water) and P Doherty (AIMS) provided inspirational and hands-on support of this work. Thanks for technical support are due to S Thomas, S Choukroun, M OÕLeary, K Hooper, J Cavanagh, D Lane-Smith and masters & crew of the RV Lady Basten. B Burnett, E Kontar, B Moore, S Krupa and other members of the SCOR/LOICZ/IOC working group 112 have provided advice on SGD issues, in particular on methods. The permission to work at Elim Beach by the traditional owners is greatly appreciated. This research is jointly funded by the Australian Research Council and the Australian Institute of Marine Science. References Anon., 1999. Nutrient balances and transport from agricultural and rainforest lands: a case study in the Johnstone River Catchment. Project No DAQ3S Final Report, Queensland Department of Natural Resources, Brisbane, 85 pp.
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