Radium and radon radioisotopes in regional groundwater, intertidal groundwater, and seawater in the Adelaide Coastal Waters Study area: Implications for the evaluation of submarine groundwater discharge

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Marine Chemistry 109 (2008) 318 – 336 www.elsevier.com/locate/marchem

Radium and radon radioisotopes in regional groundwater, intertidal groundwater, and seawater in the Adelaide Coastal Waters Study area: Implications for the evaluation of submarine groundwater discharge Sébastien Lamontagne a,⁎, Corinne Le Gal La Salle b,1 , Gary J. Hancock c , Ian T. Webster c , Craig T. Simmons b , Andrew J. Love d , Julianne James-Smith d , Anthony J. Smith e , Jochen Kämpf b , Howard J. Fallowfield f b

a CSIRO Land and Water, PMB 2, Glen Osmond, SA 5064, Australia School of Chemistry, Physics and Earth Sciences, Flinders University, GPO Box 2100, Adelaide SA 5001, Australia c CSIRO Land and Water, Black Mountain ACT 2601, Australia d Department of Water, Land and Biodiversity Conservation, GPO Box 2834, Adelaide SA 5000, Australia e CSIRO Land and Water, Wembley WA 6913, Australia f Department of Environmental Health, Flinders University, GPO Box 2100, Adelaide SA 5001, Australia

Received 12 March 2007; received in revised form 30 August 2007; accepted 31 August 2007 Available online 8 September 2007

Abstract The input of groundwater-borne nutrients to Adelaide's (South Australia) coastal zone is not well known but could contribute to the ongoing decline of seagrass in the area. As a component of the Adelaide Coastal Waters Study (ACWS), the potential for using the radium quartet (223Ra, 224Ra, 226Ra and 228Ra) and 222Rn to evaluate submarine groundwater discharge (SGD) was evaluated. Potential isotopic signatures for SGD were assessed by sampling groundwater from three regional aquifers potentially contributing SGD to the ACWS area. In addition, intertidal groundwater was sampled at two sand beach sites. In general, the regional groundwaters were enriched in long-lived Ra isotopes (226Ra and 228Ra) and in 222Rn relative to intertidal groundwater. Radium activity (but not 222Rn activity) was positively correlated to salinity in groundwater from one of the regional aquifers and in intertidal groundwater. Radium isotope ratios (223Ra/226Ra, 224Ra/226Ra and 228Ra/226Ra) were less variable than individual Ra isotope activities within potential SGD sources. Recirculated seawater (estimated from the intertidal groundwater samples with seawater-like salinities) also had distinctly higher Ra isotope ratios than the regional groundwaters. The activities for all radioisotopes were relatively low in seawater. The activity of the short-lived 223Ra and 224Ra were highest at the shoreline and declined exponentially with distance offshore. In contrast, 228Ra and 226Ra activities had a weak linear declining trend with distance offshore. Rn-222 activity was at or near background in all seawater samples. The pattern of enrichment in short-lived Ra isotopes and the lack of 222Rn in seawater suggest that seawater recirculation is the main contributor to SGD in the ACWS area.

⁎ Corresponding author. Tel.: +61 8 8303 8713; fax: +61 8 8303 8750. E-mail address: [email protected] (S. Lamontagne). 1 Current address: Laboratoire GIS, 150 Georges Basse, 30 035 Nimes, Cedex 1, France. 0304-4203/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2007.08.010

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Preliminary modeling of the offshore flux of 228Ra and 226Ra suggest that the SGD flux to the ACWS area ranges between 0.2 and 3 · 10− 3 m3 (m of shoreline)− 1 s− 1. © 2007 Elsevier B.V. All rights reserved. Keywords:

222

Rn; Radium quartet; Gulf St Vincent; SGD; Recirculated seawater

1. Introduction An excessive input of nutrients, especially nitrogen (N), is suspected to have contributed to the decline of seagrasses in Gulf St Vincent (South Australia) in the vicinity of the city of Adelaide (Fig. 1). A multidisciplinary research program, the Adelaide Coastal Waters Study (ACWS), was recently undertaken to understand the causes of the seagrass decline. One of the objectives of the program was to compile nutrient budgets for the ACWS area, including inputs from submarine groundwater discharge (SGD). The role that groundwater plays in the water balance, nutrient fluxes or the ecology of the Gulf St Vincent is not known. Elsewhere, there is evidence that fresh groundwater discharge can influence the distribution of plant communities in coastal environments (Johannes, 1980; Hatton and Evans, 1998; Kamermans et al., 2002). In addition, SGD can be a major source of N and other contaminants to coastal areas (Rutkowski et al., 1999; Smith et al., 2003; Boehm et al., 2004; Slomp and van Cappellen, 2004). Submarine groundwater discharge is often a mixture of regional groundwater and recirculated seawater (oceanic water moving in and out of porous sediments due to tides, waves, etc; Burnett et al., 2003). In this study, we evaluated the potential to estimate SGD to the ACWS area using the four radioisotopes of radium (223Ra, 224Ra, 226Ra and 228Ra) and 222 Rn (a naturally occurring radioactive gas). These radioisotopes have been used to estimate SGD both in Australia (Cook et al., 2004) and overseas (Cable et al., 1996a,b; Moore, 1996; Charette et al., 2003; Crotwell and Moore, 2003; Swarzenski et al., 2007). Both the radium quartet and 222Rn originate from the decay of uranium and thorium radioisotopes that are present in most rocks. The isotopic signature for Ra and Rn isotopes in a given source of water will be a function of several factors, including contact time with geological materials and salinity. Sources of water with a short exposure to geologic materials (hours to days) will tend to be more enriched in the short-lived 223 Ra (halflife = 11.4 days) and 224 Ra (half-life = 3.66 days) relative to the long-lived 226Ra (half life = 1600 years) and 228Ra (half-life = 5.75 years). This is due to rapid regeneration rate of short-lived Ra activity (Hancock and Murray,

1996). Longer exposures to geological materials (years to millennia) will make sources more enriched in longlived Ra isotopes (Rama and Moore, 1996). Increases in salinity (such as when an aquifer undergoes seawater intrusion) tend to increase Ra activity in groundwater. This occurs because ion-exchange mechanisms favor a greater partition of the exchangeable Ra pool in porewater than on surface exchanges sites at higher salinities (Webster et al., 1994; Hancock et al., 2000; Sturchio et al., 2001). However, the magnitude of the change in groundwater Ra activity will also be related to the frequency of exposure to more saline water because of the time required to regenerate the exchangeable Ra pool from the parent isotopes (Moore, 2003). Other factors that may contribute to differences in Ra and Rn activity between sources of SGD include variability in the concentration of uranium-bearing minerals, temperature, the geochemical environment, and porous medium porosity and size distribution (Rama and Moore, 1996; Kraemer and Genereux, 1998; Cecil and Green, 2000; Hancock et al., 2000; Sturchio et al., 2001). The objectives of this study were three-fold: 1) Characterise the Ra and Rn isotopic signatures of potential sources of SGD to the ACWS area; 2) Evaluate if salinity influences the isotopic signature in each potential sources; and 3) Obtain preliminary estimates of the flux of Ra to the ACWS area by modelling the trends in seawater Ra activity along transects perpendicular to the coastline. The potential sources of SGD evaluated included groundwater from three regional aquifers and recirculated seawater. To evaluate the impact of salinity on radioisotope activity in regional groundwater, an effort was made to sample the range in salinity found in each aquifer. In addition to the regional groundwater survey, shallow groundwater samples were collected along transects spanning the intertidal zones at two sand beach sites where regional groundwater discharge was thought to occur. These transects were used to evaluate how regional groundwater radioisotopic signatures change when mixing with seawater occurs in the subsurface. In addition, the intertidal groundwater samples with seawater-like salinities were used to evaluate the potential isotopic signature of recirculated seawater for the area. Finally, the flux of the long-lived Ra isotopes from the coastline in the Northern and Southern sections of the ACWS area was estimated by modelling

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Fig. 1. Location of the Adelaide Coastal Waters Study area and the three main hydrogeological systems potentially contributing groundwater to coastal waters. Also indicated are the location of the offshore transects for environmental tracers and the two intertidal groundwater sampling areas (West Beach and Port Willunga). The A–A' and B–B' lines represent the approximate location of the cross-sections shown on Fig. 2. More details about the bores and bore locations can be found in Lamontagne et al. (2005).

the offshore trends in seawater Ra activity using a onedimensional diffusivity model. The implications of these findings for the evaluation of SGD discharge to the ACWS area using Ra and Rn radioisotopes will be discussed. 2. Methods 2.1. Study area The Gulf St Vincent is a large coastal embayment in Southern Australia (Fig. 1). The Gulf behaves as an “inverse” estuary, with salinity generally increasing inward because of

minimal surface runoff and a semi-arid climate where potential evaporation is well in excess of precipitation (de Silva Samarasinghe et al., 2003). Ephemeral surface runoff is more common in the vicinity of Adelaide, where annual precipitation in the nearby Mt Lofty ranges is higher (550– 800 mm year− 1). Circulation patterns in the Gulf are complex but generally clockwise (de Silva Samarasinghe et al., 2003). With the exception of brief periods during neap tides, Gulf waters do not vertically density-stratify and are vertically well mixed (de Silva Samarasinghe, 1998). The ACWS area, the part of the Gulf in the vicinity of Adelaide, is approximately 120 km long by 20 km wide (Fig. 1). There are three main hydrogeological regions potentially

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Fig. 2. Geological cross-sections for the A) Metropolitan Adelaide (from Gerges, 1997) and B) Willunga Basin groundwater systems. Approximate location of the cross-sections shown on Fig. 1.

contributing groundwater to the ACWS area: the North Adelaide Plains, the metropolitan Adelaide area and the Willunga Basin (Fig. 1). The uppermost sections of the North Adelaide Plains and metropolitan Adelaide groundwater systems consist of sedimentary deposits of Tertiary and Quaternary age up to 600 m thick (Gerges, 1997, 2001; Fig. 2). Several aquifers are present in both the Quaternary (the Q aquifers) and Tertiary sediments (the T aquifers). These aquifers are numbered in order of appearance in the sequence (with the Q1 being the uppermost, etc). The main lithology of the Quaternary sediments is mottled clay and silt with interbedded sand, gravel and thin sandstone. Where these aquifers outcrop to sea is not well characterised (reviewed in Lamontagne et al., 2005). However, based on the aquifers depth interval, orientation, and the bathymetry of the Gulf, only the Q1 is expected to outcrop within the ACWS area. Salinity in the Q1 naturally varies from fresh to brackish, with fresher conditions occurring near areas of preferential recharge, such as near ephemeral streams. The extremes in salinity for the Q1 occur along the coastline. High recharge rates below sand dunes generate localised freshwater lenses in the uppermost section of the Q1 (Martin, 1996). However, deeper groundwater in the Q1 along the coastline can also have high salinities (with total dissolved solids up to 21 g L− 1) implying seawater intrusion or leakage from an underlying saline aquifer (Lamontagne et al., 2005). In the southern section of the ACWS area, the Willunga Basin is a sedimentary aquifer system that is currently

intensively used for irrigation. Its main aquifers are the Port Willunga Formation (marl limestone) and the Maslin Sands. Both aquifers outcrop at least in part in the ACWS area (Aldam, 1989; Lamontagne et al., 2005). Salinity in the Willunga Basin is naturally variable but has also been recently influenced by seawater intrusion near the coastline and by recharge of saline irrigation drainage in the centre of the basin (Lamontagne et al., 2005). The metropolitan Adelaide Q1 aquifer, the Port Willunga Formation (PWF) and the Maslin Sands (MS) were the principal aquifers investigated in this study. 2.2. Rn and Ra in regional groundwater Sampling for the radioisotopes in regional groundwater was made in a subset of the bores sampled for a regional survey of N concentration in groundwater (Lamontagne et al., 2005). Five bores were sampled from the Quaternary aquifers (four in the Q1 and one in the Q4) in the metropolitan Adelaide area, six from the Port Willunga Formation and three from the Maslin Sands Formation (Fig. 3). In order to evaluate whether groundwater salinity has an influence on Ra and Rn activities, the bores were selected to cover the range in salinity found in each aquifer using available records from the South Australian Department of Water, Land and Biodiversity Conservation. However, the most saline bores in the Q1 along the coastline could not be sampled. More detail about bore location and

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one small spring discharge with a lower salinity at the base of a cliff (TDS = 9.1 g L− 1). At the northern site, however, porewater salinity ranged from fresh to seawater. The wells (5 cm ID PVC) had a 30 cm screened interval installed 0.5 to 1 m below the water table. The wells were flushed for a minimum of three well volumes before they were sampled for EC and Ra and Rn radioisotopes. Groundwater and surf samples for 222Rn were collected following Cook et al. (2003, 2004) and 20 L samples were collected in carboys for Ra analyses. More detailed groundwater salinity and nutrient profiles were also collected at the northern site using drive points and are described elsewhere (Lamontagne et al., 2005). 2.4. Offshore transects

Fig. 3. Location of the bores surveyed in the metropolitan Adelaide area and the Port Willunga Formation.

water quality can be found in Lamontagne et al. (2005). Each bore was purged for at least three well volumes before sample collection. Electrical conductivity (EC) was measured in the field using a portable meter and converted into estimates of Total Dissolved Solids (TDS) using dilutions of a seawater standard. Samples for 222Rn were collected as described in Herczeg et al. (1994) and 10 to 20 L groundwater samples were collected in carboys for Ra isotope analyses. 2.3. Intertidal groundwater Using flow nets for the regional aquifers and preliminary sampling in the field, two sand beach sites were selected to measure radioisotope activity in intertidal groundwater. An effort was made to locate sites where mixing between freshwater and seawater occurred in the intertidal zone. One site was established in the northern (West Beach) and another in the southern part of the study area (Port Willunga; Fig. 1). Intertidal groundwater was sampled along transects spanning the edge of the beach at low tide to either the base of a dune above high tide level (West Beach) or the base of a cliff (Port Willunga). Intertidal groundwater was sampled by installing three wells at equidistant locations along each transect with the aim to capture the whole gradient in salinity present at each site. At the southern site, intertidal groundwater had seawaterlike salinities throughout the beachface, with the exception of

Seawater samples were collected during two cruises on the Flinders University Research Launch Hero. On the first cruise (4 November 03), 11 samples were taken along a transect in the northern section of the ACWS area, near Henley Beach Jetty (“Northern Transect”; Lat = −34°55'; Fig. 1). On the second cruise (26 November 03), 12 samples were collected along a transect at the southern end of the study area (“Southern Transect”; Lat = −35°16') near Port Willunga. The timing of the sampling (end of Austral spring) was chosen to coincide with the expected period of greatest regional groundwater discharge to the ACWS area (i.e., water tables tend to be highest in the region during that period). Along each transect samples were taken more frequently closer to the shoreline (every 100 to 400 m) than offshore (every 1 to 2 km). At each sampling station, vertical profiles for electrical conductivity, turbidity and temperature were measured at 1 m depth intervals (Northern Transect) or 2 m depth intervals (Southern Transect) using an AANDERAA RCM 9 (Kaempf, 2006). From the measured electrical conductivity and temperature data, salinity was calculated from the Practical Salinity Scale 1978 (Unesco, 1981). Samples for the radioisotopes were collected 1 m below the surface using a bilge pump fitted with a filter to remove large particles (N 1 mm). Note that Gulf waters were not vertically density-stratified at the time of sampling. For radium isotopes, 40 L was collected for “inshore” samples (≤ 5 km) and 80 L for “offshore” samples (N5 km) and stored in 20-L carboys. Radon-222 was concentrated in a mineral oil cocktail from 1-L samples immediately following collection using the method outlined in Cook et al. (2003). 2.5. Radium quartet and radon analyses Rn-222 samples were analysed by liquid scintillation in an LKB Quantullus counter using the pulse shape program to discriminate between alpha and beta decay (Herczeg et al., 1994). Radium isotopes were quantitatively extracted from water samples using MnO2 coated acrylic fibres (“MnO2 fibres”). Samples were gravity-fed through columns containing a glassfibre wool plug to remove small particles and between 2.0 and 3.5 g of MnO2 fibres (Moore, 1976) depending on sample size (more for the larger volume samples). Filtration rates ranged

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between 2 and 6 L h− 1. Once samples were filtered, they were backflushed with distilled water to remove the remaining seawater, tightly packed, and sent to CSIRO's laboratories in Canberra to measure Ra activity. Removal efficiency (assessed by reprocessing filtered samples) ranged between 95 and 99%. The maximum time span between sample collection and the beginning of counting for 223Ra, 224Ra and 222Rn activities was less than three days. Measurements of the activity of short-lived Ra isotopes (223Ra and 224Ra) were made by partially drying the MnO2 fibres, and placing them in an air circulation system, described by Moore and Arnold (1996). Gaseous 219Rn and 220Rn, formed by the decay of 223Ra and 224Ra, was flushed from the MnO2 fibres into a scintillation cell where alpha particles from the decay of Rn and its daughters were detected by a photomultiplier tube, and identified using a delayed coincidence system. The counting efficiency of the system was determined using MnO2 fibres containing known activities of 223 Ra and 224Ra in secular equilibrium with their parents 232Th and 227Ac. The long-lived isotopes, 226Ra and 228Ra, were determined by alpha spectrometry following radiochemical separation (Hancock and Martin, 1991). After the addition of a yield tracer (225Ra in equilibrium with its parent 229Th) the fibres were leached with hot 5 M HCl to remove Ra. The solution was then purified by co-precipitation and ion-exchange techniques. The purified solution was electroplated onto a stainless steel disc and 226Ra determined by high-resolution alpha-particle spectrometry. The disc was recounted about 6 months later, and 228Ra was determined via ingrowth of its alpha-emitting daughter, 228Th.

the absence of major river plumes. All streams discharging to the ACWS area are ephemeral and were not flowing at the time of the study. Another assumption of the model is that the water column is well mixed, which is generally the case in the ACWS area expect during periods with restricted tidal activity (de Silva Samarasinghe et al., 2003). At steady-state, Eq. (1) simplifies to:
 A AA  Do H þ kHA ¼ B: ð2Þ Ax Ax

2.6. Estimation of the offshore Ra flux

Dx ¼ Do

ð6Þ

Several steps are required to estimate SGD using trends in Ra activity in seawater. Following convention (Moore, 2000, 2003), in a first step the short-lived Ra isotopes are used to estimate the offshore coefficient of solute diffusivity (Do). Secondly, the total offshore Ra flux (Fo) is estimated using the long-lived Ra isotopes. The approach used to estimate Do and Fo was similar to the one developed by Hancock et al. (2006) and will only be briefly reviewed here. Offshore Ra activity profiles were modelled using the one-dimensional advection– dispersion equation by incorporating radioactive decay, depth and benthic flux terms:

Dx ¼ Do ½1  expðx=DÞ:

ð7Þ

AA AA A2 Do HA þ kA ¼ H 1 B þu  H 1 At Ax Ax2

ð1Þ

where A is the radium activity, t is time, x is offshore distance, u the advection velocity, H is water depth, Do is the offshore coefficient of solute diffusivity, λ the isotope decay rate, and B is the Ra benthic flux (that is, the flux of Ra from the seafloor due diffusion and bioirrigation). To solve Eq. (1), advection offshore is assumed to be negligible (u ∼ 0) so that diffusivelike processes alone control offshore transport (Moore, 2000). This assumption is reasonable for the ACWS area because of

In a first step, the short-lived isotopes, 223Ra and 224Ra, are used to estimate Do. For the short-lived isotopes, the boundary conditions for Eq. (2) are a constant Ra flux at the coastline (Fo) and a zero flux at 10 km, or: AA ¼ 0: Ax

F10km ¼

ð3Þ

Integration of Eq. (2) with respect to x yields:
 Z 10km AA 10km  Do H ¼ ð B  kHAÞdx: Ax x¼0 x¼0 The left hand side of this equation is just F10 since we assume that F10 km = 0 then: Z Fo ¼

10km

ð kHA  BÞdx:

ð4Þ km − Fo,

but

ð5Þ

x¼0

The integral on the right hand side is evaluated using the measurements of radium activity along the two transects. Two formulations for diffusivity were tested:

In both formulations Do is constant. In the second formulation, the diffusivity increases from zero at the coast to asymptote to Do further offshore. The lengthscale for this increase is Δ. Two formulations for B were also used to estimate Fo. In the first formulation, the convention used in previous studies was used (Moore, 2003), where there is no benthic flux. For this case, the Ra generation capacity of bottom sediments is assumed so low that the combined effects of molecular diffusion and bioturbation produces a negligible Ra flux. In a second formulation, a constant B flux along x was assumed. No direct measurements of B are available for the ACWS area. However, because the sediments are mostly quartz sand, B is probably low. B measurements from a sandy western Australian coastal embayment (G. Hancock, unpublished data) were used as representative values for the ACWS area (the method used to estimate B is described in Hancock et al., 2006). Four models with different combinations for the formulation of Do and B were tested. Solutions were developed for Do, Δ, and B by discretising Eq. (2) in mass-conserving form with a cell size of 50 m from the shoreline to 10 km. Depth was

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allowed to vary between cells and was approximated from depth measurements made during Ra sampling using a series of linear equations. The resulting discretised equations with their two boundary conditions were solved by LU factorisation (a form of Gaussian elimination; Schneider and Barker, 1989). Optimal parameter values for each model were evaluated by minimising the negative log-likelihood between predicted and observed activities (Hillborn and Mangel, 1997; Hancock et al., 2006). The negative log-likelihood (L) is defined as:
 X n 1 ðXi  mÞ2 L ¼ n logðrÞ þ logð2kÞ þ : 2 2r2 i¼1

ð8Þ

Where Xi is the measured Ra activity at distance i, m the predicted activity at i, n the number of observations, and σ is:

r¼

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uP un u ðYi  mi Þ2 ti¼1 n1

:

ð9Þ

The surf zone samples were not included in the negative loglikelihood minimisation procedure because they were assumed

to have a large temporal variability and were collected up to 48 h after the offshore transect samples. Using the likelihood ratio test (Hillborn and Mangel, 1997), the negative log-likelihoods were also used to evaluate what combination of parameters gave the best fit to the data. The rationale behind the test is that negative log-likelihoods will tend to decrease (i.e., indicate a better fit) when more parameters are included in a model even if the supplementary parameters have little or no relationship to the observed data. The likelihood ratio test (R) is defined as: R ¼ 2  ðL A  L B Þ

ð10Þ

where LA and LB are the negative log-likelihoods for models A and B, respectively, and where model B has more parameters than A. The test has a chi-square distribution with degrees of freedom equal to the difference in the number of parameters between models B and A. Thus, if B has one more parameter than A, R must be N 3.84 for B to be considered better than A at the 0.05 probability level. The estimates of Do obtained using the short-lived isotopes can be used to estimate Fo for the long-lived Ra isotopes. The isotopes 226Ra and 228Ra have half-lives sufficiently long that on the timescale of transport across the ACWS area decay can

Table 1 Activities of the radium quartet and of 222Rn in regional groundwater and intertidal groundwater in the vicinity of the ACWS area TDS (g L− 1)

223

Ra (mBq L− 1)

224

Ra (mBq L− 1)

226

0.87 36.5 38.5 37.1 40.1 9.3

0.42 ± 0.04 5.4 ± 0.33 6.6 ± 0.44 4.9 ± 0.32 5.2 ± 0.29 1.1 ± 0.11

5.3 ± 0.2 74 ± 1.4 106 ± 1.9 89 ± 1.8 72 ± 1.3 18 ± 0.6

0.32 ± 0.02 2.8 ± 0.17 3.3 ± 0.08 5.3 ± 0.28 4.1 ± 0.15 2.4 ± 0.14

3.3 ± 0.25 24 ± 1.5 33 ± 1.4 21 ± 1.1 15 ± 0.6 4.3 ± 0.45

0.59 ± 0.010 0.69 ± 0.012 1.0 ± 0.02 1.17 ± 0.020 1.1 ± 0.02 2.1 ± 0.04

3.6 6.7 2.2 0.53 1.2

3.4 ± 0.27 2.0 ± 0.19 1.3 ± 0.18 0.39 ± 0.044 0.93 ± 0.14

106 ± 1.6 62 ± 1.3 30 ± 1.1 7.3 ± 0.23 23 ± 0.9

23 ± 0.8 22 ± 0.8 9 ± 0.5 3.6 ± 0.35 12 ± 0.6

99 ± 3.5 54 ± 2.5 25 ± 1.3 7.8 ± 0.77 39 ± 3.4

12 ± 0.3 26 ± 0.6 7.0 ± 0.16 26 ± 0.6 20 ± 0.5

Port Willunga Formation ACW0325 0.81 ACW0339 13.3 ACW0331 1.9 ACW0333 1.1 ACW0334 0.99 ACW0335 2.0

5.2 ± 0.36 5.6 ± 0.58 0.32 ± 0.060 0.44 ± 0.059 18 ± 1.3 5.9 ± 0.45

22 ± 0.9 326 ± 6 8.4 ± 0.45 19 ± 0.7 342 ± 6 74 ± 1.8

56 ± 2.5 71 ± 2.8 5.7 ± 0.26 5.5 ± 0.28 207 ± 20 28 ± 1.2

14 ± 0.9 359 ± 12 6.0 ± 0.37 11 ± 0.9 137 ± 17 53 ± 2.1

38 ± 0.5 18 ± 0.4 2.1 ± 0.09 5.7 ± 0.14 24 ± 0.6 59 ± 1.3

Maslin Sands ACW0326 ACW0330 ACW0336

0.59 ± 0.059 0.93 ± 0.13 2.0 ± 0.25

9.2 ± 0.32 20 ± 0.9 43 ± 1.5

3.0 ± 0.13 10 ± 0.5 35 ± 2.3

3.2 ± 0.26 10 ± 0.9 70 ± 3.6

4.4 ± 0.13 9.5 ± 0.17 3.7 ± 0.17

Intertidal North 1 North 2 North 3 South 1 South 2 South 3 Quaternary (Q1) ACW0340 a ACW0345 ACW0347 ACW0349 ACW0350

0.70 1.3 0.53

Uncertainties represent ± SE. a Q4 aquifer.

Ra (mBq L− 1)

228

Ra (mBq L− 1)

222

Rn (Bq L− 1)

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Fig. 4. Activities of 223Ra, 226Ra and 222Rn in regional groundwater and intertidal groundwater as a function of TDS. A) groundwater; B) 223Ra — regional groundwater; C) 226Ra — intertidal groundwater; D) 226Ra — regional groundwater; E) groundwater; F) 222Rn — regional groundwater.

325

Ra — intertidal Rn — intertidal

223

222

be neglected. Thus, for long-lived Ra isotopes, Eq. (2) simplifies to:
 A AA Do H ¼ B: ð11Þ Ax Ax

being required to satisfy the second boundary condition. It will be demonstrated later that, as a first approximation, B does not need to be considered for the long-lived isotopes.

A key difference relative to short-lived Ra isotopes is that Fo cannot be directly estimated because F10 km N 0 for the longlived isotopes. Thus, the boundary conditions here are a constant offshore flux at the coastline and at 10 km. Eq. (11) was solved using a numerical model similar to the one used for the short-lived isotopes. However, in this case optimal solutions were sought for different combinations of Fo and the radium activity beyond 10 km offshore (A10 km), the latter

3.1. Regional and intertidal groundwater

3. Results

A wide range in Ra isotopes and 222Rn activity was found in regional and intertidal groundwater samples taken in the vicinity of the ACWS area (Table 1). In general, the range in 223 Ra and 224Ra activities overlapped between geological formations. However, 226Ra and 222Rn activities tended to be lower in intertidal groundwater (0.32–5.3 mBq L− 1 and 0.59–

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Fig. 5. Activities of 224Ra, and 228Ra in regional and intertidal groundwater as a function of TDS. A) 224Ra — intertidal groundwater; B) 224Ra — regional groundwater; C) 228Ra — intertidal groundwater; D) 228Ra — intertidal groundwater.

2.1 Bq L− 1, respectively) relative to regional groundwater (3.0–207 mBq L− 1 and 2.1–59 Bq L− 1, respectively). The Port Willunga Formation had the highest and widest range in activities for all isotopes. The activities of the Ra isotopes were positively related to salinity in intertidal groundwater and in the Q1, but not in the Port Willunga Formation or the Maslin Sands (Figs. 4 and 5). For example, in intertidal groundwater, 223 Ra activity varied from 0.42 mBq L− 1 at TDS = 0.87 g L− 1 to 6.6 mBq L− 1 at TDS = 38.5 g L− 1. Similarly, 226Ra activities also increased from 0.32 to 5.3 mBq L− 1 along the same salinity gradient. Rn-222 activity was unrelated to salinity (Fig. 4E–F). The increase in Ra activity in the Q1 was not as

large, but the salinity gradient was also smaller than in intertidal groundwater. The subset of four intertidal groundwater samples with TDS N 36 g L− 1 (that is, with similar or higher salinities than Gulf seawater) were used to estimate the potential isotopic signature of recirculated seawater. The mechanism of recirculation was probably tidal pumping because the wells were shallow and located in the intertidal zone. Thus, some of this intertidal groundwater will be exchanged twice a day and the overall average seawater residence time in the subsurface is probably in the range of days to weeks. Relative to regional groundwater, recirculated seawater was more depleted in

Table 2 Ra and Rn isotopic signatures for potential SGD sources to the ACWS area (mean ± SD)

TDS (g L− 1) Rn (Bq L− 1) 223 Ra (mBq L− 1) 224 Ra (mBq L− 1) 226 Ra (mBq L− 1) 228 Ra (mBq L− 1) 223 Ra/226Ra 224 Ra/226Ra 228 Ra/226Ra 222

Recirculated seawater (n = 4)

Q1 (n = 4)

Port Willunga Formation (n = 6)

Maslin Sands (n = 3)

Offshore ACWS

38 ± 1.6 0.99 ± 0.21 5.5 ± 0.73 85 ± 16 3.9 ± 1.1 23 ± 7.8 1.5 ± 0.51 23 ± 7.2 5.8 ± 3.6

2.8 ± 2.4 18 ± 8.4 1.6 ± 1.2 46 ± 39 14 ± 8.5 45 ± 35 0.10 ± 0.028 2.5 ± 0.68 2.7 ± 0.51

3.4 ± 4.9 24 ± 21 46 ± 39 132 ± 158 62 ± 76 97 ± 138 0.11 ± 0.068 2.3 ± 0.92 1.5 ± 0.7

0.85 ± 0.41 5.9 ± 3.1 14 ± 8.5 24 ± 17 16 ± 17 28 ± 37 0.12 ± 0.075 2.1 ± 0.94 1.4 ± 0.5

36 b0.004 0.05 0.25 1.5 1.5 0.033 0.17 1

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isotope activity in the sediment phase (Hancock and Murray, 1996). Recirculated seawater was more enriched in all isotopes relative to ACWS offshore seawater. However, the level of enrichment was inversely related to the half-life of the isotopes. The most enriched isotopes were 224Ra (340-fold) and 222Rn (N248-fold) followed by 223Ra (110-fold), 228Ra (15-fold), and 226Ra (2.6-fold). While there was a significant overlap in Ra activities between potential sources of SGD, radium isotope ratios were more distinct (Table 2). This will be illustrated here by looking at the ratios relative to 226Ra. Ratios in recirculated seawater were high, being 1.5, 23, 5.8 for 223Ra, 224Ra and 228Ra, respectively. Ratios were lower in regional groundwater and similar between aquifers, ranging from 0.10–0.12, 2.1–2.5, to 1.4–2.7 for the same isotopes, respectively (Table 2). This suggests that Ra isotope ratios may be more useful to evaluate the relative contribution of regional groundwater and recirculated seawater components to SGD than individual Ra isotope activities. 3.2. Offshore transects Fig. 6. A) Temperature (°C) and B) salinity profiles at the Northern Transect, 4 November 2003. 226

Ra, 228Ra and 222Rn but had relatively high 223Ra and 224Ra activities (Table 2). This process of short-lived Ra isotope enrichment in marine sediments relative to the long-lived isotopes has been observed elsewhere and is due to the repeated leaching of Ra from sediments subject to tidal pumping, and the subsequent rapid regeneration of short-lived

Fig. 7. A) Temperature (°C) and B) salinity profiles at the Southern Transect, 26 November 2003.

There were small horizontal temperature and salinity gradients at the two transects at the time of sampling (Figs. 6 and 7). The range in seawater temperature was small (15.4– 17.3 °C and 16.4–18.5 °C for the Northern and Southern transects, respectively) with the highest temperatures nearer to the shore. Salinities ranged from 36.60 to 37.09 at the Northern

Fig. 8. Radium isotope activities in seawater along A) the Northern Transect and B) the Southern Transect.

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Transect and from 36.37 to 37.26 at the Southern Transect. With the exception of a small decrease in salinity at the shoreline for the Northern Transect, salinities were greater inshore than offshore. While salinity gradients were present, Ra and Rn activities were not corrected for evapoconcentration because the effect would be very small (1–2%). Differences in density between surface and bottom waters in vertical profiles were small (b 0.1 to 0.3 kg m− 3), indicating that the water column was not vertically density-stratified. Most seawater samples had 222Rn activities at or below background levels (b 4 mBq L− 1; data not shown), with the exception of a few near shore and surf zone samples with low activities (5–9 mBq L− 1). However, there were negative offshore gradients in Ra isotope activity, especially for the short-lived 223Ra and 224Ra (Fig. 8). At the Northern Transect, 223 Ra and 224Ra activities were 0.6 and 8 mBq L− 1, respectively, in the surf zone and declined exponentially to b 0.1 and b 1 mBq L− 1, respectively, at offshore locations (Fig. 8A). The activity of long-lived isotopes were low and also tended to decline with distance offshore, with activities ranging between 2.1 to 1.75 mBq L− 1 and 4.1 to 2.3 mBq L− 1 for 226Ra and 228 Ra, respectively. Radium isotope activities were lower at the Southern Transect, with weaker declining offshore trends (Fig. 8B). 3.3. Estimation of Do The offshore trends in short-lived isotope activities could be reasonably well approximated by all the model representations evaluated (Table 3 and Fig. 9). With Model 1 (constant Do and B = 0), the optimised Do values ranged between 28.8 and 67.6 m2 s− 1 (Table 3). The second formulation for Do (where the diffusivity increased from zero at the coast to asymptote to Do further offshore) was essentially similar to Model 1 because the low optimised Δ values resulted in Dx ∼ Do after a few hundreds of meters from the coast (Table 3). The inclusion of B with either formulation for Do did not significantly alter predictions either (Table 3). Overall, based on the comparison of the negative log-likelihoods, the simpler Model 1 predicted short-lived Ra isotope distribution as well as the more complex model formulations. Using estimates derived from Model 1, there were differences between Do values between isotopes and between transects. At a given transect, Do values estimated with 224Ra tended to be larger (Table 3). However, they were not significantly different than the ones obtained with 223Ra based on a greater than 5% overlap in their probability distributions (Fig. 10). Thus, the average Do for the two shortlived Ra isotopes was used to estimate Do for the Northern (38 m2 s− 1) and Southern transects (62 m2 s− 1). 3.4. Estimation of Fo using

228

Ra and

226

Ra 228

226

Ra and Ra Reasonable predictions of the offshore flux were obtained by optimising Fo and A10 km using the Do values obtained with the short-lived isotopes (Fig. 11). As it was shown not to be necessary to include a B term for the

short-lived isotopes, it was also assumed that B = 0 for the long-lived isotopes as a first approximation. Unlike for Do, the optimum Fo estimates obtained with a given Ra isotope were similar between the Northern and Southern transects (Fig. 12). These ranged being between 0.0488 and 0.0547 Bq (m of shoreline)− 1 s− 1 for 228Ra and between 0.0109 and 0.0113 Bq m− 1 s− 1 for 226Ra. Combining both transects, the average Fo was 0.0518 Bq m− 1 s− 1 and 0.0111 Bq m− 1 s− 1 for 228Ra and 226 Ra, respectively.

4. Discussion 4.1. What is the isotopic signature of SGD for the ACWS area? The survey of regional and intertidal groundwater revealed a wide range in Ra and Rn isotopic signatures in the potential sources of SGD to the ACWS area. This variability appeared in part related to the geological environment and in part to variations in salinity. The highest activities were found in the limestone aquifer (PWF) as opposed to the sand or sandy silt aquifers (Q1 Table 3 Estimation of Do using offshore 223Ra and 224Ra activity profiles Do Δ (m2 s− 1) (m) Northern Ra Model 1 Model 2 Model 3 Model 4 224 Ra Model 1 Model 2 Model 3 Model 4

B L (Bq m− 2 s− 1)

P

r2

– N0.99 N0.99 N0.99

0.85 0.85 0.85 0.85

– N0.99 N0.99 N0.99

0.89 0.91 0.89 0.91

– N0.99 N0.99 N0.99

0.86 0.86 0.86 0.86

– N0.99 N0.99 N0.99

0.65 0.65 0.65 0.65

223

Southern Ra Model 1 Model 2 Model 3 Model 4 224 Ra Model 1 Model 2 Model 3 Model 4

28.8 32.0 26.2 20.7

– 8.5 – 8.6

– – 8.20 · 10− 8 8.20 · 10− 8

47.2 47.0 44.8 45.0

– 0.70 – 0.60

– – 1.30 · 10− 6 1.30 · 10− 6

57.0 57.0 52.8 53.0

– 0.60 – 0.70

– – 8.20 · 10− 8 8.20 · 10− 8

67.6 68.0 63.4 63.0

– 0.70 – 0.70

– – 1.30 · 10− 6 1.30 · 10− 6

− 23.0 − 23.2 − 23.0 − 23.2 3.57 3.57 3.55 3.55

223

− 37.8 − 37.8 − 37.8 − 37.8 − 2.22 − 2.22 − 2.22 − 2.22

Likelihood ratio tests were used to evaluate whether the two-parameter or more models had significantly lower minimum negative log-likelihoods than the one-parameter model (Model 1). Model 1 — Optimised Dx =Do only; Model 2 — Optimised asymptotic Dx only; Model 3 — Optimised Dx =Do and B; Model 4 — Optimised asymptotic Dx and B. L — Negative log-likelihood. P — Probability of R test relative to Model 1.

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Fig. 9. Comparison of measured and modelled 223Ra and 224Ra distribution for the two representations of diffusivity (the predictions from the two models are often undistinguishable). A) 223Ra — Northern Transect; B) 224Ra — Northern Transect; C) 223Ra — Southern Transect; D) 224Ra — Southern Transect. The surf zone samples (open circles) were not used in the model evaluations.

and MS), suggesting a higher concentration of uranium and thorium-bearing minerals in the former. The lack of relationship between salinity and radioisotope activity in the PWF and the MS does not imply that salinity does not influence radioisotope activity in these aquifers. The origin of the salinity and the past salinity history in different sections of the MS and PWF aquifers vary, which may have had an influence on the observed radioisotope activities. The highest salinity and some of

the highest Ra activities observed in the PWF were in bore ACW0339 in an area undergoing seawater intrusion. However, similar high Ra activities were observed in bores in the centre of the Willunga Basin where salinity is lower but increasing at an average rate (as EC) of ∼ 20 μS cm− 1 per year (Lamontagne et al., 2005). In the centre of the Willunga Basin, the mechanism of salinisation is not seawater intrusion but recharge of saline irrigation drainage. Due to the small

Fig. 10. Probability distributions for Do estimated with 223Ra and 224Ra. A) Northern Transect; B) Southern Transect.

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Fig. 11. Measured and predicted long-lived Ra isotope trends. A) 228Ra — Northern Transect; B) 226Ra — Northern Transect; C) 228Ra — Southern Transect; D) 226Ra — Southern Transect.

number of bores sampled in this study and the complexity of the salinisation process in the Willunga Basin, a more detailed assessment will be required to evaluate the influence of salinity on Ra activities in this system. The intertidal groundwater survey was only partially successful in assessing how the isotopic signature of SGD derived from regional groundwater may change during discharge to the sea. In part, this was due to the apparent lack of significant regional groundwater discharge at either location investigated. At the Northern

Transect, only a freshwater end-member and the seawater end-member were collected. The freshwater end-member had lower Ra and Rn activities relative to other samples from the Q1 and was probably derived from a local freshwater lens beneath the coastal sand dunes rather than regional groundwater. Lower Ra activities in the freshwater lens probably reflected a relatively short water residence time (compared to rest of the Q1) and perhaps a more U- and Th-poor quartz sand porous medium. Likewise, the brackish intertidal groundwater sample from the Southern Transect had

Fig. 12. Probability distributions for Fo. A) 228Ra; B) 226Ra.

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Fig. 13. Evaluation of the proportion of offshore seawater in the surf samples as a function of the proportion of SGD that could be due to seawater recirculation [that is, frec/(frec + fgw)] evaluated using mixing models. The isolines represent the 22xRa/226Ra ratios in surf samples that may arise from different combinations of frec, fgw and fsea in a given mixing model. A) 228Ra–226Ra mixing model, Northern Transect; B) 228Ra–226Ra mixing model, Southern Transect; C) 224Ra–226Ra mixing model, Northern Transect; D) 224Ra–226Ra mixing model, Southern Transect; E) 223Ra–226Ra mixing model, Northern Transect; F) 223Ra–226Ra mixing model, Southern Transect.

lower Ra activities than the regional groundwater samples, also suggesting a more local source. The limited evidence for regional groundwater discharge observed at the beach transects is consistent with the trends in declining water tables and the occurrence of seawater intrusions in many aquifers of the region (see review in Lamontagne et al., 2005).

4.2. Estimation of the source of SGD to nearshore waters using mixing models There were marked differences in the isotopic signature of different potential sources of SGD to the ACWS area. Regional groundwater was enriched in 222Rn, 226Ra and 228 Ra relative to recirculated seawater, but not in 224Ra

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and 223Ra. The Ra isotope ratios were also consistently different between sources and were less variable within sources than individual Ra isotope activities. Recirculated seawater had elevated 223Ra/226Ra, 224Ra/226Ra and 228Ra/226Ra ratios relative to regional groundwater, consistent with a shorter water residence time (Rama and Moore, 1996). Overall, the lack of 222Rn combined with elevated 223Ra/226Ra, 224Ra/226Ra and 228Ra/226Ra ratios suggest that seawater recirculation was the main source of SGD in nearshore waters. For example, the 224Ra/226Ra ratio in the surf sample at the Northern Transect (4.0) indicates that a significant contribution of recirculated seawater must occur. None of the other sources of Ra (regional groundwater or offshore seawater) have a 224 Ra/226Ra ratio above 4 (Table 2). The contribution of different sources of SGD to inshore waters can be further evaluated using mixing models (Moore, 2003). For example, for 226Ra and 228Ra, a three end-member mixing model can be formulated as: frec 226 R arec þ fgw 226 R agw þ fsea 226 R asea ¼

226

R ain

ð12Þ

frec 228 R arec þ fgw 228 R agw þ fsea 228 R asea ¼

228

R ain

ð13Þ

frec þ fgw þ fsea ¼ 1

ð14Þ

where frec, fgw and fsea are the fractions of recirculated seawater, regional groundwater and offshore seawater in inshore seawater, respectively, and where 22x Rarec, 22x Ragw, 22xRasea, and 22xRain are the activities for a given Ra isotope in recirculated seawater, regional groundwater, offshore seawater, and inshore seawater, respectively. Several assumptions must be considered when interpreting these mixing models because they do not take into account radioactive decay and transport processes. In addition, it is also assumed that the recirculated seawater signature estimated from intertidal groundwater is representative for all possible sources of recirculated seawater to the nearshore (Precht and Huettel, 2003) and that the average Ra signature from the regional groundwater survey is representative for a given aquifer. Nevertheless, at least close to the shoreline where radioactive decay will be less of a concern, the models can be used as a first approximation. In addition, mixing models using 228Ra and 226Ra will not be sensitive to changes due to radioactive decay because of their longer half-lives. As an example of the use of the mixing models, the proportion of SGD and the proportion of SGD from recirculated seawater were evaluated for the surf samples at each transect for mixing models using 223 Ra,

224

Ra, or 228 Ra with 226Ra. The average Ra isotope activities for each end-member were taken from Tables 1 and 2. The average activity in the Q1 and the PWF were used for the groundwater end-member for the northern and southern surf samples, respectively. The average recirculated seawater Ra activities for the northern and southern intertidal groundwater transects were also used as the end-member for their respective surf samples (Table 1). While in theory it should be possible to find unique values for frec, fgw and fsea using pairs of Ra isotopes (Moore, 2003), negative f values were frequently encountered. This may occur because the activity of the end-members is imperfectly known, especially when the contribution of some end-members is small. Thus, rather than seek unique solutions, the various combinations of frec, fgw and fsea that could yield the observed surf zone Ra activities were evaluated. The mixing models indicated that many combinations of frec, fgw and fsea could account for the observed surf zone Ra activities (Fig. 13). However, several inferences about the potential contribution of SGD to the surf zone can be made. According to the mixing models, the minimum proportion of SGD in the surf zone is 6–8% at the Northern Transect and b 1–3% at the Southern Transect (Fig. 13). In most cases, this minimum proportion occurs when all the SGD is derived from seawater recirculation. At the Northern Transect, the proportion of SGD will be higher if SGD is a mixture of recirculated seawater and regional groundwater (Fig. 13). In contrast, the contribution of SGD to the southern surf sample is always b 5% regardless of the source of SGD. While the mixing models could not unambiguously determine the proportion of SGD derived from regional groundwater or recirculated seawater, they showed that some recirculation input must occur and that the observed Ra isotope ratios in the surf samples are possible without regional groundwater discharge. 4.3. Preliminary estimates of SGD flux using the estimated Fo A range for the potential SGD flux to the ACWS area can be estimated by assuming that the offshore flux of long-lived Ra isotopes will be due to either regional groundwater discharge only or seawater recirculation only. Using the Fo estimates from the 1-D diffusivity model, the SGD flux can be estimated following: QSGD ¼ Fo =ASGD

ð15Þ

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where QSGD is the SGD flux along the coastline [m3 (m shoreline)− 1 s− 1] and ASGD the estimated activity of 228Ra or 226Ra in SGD. As only the magnitude of SGD was sought, two scenarios with the extremes in ASGD were compared. The low and high SGD flux scenarios were evaluated by using the average 228Ra and 226 Ra activities from the Port Willunga Formation and seawater recirculation, respectively (Table 2). Using the average Fo for a given isotope for the two transects, QSGD ranges between 1.8–5.3 · 10− 4 m3 m− 1 s− 1 when using ASGD from the PWF and 2.3–2.8 · 10− 3 m3 m− 1 s− 1 when using ASGD from recirculated seawater. There is only limited independent information to compare with these estimates. Lamontagne et al. (2005) estimated that recirculation by tidal pumping would be approximately 6 · 10− 5 m3 m− 1 s− 1 in the ACWS area, similar to values measured elsewhere (Ullman et al., 2003). Thus, SGD sources other than tidal pumping are likely to occur in the ACWS area. Based on a review of flow nets for the region, Lamontagne et al. (2005) estimated that regional groundwater discharge to the ACWS area is 5 · 10− 7 m3 m− 1 s− 1. While this estimate was considered highly uncertain, it suggests that regional groundwater discharge is a small component of the SGD. As the ACWS area is exposed to swells from the Southern Ocean, we hypothesise that porewater displacement in shallow sediments is a significant component of the SGD. 4.4. Diffusivity estimates in coastal environments When compared relative to the scale at which they were measured, the estimates of Do obtained for the ACWS area with Ra isotopes are consistent with diffusivity estimates measured elsewhere. The magnitude of Do in coastal environments is in part a function of the spatial scales over which they are measured (List et al., 1990). In this regard, Do values estimated using shortlived Ra isotopes in the ACWS area are consistent with both values of Do measured in the area using other techniques and with those measured with Ra isotopes in other coastal environments. Using drifters in the surf zone of the ACWS area (scale ∼ 100 m), Pattiaratchi and Jones (2005) found that Do was 0.13 m2 s− 1 in the cross-shore plane (that is, the same plane as the Do estimated with Ra isotopes). In offshore areas of Gulf St Vincent (scale 10–40 km), de Silva Samarasinghe et al. (2003) modelled estimates of diffusivity ranged between 25 and 160 m2 s− 1. By comparison, Do estimated using short-lived Ra isotopes in the ACWS area ranged between 29 and 68 m2 s− 1 (scale ∼ 10 km). Hancock et al. (2006) estimated that Do varied between 100 and 260 m2 s− 1 in the inner lagoon of the Great Barrier Reef

333

(scale ∼ 20 km). In the South Atlantic Bight (scale ∼ 50 km), Do estimated with short-lived Ra isotopes varied between 360 and 420 m2 s− 1 (Moore, 2000). Thus, the Do values estimated for the ACWS area using short-lived Ra isotopes are within the range expected for the scale at which the measurements were made. The sharp decline in 223Ra and 224Ra activities in the first 2 km of the ACWS transects may also be taken as evidence that mixing is more restricted along the coastline because of the horizontal salinity gradient (de Silva Samarasinghe et al., 2003). Moore (2003) observed similar 223Ra and 224Ra trends in the northeast Gulf of Mexico, where a horizontal salinity gradient also exists, and estimated that Do varied between 2.7–12 m2 s− 1 inshore (scale 3–4 km) and 168–233 m2 s− 1 offshore (scale 20–25 km). Using the same modelling technique as Moore (2003), Lamontagne et al. (2005) estimated that Do was 0.6–3.5 m2 s− 1 inshore (b 2 km) and 45–258 m2 s− 1 offshore (N 2 km) in the ACWS area, supporting the possibility that there are two scales of mixing present. However, a potential shortcoming of the modelling technique used by Moore (2003) and Lamontagne et al. (2005) is that changes in depth along the transects were not considered even though the water column was not densitystratified in both cases. The modelling approach used here indicates that the inclusion of variability in depth along the transects can also account for the observed trends in 223Ra and 224Ra activity. In other words, the rapid decline in 223 Ra and 224Ra activities inshore may also be due in large part to dilution as the water column deepens in the offshore direction. While conceptually it is reasonable to expect that Do should be smaller inshore (Hancock et al., 2006), for the ACWS area the use of a constant Do appears sufficient to describe the trends in 223Ra and 224Ra activities at a large scale (8–10 km) when the changes in activity due to variations in depth are considered. 4.5. Comparison with other SGD studies Few studies have been made on the use of Ra and Rn radioisotopes to quantify submarine groundwater discharge in Australia. In Spencer Gulf (also in South Australia), Veeh et al. (1995) reported that 228Ra and 226 Ra activities were slightly above oceanic background but tended to increase along a reverse salinity gradient. Veeh et al. (1995) postulated that input from the seafloor could account for the excess 228Ra but could not account for all the excess 226Ra. They suggested that the excess 226 Ra input could originate from periodic surface runoff from sabkhas at the northern end of the gulf or from regional groundwater discharge from the granitic basement rock. By combining offshore transects for Ra and Rn

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activities with a numerical model, Cook et al. (2004) estimated that both regional groundwater and recirculated seawater fluxes were significant in Bowling Green Bay (Burdekin Delta, Queensland). On an annual basis, groundwater discharge was estimated to range between 50 and 400 ∙ 103 ML year− 1 (5.0–40∙ 107 m3 year− 1). Unlike in the ACWS area, 222Rn activity was significant in Bowling Green Bay, ranging from 2–13 mBq L− 1 in February 2004 to 5–26 mBq L− 1 in April 2004 along transects parallel to the shoreline. The temporal variability in 222Rn activity in Bowling Green Bay indicates that the rates of submarine groundwater discharge could vary significantly during the year (Cook et al., 2004). While few studies have attempted to separate the regional groundwater and recirculated seawater component of SGD, there appears to be a large variability in the relative magnitude of the two sources from site to site (Burnett et al., 2003). For example, in Kahana Bay in Hawaii, the terrestrial (that is regional groundwater) component was estimated to be 16% of total SGD discharge (Garrison et al., 2003). However, regional groundwater still represented a significant flux of freshwater to the Bay relative to surface inputs. In contrast, from a comparison of 222Rn and 226Ra fluxes, Abraham et al. (2003) suggested that regional groundwater discharge could represent up to 88% of total SGD discharge in Waquoit Bay, Massachusetts. In Cape Henlopen, Delaware, Hays and Ullman (2007) estimated that seawater recirculation was 68% of total discharge along a sandy beachface. A number of studies have also demonstrated that enhanced porewater exchange occurs in intertidal and shallow coastal shelf zones because of the advection of water over permeable sediments generated by waves (Shum, 1992; Precht and Huettel, 2003). This enhanced porewater exchange has a significant impact on sediment–water biogeochemical processes (Jahnke et al., 2000; Werner et al., 2006). A number of mechanisms appear to contribute to the recirculation flux. For example, Precht and Huettel (2003) identified four different processes by which waves can enhance porewater exchange in shallow permeable sediments as a function of depth. Future studies in the ACWS area should seek to quantify the Ra isotopic signatures and fluxes for these different potential mechanisms of recirculation. 5. Conclusion Radium radioisotopes have provided useful information about the potential sources of SGD to the ACWS area. In particular they provide evidence that seawater

recirculation was a large component of the SGD flux at the time of study. It may be difficult to obtain an estimate of regional groundwater discharge using Ra isotopes in the ACWS area if indeed independent measurements confirm that the recirculation flux is large relative to the regional groundwater one. This study focussed on nearshore sources of SGD and, with two transects along a 120 km coastline, could not exhaustively evaluate all sources of SGD to the area. Additional transects across the study area would be required to evaluate if inshore or offshore point sources of SGD could also be present. While 222Rn was found at low activities in the areas surveyed, this tracer may still be a useful indicator of point sources of regional groundwater discharge elsewhere in the region because this source of SGD should be enriched in this isotope. Acknowledgements This work was funded by the Adelaide Coastal Waters Study and CSIRO Land and Water. We thank Peter Edwards, Mike Mellow and John Dighton for their help during field sampling on the Hero Research Launch. Jeremy Wilkinson provided field support during the beachface sampling. M.A. Charette and two anonymous reviewers made many valuable recommendations on an earlier draft of the manuscript. References Abraham, D.M., Charette, M.A., Allen, M.C., Rago, A., Kroeger, K.D., 2003. Radiochemical estimates of submarine groundwater discharge to Waquoit Bay, Massachusetts. Biology Bulletin 205, 246–247. Aldam, R.G., 1989. Willunga Basin Hydrogeological Investigations 1986/88. South Australian Department of Mines and Energy Report Book 89/22. Government of South Australia, Adelaide. Boehm, A.B., Shellenbarger, G.C., Paytan, A., 2004. Groundwater discharge: potential association with faecal indicator bacteria in the surf zone. Environmental Science and Technology 38, 3558–3566. 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. Cable, J.E., Bugna, G.C., Burnett, W.C., Chanton, J.P., 1996a. Application of 222Rn and CH4 for assessment of groundwater discharge to the coastal ocean. Limnology and Oceanography 41, 1347–1353. Cable, J.E., Burnett, W.C., Chanton, J.P., Weatherly, G.L., 1996b. Estimating groundwater discharge into the northeastern Gulf of Mexico using radon-222. Earth and Planetary Science Letters 144, 591–604. Cecil, L.D., Green, J.R., 2000. Radon-222 In: Cook, P.G., Herczeg, A.L. (Eds.), Environmental Tracers in Subsurface Hydrology. Kluwer, London, pp. 175–194.

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