Hydrol. Earth Syst. Sci., 24, 1293?1318, 2020
https://doi.org/10.5194/hess-24-1293-2020
? Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
Groundwater mean residence times of a subtropical
barrier sand island
Harald Hofmann1,5, Dean Newborn1, Ian Cartwright2, Dioni I. Cend?n3, and Matthias Raiber4
1School of Earth and Environmental Sciences, The University of Queensland, St Lucia, QLD, Australia
2School of Earth, Atmosphere and Environment, Monash University, Clayton, VIC, Australia
3Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW, Australia
4Commonwealth Scientific and Industrial Research Organisation (CSIRO), Dutton Park, QLD, Australia
5Geotechnical Engineering Centre, School of Civil Engineering, The University of Queensland, St Lucia, QLD, Australia
Correspondence: Harald Hofmann (h.hofmann@uq.edu.au)
Received: 13 June 2019 ? Discussion started: 19 August 2019
Revised: 13 January 2020 ? Accepted: 30 January 2020 ? Published: 19 March 2020
Abstract. Fresh groundwater on barrier islands is af-
fected by changing sea levels and precipitation variability
due to climate change and is also vulnerable to anthro-
pogenic processes, such as contamination and groundwater
over-abstraction. Constraining groundwater mean residence
times (MRTs) and flow paths is essential for understanding
and managing these resources.
This study uses tritium (3H) and carbon-14 (14C) to de-
termine the MRTs of groundwater along a transect across
subtropical North Stradbroke Island, south-east Queensland,
Australia. Hydraulic properties, major ion geochemistry and
stable isotopes are used to validate residence times and to
identify the processes responsible for their variability. 3H
activities range from less than 0.01 to 1 TU (tritium units),
which are values lower than those of local average rain-
fall (1.6?2.0 TU). 14C concentrations range from 62.5 to
111 pMC (percent modern carbon). Estimated MRTs deter-
mined using lumped parameter models and 3H activities
range from 37 to more than 50 years. Recharge occurs over
the entire island, and groundwater MRTs generally increase
vertically and laterally towards the coastal discharge areas,
although no systematic pattern is observed. MRTs estimated
from 14C concentrations display similar spatial relationships
but have a much greater range (from modern to approxi-
mately 5000 years). Water diversion and retention by lower-
permeability units in the unsaturated parts of the dune sys-
tems are the most likely course for relatively long MRTs to
date. The results indicate that the internal structures within
the dune systems increase MRTs in the groundwater sys-
tem and potentially divert flow paths. The structures pro-
duce perched aquifer systems that are wide-spread and have
a significant influence on regional recharge. The geochemi-
cal composition of groundwater remains relatively consistent
throughout the island, with the only irregularities attributed
to old groundwater stored within coastal peat.
The outcomes of this study enhance the understanding
of groundwater flow, recharge diversion and inhibition for
large coastal sand masses in general, especially for older sand
masses that have developed structures from pedogenesis and
dune movement. With respect to south-east Queensland, it al-
lows the existing regional groundwater flow model to be re-
fined by incorporating independent MRTs to test models? va-
lidity. The location of this large fresh groundwater reservoir,
in dry and populous south-east Queensland, means that its
potential to be used as a water source is always high. Back-
ground information on aquifer distribution and groundwater
MRTs is crucial to better validate impact assessment for wa-
ter abstraction.
1 Introduction
Barrier islands are common landforms in coastal environ-
ments. They boast some of the world?s highest biodiversity,
provide fresh groundwater resources, and have economic
value for tourism and mineral industries. The majority of
barrier islands consist of large sand dunes that have high
permeabilities. As a consequence, rainfall infiltrates quickly
Published by Copernicus Publications on behalf of the European Geosciences Union.
1294 H. Hofmann et al.: Groundwater mean residence times of a subtropical barrier sand island
and recharges the aquifer system through a large unsaturated
zone (Bryan et al., 2016). Freshwater lenses develop beneath
these islands until they reach hydrodynamic equilibrium with
the underlying saltwater (R?per et al., 2012). The dynamics
of these lenses are similar to other coastal aquifers and are
affected by changes in groundwater levels due to pumping,
land use change, climate variations and sea level fluctuations
(Austin et al., 2013; Masterson et al., 2014; Moore et al.,
2010; White and Falkland, 2009). Studies on barrier islands
from the North Sea (Europe) have indicated that the ground-
water systems there are not stratified (Houben et al., 2014;
Seibert et al., 2018; Holt et al., 2019). In comparison to those
islands, many coastal sand masses, including the barrier is-
lands in subtropical regions of Australia, have two aquifer
systems: (1) regional groundwater that is present on the entire
island and (2) small, localised perched aquifer systems and
lakes (usually sealed by the build-up of organic matter or in-
durated sands) that develop above low-permeability layers in
the unsaturated zone. Water table fluctuations in the regional
aquifer system of the island are mostly caused by the tempo-
ral variability of local rainfall, while flow patterns depend on
heterogeneity in the aquifer (Austin et al., 2013; Masterson
et al., 2014; Moore et al., 2010; White and Falkland, 2009).
Variability in the water table of the perched aquifer systems
on the other hand results from spatially variable local rainfall
and the extent or size of the perched aquifer systems. Al-
though these perched systems are generally small, they are
currently of paramount importance to regional ecology and
groundwater-dependent ecosystems. They also have the po-
tential to inhibit recharge to the regional groundwater system
and to divert the flow pattern of local groundwater.
Many studies exist on the interface of fresh groundwa-
ter and seawater in coastal environments (Post et al., 2013a;
Mahlknecht et al., 2018; Yechieli et al., 2019; Parizi et al.,
2019). The majority of these studies concentrate on either
seawater intrusion in coastal aquifers or submarine ground-
water discharge (SGD) through the seafloor into the ocean
(Stieglitz et al., 2010; Santos et al., 2009; Bryan et al., 2016;
Mahmoodzadeh and Karamouz, 2019; Parizi et al., 2019).
These studies commonly examine the groundwater?seawater
interfaces using hydrodynamic models and hydrogeochem-
istry rather than estimating recharge rates and water resi-
dence times of the terrestrial groundwater on barrier islands.
Although barrier island hydrological systems are reason-
able climate indicators, due to substantial variability in their
short- and long-term climate patterns, the current under-
standing of these systems is generally poor, especially with
respect to groundwater (Schneider and Kruse, 2003; Barr
et al., 2013, 2019). In particular, due to recharge rates and
residence times of water not being sufficiently investigated,
the extent of connectivity between regional groundwater and
perched aquifer systems is commonly not well understood.
Hydraulic connectivity and groundwater flow can be esti-
mated from groundwater bore hydrographs to some extent,
but scarce information on unsaturated zone flow and het-
erogeneities in the aquifer limits the reliability of the out-
comes. Flow paths, recharge rates and groundwater residence
times can be better constrained by using environmental trac-
ers in combination with aquifer hydraulics. Major ions, trace
elements, and stable or radioactive isotope tracers, such as
 18O,  2H, tritium (3H and 3H=3He) and 14C are valuable
tools (Voss and Wood, 1994; Han et al., 2012; R?per et al.,
2012). CFCs and SF6 have also been used extensively in
coastal aquifers (Santoni et al., 2016). In particular, 3H, 14C
and CFCs allow connectivity, recharge rates and, most im-
portantly, mean residence times (MRTs) to be assessed.
Increasing water demand from local communities, in-
creased anthropogenic nutrient loading and sea level rises,
as well as the increasing periodicity of storm surges due
to climate change threaten to deplete and contaminate fresh
groundwater bodies. A further consequence is potential dam-
age to groundwater-dependent ecosystems and the destruc-
tion of a vital groundwater source for coastal regions (Rao
and Charette, 2012; Post et al., 2013b; Parizi et al., 2019).
The purpose of this study is to determine groundwater flow
paths and MRTs in a Quaternary dune sand aquifer on the
world?s second largest sand island, North Stradbroke Island,
Queensland, Australia (Laycock, 1975; Ulm et al., 2009).
The island is an important part of the coastal environment
in south-east Queensland and provides a vital groundwater
resource, recreational space for large urban areas and, most
importantly, unique coastal ecosystems with many freshwa-
ter wetlands, lakes and marine coastal environments (Mar-
shall et al., 2011). The island also plays a major role in the
palaeoclimate reconstructions for the east coast of Australia,
as some of the longest sediment records have been extracted
from the North Stradbroke Island wetlands (Barr et al., 2013,
2019; Tibby et al., 2016, 2017). Apart from a small number
of government reports (Leach, 2011), there are few studies
on the groundwater resources of the island, and, to the best
of our knowledge, there is no prior work on MRTs on large
Early Quaternary subtropical islands. This study closes this
knowledge gap using major ion, stable isotope and radioac-
tive tracer data from 21 sites along an east?west transect
across the main centre of North Stradbroke Island (Fig. 1).
3H and 14C are used in combination with major ion chem-
istry and stables isotopes to estimate MRTs, flow paths of
groundwater and potential inter-aquifer mixing on the island.
A more thorough understanding of the islands groundwa-
ter flow paths and residence times significantly improves the
current understanding of the hydrogeology of North Strad-
broke Island and barrier islands in general. This study is rel-
evant to most barrier islands in terms of water resource man-
agement and the prediction of fresh water resources consid-
ering changing climate, sea level changes and increasing ur-
banisation in coastal environments.
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H. Hofmann et al.: Groundwater mean residence times of a subtropical barrier sand island 1295
Figure 1. (a) Location of North Stradbroke Island (NSI) in Moreton Bay, Queensland, Australia. (b) Bores sampled (by reference number
? RN) along the transect across the island. Blue arrows indicate general flow directions. (Maps were generated using GIS files from the
Queensland Government Spatial Catalogue, QSpatial, 2019.)
2 Geographical, geological and hydrogeological setting
North Stradbroke Island is located approximately 40 km east
of Brisbane in a group of sand dune islands that make up
Moreton Bay (Fig. 1). The island has an area of approx-
imately 275 km2 of which around 20 % is wetlands and
beaches with important environmental and economic value.
The abundance of fresh groundwater and its proximity to
dry and populous south-east Queensland makes it signifi-
cant as a water supply (Leach, 2011). More than 70 wetlands
and 25 groundwater-dependent ecosystems (wetlands) have
been identified on the island. The wetlands range in age with
the oldest being over 200 000 years old, which makes North
Stradbroke Island unique as a palaeoclimate archive (Tibby
et al., 2017). It is estimated that 80 % of the island will be
national park by 2026 (Cox et al., 2013). Most wetlands are
internationally important Ramsar sites that host a large vari-
ety of migrating birds and have important cultural value for
the indigenous community (Marshall et al., 2011).
The island consists predominately of large Quaternary on-
lapping transgressive parabolic sand dunes, which reach a
maximum height of 219 m a.s.l. (above sea level), and large
low-lying wetland areas around the coast (Cox et al., 2011).
There are two principal land forms: the older and higher
Pleistocene sand dunes and the fringing lower Holocene
dunes. The dunes overlie Palaeozoic and Mesozoic bedrock
units (Rocksberg Greenstone, Woogaroo Subgroup and Tri-
assic rhyolite) (Kelley and Baker, 1984; Leach, 2011) that
are almost completely covered by the dunes except for some
small outcrops in the north of the island and around Dunwich
(Fig. 1). The largest part of the island is composed of sta-
bilised Pleistocene sand dunes with uniform mineralogy. The
sand is well sorted and consists mostly of quartz ( 90 %?
99 %) with minor heavy minerals, such as rutile, zircon, il-
menite, monazite, magnetite and garnet (Laycock, 1975);
these minerals have been mined since the 1950s (Moore,
2011).
The climate is subtropical with mean daily temperatures of
15?29 C in summer and 9?20 C in winter (Australian Bu-
reau of Meteorology, 2017). Average annual rainfall ranges
across the island from 1645 to 1677 mm, with annual evap-
otranspiration ranging from 1095 to 1500 mm (Cox et al.,
2011). There are slight differences in precipitation and evap-
otranspiration between winter and summer. In general, south-
east Queensland has dry winters and most of the rainfall oc-
curs during the summer months. The mainland variability
is dampened on the coast with rainfall occurring throughout
the year. Evapotranspiration is lowest in winter (June?July)
and highest in summer (December?January). The vegetation
cover is mostly continuous across the island apart from wet-
lands, lakes, tracks, mining operations and the settlements.
It consists of native subtropical mixed-eucalypt communi-
ties. The forests and woodlands have an abundance of eu-
calypts and large Proteaceae as well as a generally sclero-
phyllous understory (Audet et al., 2013) (Figs. A1 and A2).
The associated Podzols are acidic and relatively infertile
due to the migration of clay and organic matter into the
B horizons and the general lack of nutrients. Recharge es-
timates for the island are derived from water balance models
that include climatic factors and groundwater level fluctua-
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1296 H. Hofmann et al.: Groundwater mean residence times of a subtropical barrier sand island
tions. Long-term (1889?2011) average estimates are approx-
imately 127 106 m3 yr 1. A total estimated water volume
of the groundwater mound above sea level is approximately
930 106 m3 in the regional aquifer. Of the water that leaves
the island approximately 16 % is through groundwater ex-
traction by local communities and sand mining operations,
55 % is discharged to wetlands, 2 % is discharged to Blue
Lake and 27 % is via submarine discharge at the coastline
(Leach, 2011).
The island?s groundwater system is predominately hosted
in the highly permeable, well-sorted quartz Pleistocene and
Holocene sand aquifer, constrained by the relatively imper-
meable underlying bedrock. Peat layers and indurated sands,
cemented by iron oxides and hydroxides, are widely dis-
tributed along the wetlands and swamps. They are considered
to control the regional groundwater flow from the elevated
sand dunes in the predominantly unsaturated zone (Leach,
2011). The lower-permeability units of indurated sand, peat
or sandrock/humicrete (locally known as coffee rock) as
well as clay accumulations from the weathering of the small
amounts of feldspar minerals form widely spread perched
aquifer systems. The low-permeability layers of the perched
systems are believed to originate from chemical leaching of
older dune systems in the highlands as well as inclusions
of marine muds and peats in the lowlands (Laycock, 1975;
Thompson and Ward, 1975; Brooke et al., 2008). Some of the
perched aquifers and wetland systems are disconnected from
the regional aquifer and their locations; however, their ex-
tents and genesis are generally poorly understood, with only
a few studies describing the governing processes (Leach,
2011; Barr et al., 2017; Cadd et al., 2018).
The regional groundwater system (which contains almost
all of the island?s freshwater) forms an elongated north?south
mound with the height of the water table varying from a
maximum of 42 m AHD (Australian Height Datum) near
Mt Hargrave in the centre of the island to sea level in the
coastal lowlands (Cox et al., 2011). Generally, groundwa-
ter flows east and west from the high central recharge area
to the coasts, with a larger proportion flowing east due to
the relatively elevated bedrock in the west (Fig. 1). Local
groundwater discharge points (termed ?wonky holes?) oc-
cur in the tidal areas on the island. They are round, ap-
proximately 2?10 m (in diameter) disruption features in the
tidal muds where groundwater from the regional groundwa-
ter system discharges. It is believed that groundwater is semi-
confined in palaeo-channels in the sand dune system that are
covered by fine muds in around Moreton Bay (Stieglitz et al.,
2010).
3 Materials and methods
3.1 Field sampling and analytical techniques
Groundwater samples were collected from 18 Queensland
State Government groundwater monitoring bores along an
east?west transect across the central part of the island
(Fig. 1). A number of the sample locations had nested
monitoring bores, sampling both relatively shallow perched
aquifer systems and the deeper regional groundwater. The
screen depths of the sampled bores vary across the island
from 4.6 m b.g.l. (below ground level) in the lowlands to
131 m b.g.l. in the high central dunes (Table A1). Screen
lengths of the monitoring bores are 1.5 m at the bottom of
the bore. A single surface water sample was collected from
Eighteen Mile Swamp, and two groundwater samples were
collected from two wonky holes in the tidal zone approxi-
mately 6 km north of Dunwich (Fig. 1). The wonky holes
were approximately 120 m apart and 6 m in diameter.
The groundwater bores were sampled in November 2014,
whereas the wonky holes were sampled in March 2015.
The depth to the water table was determined using an elec-
tric water level tape, and hydraulic heads were calculated
from the surveyed ground elevation of each bore taken from
the Queensland Groundwater Database (Queensland ? Gov-
ernment ? Data, 2018). Groundwater was sampled using a
Grundfos MT1 pump where the depth to the screen exceeded
20 m, while a smaller 12 V electrical impeller pump (Thermo
Fisher Inc. Super Twister) was used for the shallower bores.
The samples were collected after purging the bores for
approximately five bore volumes. Bore RN14400075 was
pumped dry and sampled using a bailer a day later. In nearly
all cases, the high hydraulic conductivities of the sands re-
sulted in no or only minor drawdown during purging. At each
bore, six 1000 mL samples were collected in high-density
polyethylene (HDPE) bottles, four of which were bottom
filled and sealed for 3H and 14C analysis. In situ measure-
ments of temperature, pH, ORP (oxidation?reduction poten-
tial), DO (dissolved oxygen) and electrical conductivity (EC)
were taken using a GPS Aquaread with an AP-800 probe
(Thermo Fisher Inc.). Samples from the wonky holes were
taken in the middle of the holes by pushing an aluminium
tube approximately 1 m into the sand to avoid sampling a
mix of fresh water and ocean water. Water was then pumped
from the tube using a Geopump peristaltic pump (Geotech
Environmental Equipment, Inc.). At the end of each sampling
day, water samples were titrated for CO2 and HCO3 concen-
trations using a Hach titration kit; the precision of these con-
centrations is 5 %. A total of 125 mL of filtered (0.45 ?m,
cellulose nitrate filters) sample was taken for stable isotope
and major ion anion analysis. Next, a total of 125 mL of sam-
ple was separated, filtered and acidified with 70 % HNO3 for
cation analysis. All samples were kept cool until analysis.
Cation concentrations were analysed using a Thermo
Finnigan quadrupole inductively coupled plasma mass spec-
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H. Hofmann et al.: Groundwater mean residence times of a subtropical barrier sand island 1297
trometer (ICP-MS) at Monash University. Anion concentra-
tions were determined using a Metrohm ion chromatograph
(also at Monash University). The precision of major ion
concentrations based on replicate analysis is 2 %?5 %. Sta-
ble isotopes were measured using Finnigan MAT 252 and
DeltaPlus Advantage mass spectrometers at Monash Univer-
sity, as described by Hofmann and Cartwright (2013). Preci-
sion based on replicate analysis is 0.15 ? for  18O and 1 ?
for  2H. Rainfall stable isotope ratios were taken from the
Global Network of Isotopes in Precipitation (IAEA ? Global
Network of Isotopes in Pricipitation, 2017). Average rainfall-
weighted isotope ratios for 18O and 2H were calculated us-
ing 18O and 2H ratios and the daily rainfall from the 1960s
to 2002.  13C values of dissolved inorganic carbon (DIC)
were analysed with a Thermo Delta V continuous flow iso-
tope ratio mass spectrometer, (CF-IRMS) coupled to a Gas
Bench II at The University of Queensland.  13C-DIC val-
ues were normalised to the V-PDB scale using international
standards NBS19 and LSVEC via two-point normalisation;
the precision of this measurement is 0.3 ?.
The Australian Nuclear Science and Technology Organi-
sation (ANSTO) conducted the analysis of the 14C and 3H
samples at Lucas Heights, Sydney. 3H was analysed via vac-
uum distillation, enriched by electrolysis, distilled further to
separate tritium and counted using three Quantulus ultra-low
background liquid scintillation counters (LSC). This anal-
ysis had a combined standard uncertainty of 0.04 TU and
a quantification limit of 0.05 TU; further analytical details
are described in Neklapilova (2008). Following acid extrac-
tion and graphitisation of inorganic carbon, 14C concen-
trations were analysed using accelerator mass spectrometry
(2MV STAR tandem accelerator). The concentrations for 14C
are expressed as percent modern carbon (pMC), and the pre-
cision of 14C=12C ratios is 0:05 %.
3.2 Estimating MRT
Tritium is a suitable tracer for determining residence times
of young groundwater. It is part of the water molecule and
once isolated from the atmosphere its activities are only af-
fected by radioactive decay. 3H has a half-life of 12.32 years
and may be used to estimate residence times of water that are
up to 100 years with a precision of a few years (Morgenstern
and Taylor, 2009; Morgenstern et al., 2010). The activities of
3H in rainfall are known with sufficient precision over time
in many areas of the world to derive a local 3H input function
(Tadros et al., 2014). Brisbane is the closest 3H monitoring
station to Stradbroke Island (approximately 35 km from the
island) and has a continuous record of 3H data from the 1960s
to 2012 (Tadros et al., 2014; IAEA ? Global Network of Iso-
topes in Pricipitation, 2017).
The 3H activities peak between the 1950s and the 1960s
due to the production of 3H by atmospheric nuclear tests
(the ?bomb pulse? 3H). Traditionally, the propagation of
the bomb pulse has been utilised to trace the flow of water
recharged during this period (Fritz et al., 1991; Clark and
Fritz, 1997). Because the bomb pulse 3H peak was several
orders of magnitude lower in the Southern Hemisphere than
in the Northern Hemisphere, 3H concentrations of remnant
bomb pulse water in the Southern Hemisphere have now de-
cayed well below that of modern rainfall. This situation al-
lows estimates of MRTs to be obtained from single 3H activ-
ities (Morgenstern et al., 2010; Morgenstern and Daughney,
2012).
14C concentrations may be used to estimate groundwater
residence times in the range from 1000 to  30000 years
(Clark and Fritz, 1997). While it is not particularly well
suited for estimating the residence times of young ground-
water, it may be used to detect the input of groundwater from
old water stores, such as low-permeability layers, within the
aquifer system (Hofmann and Cartwright, 2013). The com-
bined use of 3H and 14C may also be used to assess mixing
of older groundwater with recent recharge (Cartwright et al.,
2007). Groundwater MRTs were estimated using lumped
parameter models (LPMs; Maloszewski and Zuber, 1982;
Maloszewski, 2000). These models predict the distribution
of ages and tracer concentrations in homogenous aquifers
with simplified geometries under steady-state conditions.
The mean residence time represents the average age of the
individual water molecules in the sample. The concentration
of a radioactive tracer in the sample, Cout.t/, is related to the
input over time, Cinp.t/, via the convolution integral:
Cout.t/D
tZ
et
Cinp.t  /g. /exp.   /dt; (1)
where  is the decay constant of a radioactive tracer
(Farlin and Maloszewski, 2013), and g. ) is the transfer
function that describes the distribution of ages within the
flow system. The piston flow model (PFM), the dispersion
model (DM), the partial exponential flow model (PEM), the
exponential mixing model (EMM) and the exponential flow
model (EPM) are commonly used LPMs. The piston flow
model assumes that no hydrodynamic dispersion occurs be-
tween the recharge and discharge area, and the MRT calcula-
tion is similar to decay except that the initial rainfall activity
can be varied over time (Howcroft et al., 2017). The disper-
sion model is derived from the one-dimensional advection?
dispersion?transport equation and simulates the distribution
of a wide variety of aquifer geometries (Jurgens et al., 2012).
It allows for variable degrees of dispersion by adjusting the
dispersion parameter (Dp), which describes the ratio of dis-
persion to advection. It approaches zero when advection be-
comes the dominant process controlling the tracer transport.
The EMM is most suitable for homogeneous, unconfined
aquifers of constant thickness with uniform recharge. The
EPM applies to aquifers that have regions of confined and
unconfined flow (Maloszewski and Zuber, 1982; Zuber et al.,
2005; Jurgens et al., 2012; Atkinson et al., 2014; Cartwright
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1298 H. Hofmann et al.: Groundwater mean residence times of a subtropical barrier sand island
et al., 2017). The PEM is applicable for the same type of
aquifer as the EMM but is used when only the lower part of
the aquifer is sampled by a well (Jurgens et al., 2012). The
PEM ratio is defined as the ratio of the unsampled thickness
of the aquifer to the sampled thickness. For bores screened
across the total saturated thickness of the aquifer, the PEM
ratio equals zero and the PEM is the same as the EMM.
For this study lumped parameter models contained within
the programmable TracerLPM Excel spreadsheet (Jurgens
et al., 2012) were used. 3H activities in rainfall between 1962
and 2012 from the International Atomic Energy Agency
(IAEA ? Global Network of Isotopes in Pricipitation, 2017)
and Tadros et al. (2014) with data interpolated for missing
years were used as the input function. The highest 3H ac-
tivity in 1969 was 84 TU, and the 3H activity of post 2005
rainfall is 1.6?2.0 TU. It is assumed that pre-bomb pulse
rainfall had similar 3H activities. The rainfall input function
for 14C for the Southern Hemisphere provided by the Trac-
erLPM spreadsheet was used as the local 14C activities of
rainfall in this study. The function uses the Southern Hemi-
sphere calibration curve SHcal04 and modern tropospheric
14C data (Jurgens et al., 2012). MRTs were estimated by
matching the measured radioactive concentrations to those
predicted from the lumped parameter models. Groundwater
mean residence times in the aquifer systems are expected to
be orders of magnitude larger than sub-yearly variability in
tracer input. Therefore, tracer concentrations from accumu-
lated yearly rainfall are the best representation of the tracer
input concentration into the system (Hofmann et al., 2018).
As the sand aquifer is unconfined across most of the island
and bore screens sample only a part of the aquifer, the partial
exponential model (PEM) with PEM ratios calculated indi-
vidually for all bores is the best representation for the system.
The PEM was compared with the DM and EPM to demon-
strate the impact model selection on MRT estimations.
4 Results
4.1 Groundwater hydraulic heads and flow
Groundwater bore 14400088 is probably screened in a
perched aquifer system that surrounds Brown Lake (Fig. 1),
whereas all other bores are screened within the regional
groundwater system. The distinction between the perched
and regional aquifer system is made on the basis of nested
bores 14400088 and 14400087 in this study. On the day of
sampling, the hydraulic head in bore 14400088 was 58 m
whereas that in bore 14400087 was 35 m. This 20 m hy-
draulic head difference indicates the presence of a perch-
ing layer. Bores 14400151 and 14400152 are located in
the coastal beach/shore dunes between the Eighteen Mile
Swamp wetland and the ocean on the eastern side of the is-
land.
Hydraulic heads in the regional aquifer are highest ( 
35 m AHD) in the centre of the island, and the unsaturated
zone is approximately 30 to 60 m thick. The hydraulic head
values decline towards the coasts and reach sea level in Eigh-
teen Mile Swamp in the east and close to sea level on the
western side near Dunwich. Decadal fluctuations of regional
groundwater heads are less than 0.5 m to approximately 5 m.
These are not correlated with yearly rainfall and, therefore,
represent longer timescale climate fluctuations (Fig. 2).
4.2 Major ion chemistry and stable isotopes
Electrical conductivity (EC) of the groundwater is gener-
ally low across the island, ranging from 57 to 257 ?S cm 1
with an average of 123 ?S cm 1; TDS (total dissolved solids)
values range from 44 to 174 mg L 1 with an average of
91 mg L 1 (Table A1). These values are similar to the av-
erage TDS value (78.1 mg L 1) across the remaining parts of
the island (Queensland ? Government ? Data, 2018). Higher
salinities generally occur closer to the coast, whereas the
freshest groundwater is found in the central parts of the is-
land. The high hydraulic gradient from the centre of the is-
land towards the coastal areas inhibits an extensive saltwater
wedge developing underneath the island, and only shallow
areas in beach dunes and intertidal areas have marine ground-
water. Most of the groundwater is acidic due to the limited
buffering capacity of the relatively clean quartz sands with
pH values ranging from 3.6 to 7.5 with an average of 4.9.
Most of the groundwater is oxygenated with dissolved oxy-
gen concentrations ranging from 0.2 to 2.6 mg L 1.
In general, the geochemistry of the groundwater shows
only minor variations. Most of the samples (74 %) are Na?
Cl type groundwater, 21 % are Ca?Na?HCO3 groundwa-
ter and 5 % are Ca?HCO3 groundwater (Fig. A3). Na con-
centrations range from 9.5 to 29.8 mg L 1, Ca concentra-
tions from 0.1 to 14.1 mg L 1, Mg concentrations range
from 0.9 to 5.8 mg L 1 and K concentrations are gen-
erally below 1.2 mg L 1. Cl concentrations range from
15.8 to 46.3 mg L 1, HCO3 concentrations range from
0.4 to 47.7 mg L 1, SO4 concentrations range from 0.2 to
21.8 mg L 1 and NO3 concentrations are <0:5 mg L 1 in
most groundwater (Table A1). Groundwater samples from
the two wonky holes have a similar major ion composition to
the inland groundwater. EC values are 98 and 121 ?S cm 1,
and pH values range from 6.8 to 7.0 (Table A1). Molar Na=Cl
ratios are close to those of seawater (0.86) with a maximum
value of 1.2. Mean molar Cl=Br ratios are close to the av-
erage Cl=Br ratio of ocean water and coastal precipitation
of 650 (Davis et al., 1998) (Fig. 3a). Ca=HCO3 ratios range
from 0.01 to 0.96 with higher ratios generally occurring to-
wards the coasts.
The  18O and  2H values of inland groundwater have
a small range from  5:4 ? to  2:4 ? and  32:3 ? to
 24:4 ?, respectively (Table A1). The one rainfall sample
has  18O and  2H values of  3:2 ? and  25 ?, which
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H. Hofmann et al.: Groundwater mean residence times of a subtropical barrier sand island 1299
Figure 2. Hydraulic heads (metres Australian Height Datum, m AHD) for four selected groundwater bores, 14400051, 14400075, 14400105
and 14400153A, and yearly rainfall for the period from 2000 to 2018.
are slightly higher than those of average rainfall for Bris-
bane ( 18O of 3:98 ? and  2H of 18:4 ? (Hollins et al.,
2018). The  18O and  2H values of both wonky hole sam-
ples are 4:6 ? and 23 ?, which are within the range of
the inland groundwater. All waters plot close to the Brisbane
meteoric water line (Crosbie et al., 2012; IAEA ? Global Net-
work of Isotopes in Pricipitation, 2017; Hollins et al., 2018;
Fig. 4 in this paper).
4.3 3H, 14C and 13C
3H activities of groundwater range from below detection
(<0:05 TU) to 1.0 TU (Table A1). They are much lower
than the mean annual 3H activities in modern precipitation in
Brisbane of approximately 1.6?2.0 TU (Tadros et al., 2014).
3H activities generally decrease with depth (Fig. 5a) and in-
crease with distance from the centre of the island towards
the coastlines (Fig. 5b). The decrease is more pronounced
towards the east coast then towards the west coast. Ground-
water from bore 14400134, which is the deepest bore at
131 m b.g.l. in the central part of the island, and the ?Test
Hole C? bore, which is located close to the west coast at a
depth of 43 m b.g.l. (Fig. 1), have 3H activities that are below
detection. The water discharging from the two wonky holes
also has low 3H activities of 0.12 and 0.15 TU. The highest
3H activity of 1 TU is from groundwater in bore 14400088,
which is the shallow bore (4.6 m) in the perched aquifer sys-
tem close to Brown Lake that was discussed above.
Most 14C concentrations range from 59 to 111 pMC with
the majority of samples having 14C concentrations above
90 pMC. The lowest 14C concentrations are in groundwater
from bore 14400134 (77 pMC) and Test Hole C (63 pMC)
(Fig. 6a, b), which also have the lowest 3H activities.
Groundwater from the wonky holes has 14C concentrations
of 81 and 59 pMC. Generally, the high 3H activities and high
14C concentrations of most of the groundwater imply that it
was recently recharged.
 13C values of DIC (Fig. 6) range from  25 ? to
 10:7 ?. Most of the waters have  13C values in the range
between 24 ? and 18 ?, which are within the expected
range of  13C values of DIC derived from the dissolu-
tion of soil CO2 in an environment dominated by C3 veg-
etation (Clark and Fritz, 1997). The highest  13C values
are from groundwater from bores 14400112 ( 10:6 ?) and
14400152 ( 10:7 ?) on the eastern side of the island at
Eighteen Mile Swamp and the beach dunes. Groundwa-
ter from the shallow bore in the perched aquifer system,
bore 14400088, has the lowest  13C value of 25 ?. Water
from both wonky holes has a 13C value of 12 ? (Fig. 6a).
5 Discussion
The combined hydraulic head and geochemistry data al-
low the conceptualisation of groundwater flow across North
Stradbroke Island. Groundwater flows from the centre of
the island towards the east and the west coasts driven by
the large hydraulic gradient. There is groundwater discharge
into freshwater wetlands (e.g. Eighteen Mile Swamp), some
submarine groundwater discharge via the wonky holes into
Moreton Bay and probably offshore on the eastern side of
the island (Fig. 1). The sand dunes are thickest in the cen-
tral dune field, and the thickness gradually declines towards
both coastlines. The thinning of the aquifer and the unsatu-
rated zone allows the mixing of groundwater from the centre
of the island with more recent recharge closer to the coast
towards the discharge zones.
Most of the groundwater has low TDS and low pH. This,
on combination with the low Na=Cl ratios (<1:2), suggests
that only minor silicate weathering is occurring, which is lim-
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1300 H. Hofmann et al.: Groundwater mean residence times of a subtropical barrier sand island
Figure 3. Geochemistry of surface water and groundwater from the bores and wonky holes. (a) Na vs. Cl, (b) Ca vs. Cl, (c) Na/Cl vs. Cl,
(d) Cl/Br vs. Cl, (e) Mg vs. Cl, (f) Mg vs. SO4, (g) Ca vs. HCO3 and (h) NO3 vs. TDS.
ited by the availability of weatherable silicates. The linear
relationship between Ca and HCO3 (Fig. 3g) suggests that
some carbonate weathering has occurred. However, the ob-
servation that  13C values are generally similar to those ex-
pected for DIC derived from the soil zone implies that this is
limited. SO4 and Cl concentrations increase near the coast-
line, suggesting some marine influence (sea spray) in the
coastal lowlands of the island. However, the salinity remains
low, supporting the argument that the saltwater wedge under-
neath the island is relatively deep and close to the bedrock.
5.1 Mean groundwater residence times
The choice of the best fit lumped parameter model requires
conceptualisation of the flow system. The exponential pis-
ton flow model is commonly used to calculate MRTs in flow
systems that have near-vertical recharge through the unsat-
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H. Hofmann et al.: Groundwater mean residence times of a subtropical barrier sand island 1301
Figure 4.  18O and  2H values of groundwater, surface water and
water from the wonky holes on North Stradbroke Island. The black
point represents the rainfall-weighted average composition of  18O
and 2H for rainfall from the Global Network of Isotopes in Precipi-
tation for Brisbane from 1962 to 2014 (Hollins et al., 2018). LMWL
denotes the local meteoric water line for Brisbane, and GMWL
refers to the global meteoric water line
urated zone overlying a flow system with an approximately
exponential distribution of flow paths (Jurgens et al., 2012;
Morgenstern and Daughney, 2012). However, the exponen-
tial piston flow model assumes that the entire aquifer thick-
ness is sampled, which is not the case. Here we use the partial
exponential model and calculate the PEM ratio for each bore
using the screen width and depth below the water table. This
model assumes that the flow system is exponential and, there-
fore, ignores the likelihood of piston flow in the unsaturated
zone. However, as the thickest unsaturated zone is approxi-
mately 60 m and the total flow path length is 5000 m, the
proportion of the piston flow component is very small.
MRTs calculated using the partial exponential flow model
range from 37 to more than 150 years. Below background 3H
activities in groundwater samples from Test Hole C and bore
14400134 suggest MRTs outside of the age range of tritium
(Table A2). Deep groundwater in the centre of the island
where the sand dunes overlay the bedrock has MRTs of more
than 150 years (Fig. 8b). The MRTs in the eastern part of the
island are generally younger (50?124 years) than in the west-
ern part (37?>150 years). There are several uncertainties in
estimating MRTs. Varying the 3H activity of modern rainfall
between 1.6 and 2.0 TU has little effect on the MRT estimate
with less than a 1 % difference in MRT. Macroscopic mix-
ing within aquifers (aggregation) may affect MRTs (Stewart
et al., 2017), but this mainly occurs where bimodal mixing of
young and old waters occurs. The more complex mixing of
water of different MRTs in aquifers reduces the error associ-
ated with aggregation, as this is similar to the flow simulated
by lumped parameter models (Cartwright and Morgenstern,
2016). The main uncertainty in the MRT calculations, espe-
cially for older waters, is the choice of lumped parameter
model (Cartwright et al., 2018). While the partial exponential
model accords with the conceptualisation of the flow system,
it will not represent it in detail. To demonstrate the effect of
LPM choice, MRTs were also calculated using the dispersion
model with dispersion parameters of 0.05 (mostly advection)
and 0.5, as well as an exponential piston flow model with a
piston flow to exponential flow ratio of 0.5. The difference in
MRTs between the models is low when 3H activities are high
(>1 TU) but increase markedly as 3H activities decrease. For
3H activities less than 0.2 TU, the range of MRTs is from 75
to 290 years, indicating that MRTs cannot be reliably cal-
culated. Additionally, low-3H waters are more susceptible to
contamination during sampling or analysis. While determin-
ing MRTs is subject to uncertainties, the relative distribution
of older and younger water does not change according to
which lumped parameter model is used (Fig. 7a).
While groundwater was expected to have young MRTs,
some of the groundwater has relatively low 14C activities.
Estimating MRTs using 14C activities requires that the ad-
dition of 14C-free carbon from the groundwater flow sys-
tem be accounted for. Significant addition of 14C-free carbon
may dilute the 14C activities, potentially resulting in MRTs
being overestimated (Coetsiers and Walraevens, 2009). Ma-
jor ion geochemistry and the  13C values of DIC indicate
minor calcite dissolution in some of the groundwater. The
proportion of 14C derived from recharge (q) is calculated
from a  13C mass balance using the measured  13C of the
groundwater samples and estimated  13C value for DIC in
recharge and carbonates (Clark and Fritz, 1997; Cartwright
et al., 2017; Hofmann and Cartwright, 2013). Most of the
 13C values of DIC are close to those expected from the dis-
solution of CO2 in soils dominated by C3 vegetation, and
q values are close to 0.95 (with a range of 0.61?1.0 and
a median of 0.95) (Clark and Fritz, 1997). These q values
are higher than those generally proposed for sediments con-
taining fine-grained carbonates (0.75?0.9) and more simi-
lar to those in silicate-dominated crystalline rocks (0.9?1.0)
(Clark and Fritz, 1997). They are consistent, however, with
silica-rich sands and the limited carbonate dissolution im-
plied by the geochemistry. The Ca and HCO3 concentrations
in groundwater are also similar to those in groundwater from
crystalline rocks (Tweed et al., 2005; Le Gal La Salle et al.,
2001; Coetsiers and Walraevens, 2009). Groundwater from
bores 14400152 and 14400112, Test Hole C and the wonky
holes has higher  13C values, and Ca and HCO3 concentra-
tions and may record a higher degree of calcite dissolution.
In addition to calcite dissolution, microbial degradation of
organic matter in the aquifer or the very low-permeability
sediments around the lakes and wetlands may contribute 14C-
free carbon (Table A2). Most of the deeper groundwater has
little dissolved organic carbon (DOC ? which is assessed
by the clarity of the water) but the groundwater recharged
through the peat-rich wetlands often contains larger amounts
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1302 H. Hofmann et al.: Groundwater mean residence times of a subtropical barrier sand island
Figure 5. (a) Variation in 3H activities with bore depth, (b) 3H (tritium) activities vs. distance from the centre of the island, which is
approximately the groundwater flow divide between westward and eastward flow.
Figure 6. (a) 14C concentration vs.  13C values of groundwater and water from the wonky holes. The shaded grey area represents the
predicted range of  13C values of DIC in the soils. (b) 14C concentrations vs. 3H activities of groundwater and the water from the wonky
holes. The curves show the predicted covariance from the lumped parameter models: PEM represents the partial exponential model with
dilution factors ofqD0:95 andqD0:85, DM represents the dispersion model (with dispersion parameters of 0.05 and 0.5), EPM represents
the exponential piston flow model (with an EPM factor of 0.8).
of DOC. The breakdown of organic matter as an origin of
14C-free carbon is more likely, but it would not have oc-
curred in situ where the samples were taken for a number
of reasons: (1) most of the waters are slightly oxygenated;
(2) while there is very little NO3, concentrations of SO4 are
relatively high; and (3) the isotopic shift of  13C values to-
wards more enriched values would be more pronounced. It
seems more likely that organic matter degradation processes
occurred in the peat sediments around lakes and wetlands,
and the seepage from those mixed with existing water in the
sand aquifer. The maximum amount of dilution by 14C-free
carbon derived from calcite dissolution organic matter may
be estimated from the covariance of 3H and 14C (Fig. 6;
Cartwright et al., 2013). Reducing q values displaces the co-
variance curves to lower 14C. While mixing with old ground-
water can result in waters lying to the left of the covariance
curves, it is not possible for waters to have higher 14C as that
would require the initial 14C activity to be greater than that
recorded in the atmosphere. In the case of Stradbroke Island
groundwater, this implies that q values cannot be substan-
tially lower than 0.8.
MRTs were calculated by adjusting the 14C input function
using q values of between 0.85 and 0.95. Some of the ad-
justed groundwater 14C MRTs are younger than 200 years
and are therefore considered to be modern (Fig. 7b). MRTs
calculated using the partial exponential model range from
modern to 4800 years. MRTs of groundwater are generally
higher on the eastern side of the island. The MRTs in the
wonky holes (Wonky Hole South 4800 years and Wonky
Hole North 1600 years), Test Hole C (4100 years), and bores
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H. Hofmann et al.: Groundwater mean residence times of a subtropical barrier sand island 1303
Figure 7. (a) Comparison of calculated MRTs for the partial exponential model (PEM), the dispersion model (DM with dispersion parameters
of 0.05 and 0.5) and the exponential piston flow model (EPM) using 3H. (b) Comparison of calculated MRTs for the partial exponential
model (PEM), the dispersion model (DM with dispersion parameters of 0.5 and 0.05) and conventionally calculated carbon ages using the
decay function. The model MRTs were also calculated using 14C dilution models with q factors of 0.95 and 0.85.
14400134 (2100 years), 14400092 (1600 years), 14400105
(1300 years) and 1440075 (1300 years) are older than the
majority of groundwater in the main aquifer system. The
partial exponential flow model produces the youngest MRTs
using 14C (Fig. 7b; Table A2). Differences are more pro-
nounced in the older waters with a maximum of 1100 years
difference (partial exponential model vs. dispersion model
with a dispersion parameter of 0.5), but relative MRT dis-
tributions across the models remain the same. Applying the
dilution factors of 0.85 and 0.95 to the calculations has a
much stronger effect on the MRTs. The range of MRTs of
groundwater using the partial exponential model with the
dilution factor of 0.85, for example, is between modern
and 3100 years, with Wonky Hole South, Test Hole C, and
bores 14400134 and 14400092 still having the largest MRTs
of 3100, 2500, 750 and 420 years, respectively.
5.2 Disparities in groundwater MRTs
The calculated groundwater MRTs from the radioisotope
tracers are generally higher than what was initially hypoth-
esised. Existing MRTs were based on flow rates estimated
using the regional groundwater model (Leach and Gallagher,
2013). This assumed lateral hydraulic conductivities of 1 to
40 m d 1 based on Leach and Gallagher (2013). The large
variance in hydraulic conductivities comes from the inclu-
sions of isolated peat and clay layers (Leach and Gallagher,
2013). By contrast, hydraulic conductivities calculated via
Darcy?s law with velocities from the 14C MRTs, porosities
from Leach (2011) and the measured hydraulic gradients are
generally between 0.25 and 1 m d 1. There are two likely ex-
planations for these lower hydraulic conductivities: (1) some
groundwater discharge to the unconfined sand aquifer from
the basement units underneath the sand dunes, and (2) a
larger volume of geological units with lower permeability,
such as peat, coffee rock and clay, which have more control
on groundwater flow than prior studies have suggested.
5.3 Potential influence of the geological basement
The geological basement underneath the island is comprised
of the Woogaroo Subgroup, Rocksberg Greenstone and rhy-
olitic intrusions. A regional evaluation of aquifer storage
and retention for south-east Queensland identified aquifers
within these units and summarised some of the general hy-
drogeological characteristics, such as permeabilities, trans-
missivities and general groundwater flow (Helm et al., 2009).
In the case of the Rocksberg Greenstone, water is stored in
fractures infilled with clay, and the typical yield is less than
0.2 L s 1. The Woogaroo Subgroup sandstone has a typical
yield ranging from less than 1.5 to 6 L s 1 and has been
identified to have a moderate groundwater storage poten-
tial (Neuman, 2005). There is a possibility that the under-
lying basement units on the island are connected to the lower
sand dunes and groundwater enters the young dune water
from these formations. The basement isopach, which was
extrapolated from the regional geological model for south-
east Queensland, indicates the proximity of each sampling
location to the bedrock (Fig. 8b). The closer the groundwater
samples are to the basement geology, the older their respec-
tive MRTs. This in itself is not a compelling argument, as it
is also in line with the progression of MRTs from shallow
to deeper sections of the aquifer; however, higher MRTs in
the west of the island where the bedrock is shallow might
indicate a small degree of discharge from the basement to
the dune aquifer. To the best of our knowledge, there are no
groundwater bores in the basement on the island, but general
groundwater heads on the mainland suggest that this could
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1304 H. Hofmann et al.: Groundwater mean residence times of a subtropical barrier sand island
Figure 8. (a) Vertically exaggerated cross section along the sample transect with bore locations, depth, pMC (black), and tritium activities
(red). (b) Estimated MRTs calculated for tritium activities (rainfall 1.6 TU) with an exponential piston flow model, and 14C ages after
correction (qD0:95).
be a possibility; however, the volumes would be minor, if ex-
istent, compared with the volume of the sand aquifer.
5.4 Potential influence of lower-permeability units
on MRTs
Over the course of the Quaternary, the accumulation of sand
along the coast of south-east Queensland formed the large
sand masses in the area. The dune formation came with pe-
riods of dune migration and relatively stable periods where
dunes were stagnant (Barr et al., 2013; Ellerton et al., 2018).
During these periods, the decomposition of organic material,
the weathering of minerals and fluctuations in the water ta-
ble with subsequent redox reactions have led to the formation
of lower-permeability units within the otherwise relatively
permeable dune systems. The geomorphological, geochem-
ical and environmental processes around the formation of
these lower-permeability units are not entirely constrained,
but they are often linked to pedogenic processes or the ac-
cumulation of fine materials in surface depressions. They led
to the occurrence of the aforementioned iron crusts, coffee
rocks and thick peat sections in the dune stratigraphy. The
iron crusts in particular are believed to be linked to the soil
B horizons and are expected to divert groundwater flow in
some areas of the island, whereas the coffee rocks and peats
are formed around wetland and lakes (Cend?n et al., 2014).
All of these formations most likely have intermediate wa-
ter storages with much higher MRTs than the sand aquifer.
Permeabilities for the iron crusts are not known, and thick-
nesses are variable; however, some peat formations are more
than 10 m thick, and permeabilities in the peat decline rapidly
once a certain thickness (below a depth of 2?3 m) is reached,
resulting in the storage of water over long residence times
(millennial). Slow leakage or seepage, particularly with hy-
draulic loading under increased recharge events may add wa-
ter with a very long MRT to the general water pool in the
sand aquifer system. While this is a potential cause of mixed
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H. Hofmann et al.: Groundwater mean residence times of a subtropical barrier sand island 1305
groundwater from sources with very distinctly different res-
idence times, it would most likely occur in the proximity
of these peat units. There is proof of this type of scenario
when comparing bores 14400114 and 14400112: two nested
bores on the edge of Eighteen Mile Swamp, a large freshwa-
ter peatland on the eastern side of the island. Bore 14400114
is the shallower bore but has a much older MRT (pMC of
91) than the deeper bore 14400112 (pMC of 105). A com-
parison of major ion and trace element concentrations indi-
cates that the groundwater sample from bore 14400114 has
uncharacteristically high concentrations of Na, Mg, Sr, Si,
Mn, Fe, Cl, Br and HCO3. Almendinger and Leete (1998)
found elevations in the same major ions and trace elements
when comparing wells above peat layers to wells below peat
layers. The high enrichment of elements seen in this sample
and its disparity to other MRTs suggests that this bore re-
ceives groundwater from the peat unit forming Eighteen Mile
Swamp, where the hydraulic conductivities are relatively low
and the concentrations of the aforementioned ions and ele-
ments are high.
5.5 Conceptualisation of groundwater flow
Variations in groundwater MRTs estimated from 3H and 14C
throughout North Stradbroke Island support the current idea
of groundwater flow from the centre of the island towards
both coasts. There is also robust evidence that groundwa-
ter has a large vertical flow component (Fig. 9) with MRTs
generally increasing with depth (Fig. 8a, b). While the MRT
variation is not as strongly pronounced in the horizontal flow
direction, the combination of known hydraulic gradients and
known areas of discharge support the idea that lateral flow
is consistent with MRT distributions. Modelling by Chen
et al. (2003) and Leach and Gallagher (2013) suggests that
groundwater on North Stradbroke island is discharged via
(a) movement into coastal wetlands (55 %), (b) submarine
discharge around the coastline (27 %), (c) groundwater ex-
traction (16 %) and (d) discharge into Blue Lake (2 %) (Chen
et al., 2003). The conceptual model of groundwater flow
along the sampled transect is divided into two areas: an area
where groundwater flows to the west and an area where it
flows to the to the east (Fig. 9). There are minor flow diver-
sions to the north and the south, resulting from the centre of
the transect also being the highest elevation on the island ?
topography drops from there in all four directions. It is as-
sumed that recharge occurs across the whole island and, as
such, recent recharge is added to the lateral groundwater flow
at all points along the cross section.
The east coast of the island is comprised of a large fresh-
water wetland system at the foothills of the major dune
system. This wetland, Eighteen Mile Swamp, separates the
major Quaternary dunes from the foredunes. Groundwa-
ter from the main dunes discharges into the wetland and
partially re-enters into the foredunes along the coastline
(Fig. 9). However, it appears that there are at least two
aquifer systems that are separated by a lower-permeability
unit. This lower-permeability unit was identified as a ma-
rine clay underneath the peat sequences of Eighteen Mile
Swamp by Mettam et al. (2011). Nested bores 14400151
(67 m) and 14400152 (17 m) indicate younger MRTs and
higher salinity (173 mg L 1 TDS) in the shallow part of the
foredune system and older, lower salinity in the deeper parts;
this is also reflected in bore 14400051 in the main dune sys-
tem (Fig. 1). Moreover, there is an upward gradient indicat-
ing a hydraulic connection of the deeper foredune to the re-
gional groundwater system and potentially to Eighteen Mile
Swamp (Fig. 9).
The west coast of the island generally has lower topo-
graphic gradients and extended salt marshes that continue
into the mangrove tidal areas of Moreton Bay. Sequences of
fine, organic-matter-rich muds overlay the dune sands and
lead to semi-confined conditions in the sand aquifer in the di-
rect proximity of the coast underneath the muds (Fig. 9). The
wonky holes are circular disturbances in the mud sequences
where fresh groundwater from the island?s sand dunes dis-
charges into the saltwater environment of the bay. The con-
trols surrounding the formation of these discharge points is
unknown, but an upward head gradient in the underlying con-
fined system, bioturbation and heterogeneities in the mud
sediments possibly created an opening to the surface. The
long MRT of water discharging through the wonky holes sug-
gest that most of the water derives from the deeper confined
sand aquifer units that are linked to the centre of the island
(Fig. 9).
The results of this study indicate that the perched ground-
water systems have a significant effect on groundwater flow,
recharge inhibition and intermediate water storage. While
this study focuses on a transect of the island, similar stratig-
raphy and dune heterogeneity is widespread across North
Stradbroke Island (Leon Leach, personal communication,
2018) and across all of the sand masses on the east coast of
Australia (Ellerton et al., 2018), which highlights the trans-
ferability of this study. Many lake and wetland systems ex-
ist in this environment around perched aquifer system and
are sometimes the cause of the perching layer formation. De-
pending on the age and location of the perched systems, the
perching layer varies largely in thickness. Permeabilities de-
crease with thickness, and some of the systems may have
very slow flow velocities through the perching layers, result-
ing in groundwater that potentially has a MRT of thousands
to tens of thousands of years leaking into the main aquifers.
Even small amounts of leakage from these systems have the
ability to lower the overall groundwater MRT.
The groundwater MRTs observed in this study differ from
those found in similar studies of barrier island groundwater
systems, as the MRTs in the North Stradbroke Island aquifer
systems are less consistently distributed from the centre of
the island towards the coastal discharge areas (R?per et al.,
2012; Houben et al., 2014; Seibert et al., 2018; Holt et al.,
2019). The reason for this is probably the age of the dune
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1306 H. Hofmann et al.: Groundwater mean residence times of a subtropical barrier sand island
Figure 9. Conceptual sketch of groundwater flow across the island from the centre of the island to the ocean in the east and to Moreton Bay in
the west. Parts of the centrally recharged groundwater also flow to the north and the south (Leach and Gallagher, 2013). Lower-permeability
units in the unsaturated zone can have longer residence times than direct recharge. There may also be groundwater contributions from the
bedrock. On the western side, some of the groundwater is discharged directly into Moreton Bay, while some flows underneath the tidal mud
flats in semi-confined conditions and is discharged partly through wonky holes into the bay.
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H. Hofmann et al.: Groundwater mean residence times of a subtropical barrier sand island 1307
systems. Older dunes systems have undergone stable periods
with pedogenesis, especially in subtropical and tropical envi-
ronments, and periods of dune migration, burying former soil
horizons in the deeper units of the dune stratigraphy. This
leads to a more heterogeneous dune stratigraphy with the in-
clusion of peat, clay and indurated sands compared with the
relatively young, homogeneous Holocene islands mentioned
in the studies above. Furthermore, the island is much larger
with a more variable topography, geomorphology and denser
vegetation than the islands described in R?per et al. (2012),
Houben et al. (2014), Seibert et al. (2018) and Holt et al.
(2019). The best comparison to this study is the study by
Stuyfzand (1993) where MRTs were estimated using a sim-
ple analytical solution. However, this method revealed much
shorter MRTs in a dune system with similar stratigraphic
complexity and dune ages to North Stradbroke Island.
6 Conclusions
A combination of cosmogenic isotopes, major ion chemistry
and stable isotope geochemistry was used to conceptualise
groundwater flow on North Stradbroke Island, with varying
groundwater MRTs in different parts of the island, and to de-
termine groundwater flow paths through the aquifer systems.
MRTs estimated using 3H indicate a strong vertical strati-
fication from 37 years to more than 150 years. 14C MRTs dis-
play similar temporal relationships with much greater ranges.
This MRT discrepancy is attributed to different groundwa-
ter reservoirs. This study did not produce evidence for con-
tributions from the fractured Woogaroo Subgroup sandstone
aquifer, but the possibility remains. Water diversion and re-
tention by low-permeability units in the dune systems are
currently the most likely course for relatively long MRTs.
The geochemical composition of groundwater remains rela-
tively consistent throughout the island, with the only irreg-
ularities attributed to old groundwater stored within coastal
peat. The stable isotope composition of North Stradbroke Is-
land groundwater is similar to Brisbane precipitation without
any indication of evaporative enrichment. The outcomes of
this study can be incorporated in regional groundwater flow
models to refine the potential inhibition and retardation of
recharge to test models? validity. The position of the islands
large fresh water reservoir in dry and populous south-east
Queensland means its potential to be used as a water resource
is always high; thus, background information on aquifer dis-
tribution and groundwater MRTs is crucial to better validate
impact assessment for water abstraction.
www.hydrol-earth-syst-sci.net/24/1293/2020/ Hydrol. Earth Syst. Sci., 24, 1293?1318, 2020
1308 H. Hofmann et al.: Groundwater mean residence times of a subtropical barrier sand island
Appendix A
Figure A1. North Stradbroke Island wetland with typical vegetation of eucalypt species on the surrounding dunes.
Figure A2. The North Stradbroke Island east coast at Point Lockout at the northern tip of the island looking south-west. The dunes in the
background are covered by typical native eucalypt forest.
Hydrol. Earth Syst. Sci., 24, 1293?1318, 2020 www.hydrol-earth-syst-sci.net/24/1293/2020/
H. Hofmann et al.: Groundwater mean residence times of a subtropical barrier sand island 1309
Figure A3. Piper diagram showing inland groundwater samples (red), samples from the two wonky holes (yellow) and the rain water sample
(blue).
www.hydrol-earth-syst-sci.net/24/1293/2020/ Hydrol. Earth Syst. Sci., 24, 1293?1318, 2020
1310 H. Hofmann et al.: Groundwater mean residence times of a subtropical barrier sand island
Table
A1.
Site
information,
chemistry
data
and
major
ion
data.
?Lat?
stands
for
latitude,
?Long?
is
longitude,
?W
ater
lev
el?
is
the
water
lev
el
belo
w
the
natural
surf
ace,
?Hydr
.he
ad?
is
the
hydraulic
head,
Cl
=Br
and
Na
=Cl
are
molar
ratios,
CBE
is
the
char
ge
balance
error
,?n/a?
is
not
applicable,
?nd?
is
not
determined
and
?bd?
is
belo
w
detection.
RN
Lat
Long
Bore
depth
Water
lev
el
Hydr
.head
Temp
pH
TDS
DO
ORP
EC
 18
O
 2
H
Units
( )
( )
(m)
(m)
(m)
( C)
(mg
L 
1)
(mg
L 
1)
(mv)
(?S
cm
 1
)
(?)
(?)
14400051
 27
:521831
153.496183
36.5
0.87
5.7
22.6
5.3
71
1.94
206.3
124
 4
:9
 23
:5
14400067
 27
:517191
153.488406
59.0
30.45
15.9
24.6
4.4
56
2.17
238.8
94
 4
:7
 22
:3
14400075
 27
:501655
153.436704
67.5
42.23
29.0
27.1
5.1
106
0.5
98.7
104
 4
:6
 22
:0
14400087
 27
:497494
153.430401
73.9
24.77
35.9
21.4
3.9
98
1.38
187.3
67
 3
:7
 19
:6
14400088
 27
:497503
153.430370
4.6
2.13
58.3
19.1
3.7
44
1.67
74.8
77
 4
:2
 14
:1
14400091
 27
:500095
153.430138
36.3
28.90
35.4
22.9
4.3
63
0.56
224.6
57
 5
:4
 15
:9
14400092
 27
:500158
153.430148
84.0
28.90
34.9
23.0
5.2
92
1.39
1.5
105
 4
:4
 21
:7
14400093
 27
:500231
153.430158
57.0
28.96
35.0
23.0
5.4
97
1.31
 19
:7
105
 3
:8
 18
:6
14400094
 27
:497349
153.430613
45.8
25.38
35.7
21.6
4.4
78
1.24
35.5
66
 3
:6
 15
:9
14400105
 27
:484070
153.412593
9.4
3.18
12.5
21.5
4.0
173
0.74
 7
:6
150
 3
:6
 18
:8
14400112
 27
:522101
153.498858
36.7
1.78
4.3
20.4
5.1
219
2.26
 52
:5
257
 3
:1
 14
:1
14400114
 27
:522119
153.498868
14.5
1.72
4.4
22.3
4.4
68
2.51
206
124
 4
:3
 22
:3
14400133
 27
:515712
153.485342
92.1
36.48
42.1
22.9
4.9
50
1.85
233.5
73
 4
:2
 21
:4
14400134
 27
:508440
153.446380
131.0
63.46
34.5
24.6
4.5
67
0.84
204.9
75
 4
:4
 21
:4
14400151
 27
:522841
153.501403
66.9
3.10
2.3
nd
nd
59
nd
nd
nd
 4
:1
 23
:4
14400152
 27
:522850
153.501372
16.9
3.62
1.8
21.1
5.8
174
2.6
62
220
-4.0
-20.1
14400153A
 27
:510026
153.469168
71.5
8.91
31.0
22.0
4.2
62
1
217.1
217.1
 4
:3
 21
:8
Test
Hole
C
 27
:505453
153.413512
43.5
13.60
21.4
23.0
5.4
104
0.94
79.8
114
 2
:4
 11
:7
Eight.
Mile
Sw
amp
 27
:521715
153.497049
nd
nd
0.0
24.2
7.5
115
1.47
3.2
199
 3
:2
 13
:7
Rain
 27
:496514
153.400427
n/a
n/a
n/a
n/a
n/a
49
nd
nd
nd
0.2
 16
:0
Wonk
yH.
South
 27
:451400
153.428417
0.0
n/a
nd
23.5
6.8
nd
2.46
30.7
98
 4
:6
 23
:0
Wonk
yH.
North
 27
:450828
153.428233
0.0
n/a
nd
23.9
7.0
nd
2.6
34.9
121
 4
:7
 23
:0
Hydrol. Earth Syst. Sci., 24, 1293?1318, 2020 www.hydrol-earth-syst-sci.net/24/1293/2020/
H. Hofmann et al.: Groundwater mean residence times of a subtropical barrier sand island 1311
Table
A1.
Continued.
RN
 13
C
14
C
14
C
error
3H
3H
error
HCO
3
F
Cl
Br
NO
3
SO
4
Na
Mg
Units
(?)
(pMC)
(1
s)
(TU)
(1
s)
(mg
L 
1)
(mg
L 
1)
(mg
L 
1)
(mg
L 
1)
(mg
L 
1)
(mg
L 
1)
(mg
L 
1)
(mg
L 
1)
14400051
 21
:2
97.65
0.27
0.56
0.04
1.7
bd
26.91
0.09
0.51
3.74
16.51
1.75
14400067
 21
:9
98.75
0.26
0.60
0.04
0.4
bd
19.84
0.06
0.41
3.46
12.34
1.42
14400075
 21
:7
84.29
0.27
0.06
0.03
25.9
0.02
24.51
0.07
1.71
3.89
14.93
1.41
14400087
 23
:9
111.38
0.33
0.58
0.04
30.8
bd
16.51
0.06
0.16
2.54
10.90
0.88
14400088
 25
:0
103.68
0.29
1.00
0.05
0.4
bd
15.87
0.05
0.25
0.42
9.64
1.27
14400091
 24
:2
101.45
0.27
0.90
0.05
16
bd
15.76
0.09
0.09
1.23
9.48
1.12
14400092
 19
:3
80.81
0.24
0.17
0.03
12
0.01
23.26
0.07
0.30
2.75
13.92
0.93
14400093
 22
:0
91.50
0.25
0.58
0.04
21.2
bd
17.01
0.06
0.03
1.08
12.45
2.04
14400094
 24
:0
103.85
0.27
0.93
0.05
22.4
bd
15.89
0.07
0.04
0.68
9.85
1.37
14400105
 24
:1
84.09
0.27
0.99
0.05
44.3
bd
30.35
0.15
0.04
21.76
21.54
3.43
14400112
 10
:6
104.83
0.32
0.67
0.04
47.7
0.03
46.33
0.21
0.07
0.15
29.76
5.82
14400114
 22
:2
90.99
0.28
0.34
0.03
0.4
bd
26.95
0.09
0.39
3.44
16.07
1.74
14400133
 21
:9
95.67
0.26
0.44
0.04
0.8
bd
16.98
0.06
0.45
2.11
9.96
1.20
14400134
 22
:0
76.56
0.24
0.01
0.02
13
0.01
18.20
0.07
0.09
3.28
11.17
1.25
14400151
 21
:2
91.32
0.32
0.23
0.03
0.9
bd
20.92
0.04
0.69
3.04
12.73
1.44
14400152
 10
:7
91.87
0.31
0.49
0.04
38.8
0.02
33.72
0.14
0.29
4.42
20.84
3.09
14400153A
 21
:3
92.71
0.32
0.08
0.03
0.9
bd
22.19
0.07
0.89
3.61
13.01
1.71
Test
Hole
C
 17
:7
62.54
0.22
0.01
0.02
20.6
0.08
19.48
0.07
0.01
1.69
15.62
2.38
Eight.
Mile
Sw
amp
nd
nd
nd
nd
nd
11.5
0.01
41.94
0.13
0.13
3.25
23.77
3.32
Rain
nd
nd
nd
nd
nd
0
0.01
19.36
0.03
3.15
5.15
11.05
1.38
Wonk
yH.
South
 12
:0
58.83
0.21
0.12
0.03
0
0.06
18.07
0.13
0.83
1.99
11.29
1.46
Wonk
yH.
North
 12
:0
80.87
0.23
0.15
0.03
0
0.06
17.83
0.17
1.09
2.04
11.23
1.36
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1312 H. Hofmann et al.: Groundwater mean residence times of a subtropical barrier sand island
Table
A1.
Continued.
RN
K
Ca
Sr
Ba
Al
Si
Mn
Fe
Cl
=Br
Na
=Cl
CBE
Units
(mg
L 
1)
(mg
L 
1)
(mg
L 
1)
(mg
L 
1)
(mg
L 
1)
(mg
L 
1)
(mg
L 
1)
(mg
L 
1)
(molar)
(molar)
(%)
14400051
0.57
0.28
0.01
0.00
0.01
4.52
0.00
0.01
653
0.95
1.39
14400067
0.47
0.24
0.01
0.01
0.01
4.54
0.02
0.02
759
0.96
 1
:03
14400075
0.98
1.72
0.01
0.02
0.10
4.79
0.05
0.22
742
0.94
2.64
14400087
0.25
0.25
0.00
0.00
0.02
4.25
0.01
0.19
645
1.02
 1
:83
14400088
0.39
0.42
0.01
0.00
0.46
1.56
0.00
0.27
788
0.94
 3
:52
14400091
0.35
0.39
0.00
0.00
0.19
1.99
0.00
0.17
379
0.93
2.40
14400092
0.63
3.00
0.02
0.03
0.01
5.51
0.10
4.80
738
0.92
9.33
14400093
0.36
0.76
0.01
0.01
0.09
5.05
0.04
3.31
616
1.13
12.59
14400094
0.35
0.27
0.01
0.00
0.80
2.36
0.00
1.56
518
0.96
 3
:90
14400105
0.40
0.56
0.02
0.01
0.88
3.15
0.01
1.73
451
1.09
 0
:73
14400112
1.10
5.69
0.05
0.00
0.07
13.20
0.18
8.40
492
0.99
10.49
14400114
0.54
0.25
0.01
0.01
0.01
4.62
0.00
0.01
676
0.92
0.70
14400133
0.37
0.07
0.00
0.01
0.05
4.62
0.00
0.01
606
0.9
3.27
14400134
0.70
0.37
0.01
0.02
0.02
5.40
0.03
0.01
608
0.95
 1
:22
14400151
0.74
0.27
0.01
0.01
0.01
4.72
0.01
0.00
1069
0.94
1.04
14400152
0.72
14.10
0.08
0.00
0.05
5.65
0.02
0.50
553
0.95
7.59
14400153A
0.64
0.45
0.01
0.01
0.02
4.69
0.03
0.72
752
0.9
2.20
Test
Hole
C
1.26
3.64
0.04
0.08
0.02
5.68
0.11
0.02
634
1.24
0.29
Eight.
Mile
Sw
amp
0.85
2.27
0.03
0.01
0.00
3.60
0.03
0.04
725
0.87
6.10
Rain
0.83
8.18
0.01
0.01
0.00
0.06
0.00
0.00
1431
0.88
 7
:57
Wonk
yH.
South
0.52
2.12
nd
nd
nd
4.43
0.01
0.00
318
0.95
 5
:58
Wonk
yH.
North
0.49
1.64
nd
nd
nd
4.29
0.01
0.01
237
0.96
 4
:14
Hydrol. Earth Syst. Sci., 24, 1293?1318, 2020 www.hydrol-earth-syst-sci.net/24/1293/2020/
H. Hofmann et al.: Groundwater mean residence times of a subtropical barrier sand island 1313
Table
A2.
3H
residence
times
calculated
with
an
input
function
with
modern
tritium
rainf
all
acti
vities
of
1.6
and
2.0
TU
as
well
as
the
av
erage
between
the
tw
oresults.
14
C
ages
are
reported
as
con
ventional
ages
from
the
laboratory
,and
further
ages
are
calculated
using
astati
stical
qf
actor
of
0.85
(Clark
and
Fritz
,1997
)and
acorrection
factor
of
qD
0:95.
?bd?
denotes
belo
w
detection,
and
?n/a?
refers
to
not
applicable.
Modern
rainf
all
input
2TU
Modern
rainf
all
input
1.6
TU
RN
3H
(TU)
14
C
(pMC)
PEM
ratio
PEM
DM
0.05
DM
0.5
EPM
PEM
DM
0.05
DM
0.5
EPM
14400051
0.56
97.65
0.84
57
58
83
72
57
58
82
72
14400067
0.60
98.75
0.44
58
57
77
69
58
57
76
69
14400075
0.06
84.29
0.20
136
88
290
88
136
88
290
88
14400087
0.58
111.38
0.13
57
57
80
71
57
57
79
71
14400088
1.00
103.68
0.35
37
45
32
34
37
45
27
34
14400091
0.90
101.45
1.24
50
48
41
45
50
48
38
45
14400092
0.17
80.81
0.12
143
75
190
86
143
75
190
86
14400093
0.58
91.50
0.25
57
57
80
71
57
57
79
71
14400094
0.93
103.85
0.37
40
47
38
42
40
47
34
42
14400105
0.99
84.09
0.16
33
45
33
36
32
45
28
36
14400112
0.67
104.83
0.09
50
54
67
64
50
54
66
64
14400114
0.34
90.99
0.24
83
66
130
82
83
66
130
82
14400133
0.44
95.67
0.11
73
62
107
78
73
62
106
78
14400134
0.01
76.56
0.10
bd
bd
bd
bd
bd
bd
bd
bd
14400151
0.23
91.32
0.09
124
71
165
85
124
71
165
85
14400152
0.49
91.87
0.45
65
60
95
76
65
60
95
76
14400153A
0.08
92.71
n/a
200
84
260
88
200
84
260
88
Test
Hole
C
0.01
62.54
n/a
bd
bd
bd
bd
bd
bd
bd
bd
Wonk
ySouth
0.12
58.83
n/a
180
79
220
87
180
79
220
87
Wonk
yNorth
0.15
80.87
n/a
157
77
200
87
157
77
200
87
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1314 H. Hofmann et al.: Groundwater mean residence times of a subtropical barrier sand island
Table
A2.
Continued.
qD
1
qD
0:95
qD
0:85
RN
14
C
(pMC)
Con
v.
ages
PEM
DM
0.5
DM
0.05
EPM
PEM
DM
0.5
DM
0.05
EPM
PEM
DM
0.5
DM
0.05
EPM
14400051
97.65
197
100
320
140
2
9
190
9
9
24
49
23
28
14400067
98.75
104
3
280
3
3
10
170
10
130
26
49
25
32
14400075
84.29
1412
1300
1500
1300
1400
860
1000
910
970
4
270
4
4
14400087
111.38
0
20
28
20
20
28
49
26
44
37
49
42
44
14400088
103.68
0
10
10
10
10
17
20
17
17
32
49
30
44
14400091
101.45
0
7
7
7
7
14
15
14
14
29
49
28
44
14400092
80.81
1761
1600
1900
1700
1800
1300
1400
1200
1300
420
450
460
430
14400093
91.50
734
640
750
710
730
360
380
360
310
15
17
15
15
14400094
103.85
0
10
10
10
10
17
21
17
17
32
49
31
44
14400105
84.09
1432
1300
1500
1300
1400
960
1000
930
990
60
280
4
4
14400112
104.83
0
12
160
80
11
18
24
18
19
33
49
32
44
14400114
90.99
780
730
800
740
780
370
410
400
360
14
130
14
14
14400133
95.67
366
480
410
410
370
6
230
6
6
22
35
21
22
14400134
76.56
2207
2100
2500
2200
2300
1600
2000
1700
1800
750
890
810
860
14400151
91.32
750
720
770
720
750
360
390
370
330
15
16
15
15
14400152
91.87
701
700
710
680
700
220
360
320
260
16
120
16
110
14400153A
92.71
626
610
640
620
630
100
320
140
170
17
20
17
17
Test
Hole
C
62.54
3879
4100
5000
4200
4500
3600
4300
3700
3900
2500
3000
2600
2700
Wonk
yH.
South
58.83
4384
4800
5900
4900
5200
4300
5200
4300
4600
3100
3700
3200
3300
Wonk
yH.
North
80.87
1755
1600
1900
1700
1800
1200
1400
1200
1300
390
450
450
420
Hydrol. Earth Syst. Sci., 24, 1293?1318, 2020 www.hydrol-earth-syst-sci.net/24/1293/2020/
H. Hofmann et al.: Groundwater mean residence times of a subtropical barrier sand island 1315
Data availability. The chemical data are made available in Ta-
bles A1 and A2 in the Appendix of this paper.
Author contributions. Harald Hofmann designed the project. HH,
DN (honours student) and IC executed the field work and data anal-
ysis. HH and IC developed the model. DIC assisted with 3H and
14C analysis and interpretation. MR assisted with conceptualisation
of the geology and the regional context. HH prepared the paper with
contributions from all co-authors.
Competing interests. The authors declare that they have no conflict
of interest.
Acknowledgements. We thank the traditional owners of the land
and water around Moreton Bay and North Stradbroke Island (Min-
jerribah), represented by the Quandamooka Yoolooburrabee Abo-
riginal Corporation (QYAC), for their continuing support and col-
laboration on their land. We also want to thank the staff of The Uni-
versity of Queensland Moreton Bay Research Station in Dunwich
for their continuous support and help in preparing and executing
field work.
Review statement. This paper was edited by Christine Stumpp and
reviewed by two anonymous referees.
Financial support. This research project was partially funded by
the School of Earth Sciences at The University of Queensland and
the School of Earth, Atmosphere and Environment at Monash Uni-
versity. 3H and 14C analyses were funded by the Australian Nuclear
Science and Technology Organisation (ANSTO; award no. AL-
NGRA14530).
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