Evolution of dissolved inorganic carbon in groundwater recharged by cyclones and groundwater age estimations using
the 14 C statistical approach
K.T. Meredith
a,⇑, L.F. Han
b, D.I. Cendo´n
a, J. Crawford
a, S. Hankin
a, M. Peterson
a, S.E. Hollins
aaThe Environment, Australian Nuclear Science and Technology Organisation, New Illawarra Road, Lucas Heights, NSW 2234, Australia
bHydrology and Water Resources Department, Nanjing Hydraulic Research Institute, Guangzhou Road 223, P.O. Box 210029, Nanjing, China Received 24 November 2016; accepted in revised form 2 September 2017; available online 12 September 2017
Abstract
The Canning Basin is the largest sedimentary basin in Western Australia and is located in one of the most cyclone prone regions of Australia. Despite its importance as a future resource, limited groundwater data is available for the Basin. The main aims of this paper are to provide a detailed understanding of the source of groundwater recharge, the chemical evolution of dissolved inorganic carbon (DIC) and provide groundwater age estimations using radiocarbon (14CDIC). To do this we combine hydrochemical and isotopic techniques to investigate the type of precipitation that recharge the aquifer and identify the carbon processes influencing14CDIC,d13CDIC, and [DIC]. This enables us to select an appropriate model for calculating radiocarbon ages in groundwater. The aquifer was found to be recharged by precipitation originating from tropical cyclones imparting lower averaged2H andd18O values in groundwater (56.9‰and7.87‰, respectively). Water recharges the soil zone rapidly after these events and the groundwater undergoes silicate mineral weathering and clay mineral transformation processes. It was also found that partial carbonate dissolution processes occur within the saturated zone under closed system conditions. Additionally, the processes could be lumped into a pseudo-first-order process and the age could be estimated using the14C statistical approach. In the single-sample-based14C models,14C0is the initial14CDICvalue used in the decay equation that considers only14C decay rate. A major advantage of using the statistical approach is that both14C decay and geochemical processes that cause the decrease in14CDICare accounted for in the calculation. The14CDICvalues of groundwater were found to increase from 89 pmc in the south east to around 16 pmc along the groundwater flow path towards the coast indicating ages ranging from modern to 5.3 ka. A test of the sensitivity of this method showed that a15% error could be found for the oldest water. This error was low when compared to single-sample-based models. This study not only provides the first groundwater age estimations for the Canning Basin but is the first groundwater dating study to test the sensitivity of the statistical approach and provide meaningful error calculations for groundwater dating.
Crown CopyrightÓ2017 Published by Elsevier Ltd. All rights reserved.
Keywords: Carbon isotopes; Radiocarbon; Hydrochemistry; Canning Basin; Episodic recharge; Wallal Sandstone
1. INTRODUCTION
Groundwater is often the only reliable source of water in arid regions throughout the world. The Canning Basin located in Western Australia, is remote and one of the
https://doi.org/10.1016/j.gca.2017.09.011
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Geochimica et Cosmochimica Acta 220 (2018) 483–498
hottest and driest areas of Australia, experiencing extreme temperatures (up to 49°C) but it is also one of the most economically important. The prediction that average tem- peratures will rise by 1.5°C by 2030 (McFarlane, 2015) will directly increase the evaporation rates that greatly exceed the often seasonal and highly variable rainfall and thus reduce the surface water availability for the region.
Although the direct impact of these changes on surface water resources is well known, the impact of longer drought periods and higher intensity rainfall on groundwater stor- age remains largely unknown. It has been identified that cli- mate extremes will influence groundwater recharge (Taylor et al., 2013) and that there is a need to better understand available groundwater storage as a result of rainfall intensi- fication (Taylor, 2014).
The Canning Basin is the largest sedimentary basin in Western Australia and is second in size to the Great Arte- sian Basin in Australia. Gravity Recovery and Climate Experiment (GRACE) satellite mission data showed that the Canning Basin is a variably stressed aquifer system with a positive value of recharge and negative value of use (Richey et al., 2015). The high stress ratio given to the Basin suggests that about 150% more water is being depleted than is naturally available. This assessment contradicts other global studies that suggest that the aquifers of the Canning Basin are renewable groundwater resources (Gleeson et al., 2012).
The conflicting global scale studies highlight our limited knowledge of the hydrological balance of the Canning Basin. Rainfall sources to the Canning Basin are well known, particularly in the coastal areas, as being influenced by both tropical maritime air from the Indian Ocean and continental air from inland. These weather patterns result in extreme rainfall conditions ranging from severe droughts to cyclonic rainfall events. The rainfall in summer can be widespread, associated with cyclonic weather from tropical low-pressure systems with centres formed to the north-west of the coast (Sturman and Tapper, 1999; Dogramaci et al., 2012). The Pilbara coast, where the Canning Basin is located, experiences more cyclones than any other part of Australia. Since 1910, there have been 48 cyclones (i.e. on average about one every two years (BOM, 2016)). The win- ter rainfall for the region is relatively low, with the majority of which (about 70–90%) resulting from north-west cloud bands (Wright, 1997; Sturman and Tapper, 1999). Based on the climatic data, it seems reasonable to suggest that cyclone events are the dominant groundwater recharge sources but understanding how this water is recharged and the volume being recharged is still poorly understood for the region.
Sustainable groundwater use requires quantification of fluxes and groundwater storage (Gleeson et al., 2016). A groundwater flow model was created for the site (Aquaterra, 2009), which can be used to give an indication of the groundwater storage volume. However, estimating groundwater storage is challenging in most arid zone sys- tems (Scanlon, 2000) and models require data for valida- tion. Monitoring and sampling groundwater recharge from high volume rainfall events is difficult especially in remote arid regions. This is where isotope hydrology tech-
niques that measure the water sample directly (Fontes, 1980) may prove more robust, and will provide a greater understanding of the hydrological response of large groundwater basins.
Estimating groundwater age using radio-isotope tracers such as radiocarbon (14C) is important for any groundwater resource assessment because they can offer guidance on the sustainability of a groundwater resource and can also be used to calibrate groundwater flow models that can be used as tools to give an understanding of groundwater resources.
But in order to use the14C content of dissolved inorganic carbon (14CDIC) to calculate groundwater ages, the initial
14CDIC value (14C0) must be known. The initial 14CDIC
value can be estimated provided that the carbon sources and reactions that affect carbon mass transfer are known or there is strong geochemical evidence for the assumptions used (e.g.Ingerson and Pearson, 1964; Gonfiantini, 1972;
Geyh, 2000; Han and Plummer, 2013; Plummer and Glynn, 2013). These single-sample-based models rely on mass balances of major carbon species or carbon isotopes (14C and13C) of DIC in a single water sample. Therefore, an understanding of evolution of DIC in groundwater is important for the successful application of14C in estimating groundwater age. The graphical analysis methodHan et al.
(2012)can be used to aid in revealing the complexity of the geochemical environment, conceptualising the processes that affect 14CDIC in aquifers, and, determining which approach is most appropriate for the estimation of the ini- tial14CDIC.
The main aims of this paper are to provide a detailed understanding of the source of groundwater recharge, the chemical evolution of DIC and provide the first groundwa- ter age estimations for the Canning Basin. To do this we combine hydrochemical and isotopic techniques to investi- gate the type of precipitation that recharge the aquifer and identify the carbon processes influencing14CDIC,d13- CDIC, and [DIC]. This enables us to select an appropriate model for radiocarbon dating of DIC in groundwater. In addition, this paper not only presents the first hydrochem- ical and isotope dataset for groundwater in the Canning Basin but is also the first study to use the statistical mod- elling approach for calculating a14C age for groundwaters that were recharged by cyclone sourced rainfall. Further- more, this is the first study to test the sensitivity of the sta- tistical approach and provide meaningful error calculations for this groundwater dating method.
2. ENVIRONMENTAL SETTING 2.1. Study site
The West Canning Basin (WCB) covers an area of 10,000 km2and is a subset of the Canning Basin, Western Australia. The study site is situated between Pardoo Station in the north-western corner and Shay Gap in the south- eastern corner (Fig. 1). The nearest high quality long-term climate monitoring site is Port Hedland (Bureau of Meteo- rology site number: 004032) where a mean annual rainfall of 318.5 mm yr1 (1942–2013) and potential evaporation of 3285 mm yr1 (1967–2013) was recorded, this is repre- 484 K.T. Meredith et al. / Geochimica et Cosmochimica Acta 220 (2018) 483–498
sentative for the study area including the recharge area. The lowest mean monthly minimum temperature of 12.3°C occurs in July (1948–2013) and the highest mean monthly maximum temperature of 36.7°C, occurs in March (BOM, 2013). The area has a mean daily relative humidity of41%.
The study site is situated on a plain ranging in elevation from sea level at the coast to 100–200 m above sea level inland (Fig. 1). The coastal plain is a grassed area with tidal salty flats and recent sand dunes, while the inland area is covered by terrigenous and pedogenic carbonate sediments with occasional mesas and fixed seif dunes (Leech, 1979).
The terrigenous sediments are dominated by red siliclastic and lithoclastic sands and clay mineral muds, derived from the Precambrian hinterland (Semeniuk, 1996). Soil profiles are weakly developed because of limited vegetation cover.
The vegetation present includes Mulga (Acacia aneura), woodlands and shrublands with hummock grass on the coast and inland over the sandplain (McFarlane, 2015).
2.2. Hydrogeology
The West Canning Basin (WCB) is a multilayered aqui- fer system, with the main aquifers being the Broome and Fig. 1. (a) Elevation and location map of the study site with regards to the location of the West Canning Basin containing the hydrology, well locations, meteorological stations, and the extent of the unconfined and artesian areas as identified byAquaterra (2010)for the Wallal Sandstone. The elevation ranges from 0 m asl depicted in blue along the coast line to 275 m asl in brown for inland areas in the vicinity of Shay Gap and (b) the south (A’) to north (A) geological cross section afterHaig (2009)showing the location of wells and the screen depths shown as light grey rectangles.
Wallal Sandstones. These units are separated by the Jar- lemai Siltstone, which acts as an aquitard (Fig. 1b). The Wallal Sandstone is the focus of this study. It is present across the entire Basin as a semi-confined artesian aquifer that comprises sandstone with rare siltstones and gravel interbeds. Sediments are very coarse to fine grained, poorly consolidated, and are fawn to light-grey in colour. It com- prises poorly to well sorted, sub-angular to rounded grains.
The quartz sandstone contains traces of carbonate miner- als, pyrite, black heavy minerals and rose coloured garnets and forms the basal unit of a marine transgression (Leech, 1979). The Wallal Sandstone unconformably overlies Pre- cambrian rocks and reaches a maximum recorded thickness of 420 m to the north-east of the project area (not shown in cross section). It is confined by the Jarlemai Siltstone, except in the south where it is in direct contact with the Broome Sandstone (depicted in the southern section of the cross section inFig. 1b). The aquifer is artesian along the coastal zone, with potentiometric heads of up to 50 m above the ground surface. Groundwater flow direction fol- lows a general west-north-west direction (Fig. 2) (Aquaterra, 2009), which is oblique to the available sam- pling transects.
Recharge to the Wallal Sandstone can only occur inland beyond its artesian boundary and most likely where the confining layer of the Jarlemai Siltstone is absent near the limit boundary of the Basin to the south inFig. 1. The dom- inant lateral groundwater contribution from the wider Can- ning Basin recharge area is not investigated in this study but is likely to originate to the south-east of the study site as suggested by the potentiometric head configuration.
Groundwater discharge is observed near the coast as small springs but predominately occurs as submarine discharge and ranges between 4.6107 and 2.4108m3yr1 (McFarlane, 2015).
3. METHODS
Four field trips in December 2008, October 2009, June 2012 and May 2013 were conducted to collect groundwater samples. During these, 28 groundwater samples were col- lected from groundwater monitoring wells for hydrochemi- cal analysis. Sampling methods differed depending on whether the aquifer was pressurised, and if not, the method depended on the depth of the standing water levels (SWLs).
In the case of artesian wells, groundwater was purged through the main pressure valves. A minor outflow valve was plumbed into a flow cell allowing for simultaneous purging and monitoring of field parameters. Wells with SWLs (<44 m bgs) were sampled using low-flow methods (Puls and Barcelona, 1996) and others were sampled using a Grundfos MP1 submersible pump following the standard 3 well volume method.
Groundwater samples were generally collected via an in- line, 0.45lm filter, with d13CDIC samples further filtered through 0.22lm. Total alkalinity was determined in the field by acid-base titration using a HACH digital titrator.
Samples were collected, preserved and measured according to Appendix A. Specifically, cations and anions were mea- sured by inductively coupled plasma – atomic emission spectroscopy and ion chromatography, respectively. The d18O and d2H were analysed by IRMS and are reported
Fig. 2. The spatial distribution of Cl(mmol L1) in groundwaters for the study area compared with 2 m piezometric surface contours for the Wallal Sandstone (Aquaterra, 2009). Polygons represent groupings identified by the cluster analysis. Note: group 4a inland includes all the wells that are not in a polygon.
486 K.T. Meredith et al. / Geochimica et Cosmochimica Acta 220 (2018) 483–498
as per mil (‰) deviations from the international standard V-SMOW and were reproducible to ±0.1‰ and ±1.0‰, respectively.
Thed13C signatures of waters were analysed by IRMS and results were reported as‰deviation from the interna- tional carbonate standard, NBS19 with a precision of
±0.1‰ according to methods reported in Meredith et al.
(2016). The14CDICcontent was determined by accelerator mass spectrometry after samples were processed according to the methods outlined inMeredith et al. (2012). Briefly, the total DIC or carbonate was processed into CO2by acid- ifying the samples and extracting the liberated CO2gas. The CO2sample was then heated with CuO, Ag and Cu wire, at 600 C for 2 h and then converted into graphite by reducing it with excess hydrogen gas in the presence of an iron cata- lyst at 600 C. The14C results reported from the laboratory were in percent Modern Carbon (pMC) normalised against thed13C of the graphite. We use the ‘un-normalised’ 14C values for groundwater and report them as pmc which was calculated using the 14C/12C ratio (Plummer and Glynn, 2013).
Cations and anions were assessed for accuracy by eval- uating the charge balance error percentage (CBE%;
Table 1). Most samples (except WCB15Y = 7.8%) fell within the acceptable ±5% range. Hierarchical cluster anal- ysis was carried out to validate the hydrochemical observa- tions and the following parameters were selected; HCO3, Cl, SO4, SiO2, Na, Ca, Mg, K and Sr. To reduce the impact of the magnitude of each parameter, the mean and standard deviations of each parameter were calculated, then each parameter’s value was normalised by subtracting the mean and dividing by the standard deviation. Following this, hierarchical cluster analysis was carried out, where the sim- ilarity between two sites was calculated using Euclidian dis- tance between the parameters, with each parameter having the same weighting. In hierarchical cluster analysis, initially each object is assigned to its own cluster and the distances between clusters are calculated. The algorithm then pro- ceeds iteratively; at each stage joining the two clusters with the shortest distance. The distances between the resulting clusters are then re-calculated and the iterations are contin- ued until all clusters have been merged into one cluster. The optimum number of clusters can be chosen when the cost of merging two clusters is high (e.g. when objects with large difference are grouped into the one cluster) and/or by anal- ysis the cluster dendrogram (cluster tree). The cluster anal- ysis was run with and without 14CDIC, d2H andd18O as variables, resulting in no change in the groupings. A princi- ple component analysis (PCA) was completed on ground- waters from cluster groups 3, 4a and 4b (Fig. 2).
Parameters used include HCO3, Cl, SO4, SiO2, Na, Ca, Mg, K and Sr. Principle Component Analysis (Jollife, 1986) and cluster analysis (using the K-means clustering algorithm; (Hartigan and Wong, 1979) were performed using the R programming environment (Venables et al., 2016).
Saturation indices (calcite, dolomite and siderite), con- centration of dissolved inorganic carbon [DIC], carbon dioxide [CO2], carbonate [CO32] and bicarbonate [HCO3] were calculated using the WATEQ4F thermodynamic data-
base in the PHREEQC programme (Parkhurst and Appelo, 1999). We use the graphical method developed byHan et al.
(2012)to interpret groundwater evolution based on the con- centration and the isotopic composition of the DIC. The
87/86Sr ratios were converted into per mil (d87Sr) (Elderfield, 1986). All the methods including references, col- lection vessels, reference material and errors are reported in Appendix A.
4. RESULTS
A range of hydrochemical (major ions and trace ele- ments) and environmental isotopes were measured on groundwaters from the Wallal Sandstone over the duration of the study. Four major groups of groundwaters were identified by using classical hydrochemical X-Y plot rela- tionships that show the addition or removal of groundwater ions and can then be used to infer geochemical processes that have led to the evolution of groundwater. The location of the different hydrochemical groups were similar to those identified with cluster analysis. The relationship between these ions also relates to the geographical location of the groundwaters in the aquifer therefore the cluster analysis was used as an additional line of evidence for hydrochemi- cal interpretation. These groups include: (1) Group 1 - the well located in the far north-west (WCB04), (2) Group 2 - wells located in the north-west (WCB08, WCB09, WCB10), (3) Group 3 - wells located inland (PB1 and PB3) and (4) Group 4 - wells also located inland and along the coast (Fig. 2). Based on the cluster analysis, Group 4 was further sub-divided into group 4a located inland (WCB20Y, WCB18, WCB19, WCB15Y, MMCA03E, WCB23B), group 4b (WCB25X, WCB25Y, Cooragoora, WCB24Y, WCB21Y) and group 4c located on the coast (WCB22, WCB17 and Pardoo) (Fig. 2). Groundwaters were also assessed with distance from Shay Gap (Fig. 1).
This was done because the major recharge area appears to be located to the south east of the study site according to the potentiometric contours (Fig. 2). This transect does not strictly represent the groundwater flow path but a flow path from WCB23B?WCB20?Pardoo wells is shown in Figs. 4–6for reference.
The groundwater Cl concentration in the Wallal Sand- stone is generally below 5 mmol L1 but rises to over 10 mmol L1, reaching a high of 21.4 mmol L1 in the western coastal area (Groups 1 and 2) (Table 1,Fig. 2).
Groundwaters from Group 3, 4a and 4b have the lowest Cl concentrations below 3.5 mmol L1(Fig. 2) and the low- est ion concentrations (Table 1). The Na/Cl ratios are gen- erally around 1 for Group 1 and 2 groundwaters but increase to greater than 1 in Groups 3 and 4, indicating there is a source of Na relative to Cl in these groundwaters (Fig. 3a). Groundwaters located closer to the coast (Group 4c) have higher Cl concentrations between 3.5 and 8 mmol L1. The Cl/Br ratios for all groundwaters (except those from Groups 1 and 2) suggest Cl is derived from mar- ine aerosols with ratios close to marine signatures of 655 (Fig. 3a). The increase in Cl concentration across the site is broadly reflected in the concentration of other ions, with an excellent correlation (r2greater than 0.9) with Cl, Na,
Table 1
Hydrochemical results for groundwater samples.
ID Date Depth Distance T pH DO Na Ca Mg K Sr Cl HCO3 SO4 SiO2 CBE
m bgs km mg L1 mmol L1 mmol L1 mmol L1 mmol L1 umol L1 mmol L1 mmol L1 mmol L1 mmol L1 %
Cooragoora 7/06/2012 240 39 34.4 6.6 0.05 4.2 0.4 0.4 0.2 2.4 2.4 2.5 0.5 0.1 1.1
MMCA03E 25/05/2013 102 5 30.72 6.4 5.19 2.3 0.6 0.6 0.1 2.3 2.5 1.8 0.1 0.5 4.1
Pardoo 7/06/2012 142 35 34.6 6.5 0.05 6.7 0.8 0.7 0.2 4.0 5.4 2.5 1.0 0.1 0.1
PB1 12/02/2008 112 2 33.5 6.5 6.71 3.0 0.5 0.6 0.1 3.3 3.1 1.6 0.2 1.0 2.9
PB3 12/02/2008 111 2 33.6 6.2 6.54 1.7 0.4 0.4 0.1 2.4 1.7 1.0 <0.1 1.0 7.7
PB3 10/12/2009 111 2 33.4 6.4 6.23 1.7 0.4 0.4 0.1 2.3 1.7 1.0 <0.1 1.0 8.8
WCB04 8/06/2012 82 10 32.3 6.3 0.4 18.7 2.1 1.8 0.5 14.0 20.6 1.1 2.5 0.1 0.6
WCB04 12/04/2008 82 10 32.4 6.2 1.18 18.7 2.1 1.9 0.5 13.7 21.4 0.9 2.4 0.2 0.0
WCB08 12/03/2008 85 26 32.8 6.3 0.68 10.0 1.5 1.3 0.3 7.8 10.7 2.5 1.7 0.3 -2.2
WCB09 12/03/2008 136 37 32.5 6.2 0.21 9.1 1.6 1.3 0.3 8.2 12.7 0.9 0.8 0.2 0.1
WCB10 12/04/2008 72 18 31.9 6.3 0.34 11.7 1.7 1.4 0.3 9.2 14.1 1.2 1.5 0.3 -0.2
WCB15Y 5/06/2012 107 16 34.7 6.8 4.21 1.9 0.7 0.5 0.1 2.6 1.7 1.9 0.1 0.5 7.8
WCB17 14/10/2009 140 39 34.28 6.8 0.24 6.1 0.5 0.5 0.2 2.7 4.2 2.8 0.8 0.3 -3.2
WCB17 12/03/2008 140 39 34.2 6.5 0.43 6.5 0.5 0.5 0.2 2.7 3.9 4.0 0.8 0.3 -5.3
WCB18 23/05/2013 119 3 31.98 6.5 3.98 3.0 0.8 0.6 0.1 3.2 3.1 2.3 0.1 0.6 1.4
WCB19 23/05/2013 150 15 32.54 6.6 2.55 1.8 0.7 0.5 0.1 2.2 1.8 2.2 0.1 0.5 2.8
WCB20Y 24/05/2013 195 23 32.34 6.8 3.79 2.8 0.6 0.4 0.1 1.9 1.9 2.5 0.1 0.4 2.7
WCB21Y 22/05/2013 217 30 35.21 6.4 2.51 3.2 0.4 0.3 0.1 2.0 2.1 2.0 0.3 0.3 1.9
WCB22 12/03/2008 144 37 32.9 6.4 0.77 7.0 0.7 0.6 0.2 4.1 4.8 3.4 1.0 0.3 -1.3
WCB23B 25/05/2013 106 18 33.62 6.4 0.06 2.0 0.7 0.4 0.1 2.2 1.5 2.1 0.2 0.4 2.5
WCB24Y 21/05/2013 204 28 34.77 6.5 0.6 2.7 0.4 0.2 0.1 1.6 1.5 2.0 0.2 0.2 0.8
WCB25X 6/06/2012 262 38 35.12 7.0 0.03 3.8 0.3 0.2 0.1 1.5 1.7 2.5 0.3 0.1 0.9
WCB25Y 6/06/2012 324 38 34.6 6.8 0.08 4.2 0.3 0.3 0.1 1.7 2.5 2.3 0.4 0.1 -0.1
488K.T.Meredithetal./GeochimicaetCosmochimicaActa220(2018)483–498
SO4, Br, Ca, Mg, K, Sr and Rb. The high correlation coef- ficients imply a similar source of ions for the fresher waters.
The Cl/Br and Na/Cl ratios also show the difference in groundwaters from Groups 1 and 2 which have higher Cl/Br ratios (800–1200) suggesting an alternative source of salinity for this section of the aquifer but the Na/Cl ratios are within the marine range (0.8–1) (Fig. 3a). These finding are very important because they show that there is not a single source of Cl within the aquifer.
The d2H and d18O values have an average of56.9‰ and7.87‰, respectively (n = 33) (Table 2) with a stan-
dard deviation of 1.7 and 0.30, respectively, suggesting a common origin of water. Groundwaters for the region plot to the right of the Darwin Local Meteoric Line (LMWL) (Fig. 3b) on a regression line described by d2H = 4.08 d18O – 24.72. The only rainfall study for this region was from Dogramaci et al. (2012) for rainfall events >20 mm (d2H‰= 7.0d18O – 4.8), this line is similar to the Darwin LMWL (Fig. 3b).
A principle component analysis was used in this study to confirm the relationships that were identified from classical hydrochemical graphical methods and were only under- Fig. 3. Bivariate plots of (a) Cl/Br vs. Na/Cl (molar concentration) and (b)d2H vs.d18O compared with the Darwin LMWL_PWLSR (d2H‰= 7.69d18O + 9.75 [black solid line]), Local Meteoric Water Line (LMWL) for rainfall events >20 mm for Hamersley Basin (n = 8;
d2H‰= 7.0d18O + 4.8 [grey dashed line]) (Dogramaci et al., 2012), and a Local Evaporation Line (LEL) (d2H‰= 5.2d18O – 14.4 [black dashed line] (Dogramaci et al., 2012).
Fig. 4. Distance from the south eastern corner of the study area (Shay Gap inFig. 1) for groundwaters from cluster groups 3 and 4 with respect to (a) SiO2concentration (r2= 0.8), (b) Ca and Mg concentration (both r2= 0.0) and (c) Na concentration (r2= 0.6). The linear fit is depicted by a solid black line. This figure does not represent the groundwater flow path as shown inFig. 2. A groundwater flow path from wells located at WCB23B?WCB20?Pardoo shows how parameters change.
taken on groundwaters from cluster Groups 3, 4a and 4b.
This was done to help identify the major hydrochemical processes leading to the formation of the observed ground- water chemistry across the site. Component 1 (PCA1) explained 50% of the variation. Larger SiO2, Ca, Mg and Sr values were accompanied by lower HCO3, SO4, Na and K values. The positive loadings are associated with a linear decrease in SiO2 concentrations, ranging from 1 mmol L1at PB1 and PB3, decreasing towards the coast to 0.1 mmol L1(r2= 0.8). The negative weighted variables of Na, HCO3, K and SO4increase in concentration similar to Na concentration with distance (r2= 0.6) (Fig. 4c). Com- ponent 2 explained 28% of the variation and most variables
were negatively loaded, except SiO2. Cl had the highest absolute loading with Ca, Mg and Sr being the most related.
Thed87Sr values of groundwater from the Wallal Sand- stone are significantly elevated (+6.6 to +13.7‰) compared to seawater (0‰). Even though these values are high, they are not to the levels found for the inland Pilbara region groundwaters (+27.2 to +40.2‰) (Dogramaci and Skrzypek, 2015). No relationship between Sr concentration and d87Sr values (R2= 0.002) is observed in this study.
However, an increase in Na/Cl ratios (Fig. 5a) and d87Sr values (Fig. 5b) is observed with distance from the Shay Gap area (r2= 0.6 and 0.7, respectively), suggesting a Fig. 5. Distance from the south eastern corner of the study area (Shay Gap inFig. 1) for groundwaters from cluster groups 3 and 4 with respect to (a) Na/Cl (r2=0.6), (b)d87Sr values (r2=0.7) and (c) Sr concentration (r2=0.0). The linear fit is depicted by a solid black line. This figure does not represent the groundwater flow path as shown inFig. 2. A groundwater flow path from wells located at WCB23B?WCB20
?Pardoo shows how parameters change.
Fig. 6. Distance from the south eastern corner of the study area (Shay Gap inFig. 1) for groundwaters from cluster groups 3 and 4 with respect to (a) DIC (r2= 0.3), (b)d13CDIC(r2= 0.4) and (c)14CDIC(r2= 0.9). The linear fit is depicted by a solid black line. This figure does not represent the groundwater flow path as shown inFig. 2. A groundwater flow path from wells located at WCB23B?WCB20?Pardoo shows how parameters change.
490 K.T. Meredith et al. / Geochimica et Cosmochimica Acta 220 (2018) 483–498
source of Na together with the addition of radiogenic Sr.
The Sr concentrations are generally relatively consistent as shown with the trend line in Fig. 5c, unlike the d87Sr values.
The d87Sr values suggest that the weathering of old rubidium-rich primary silicate minerals (Faure and Mensing, 2005) is a dominant process in this system. The addition of radiogenic Sr into the groundwater occurs close to the unconfined area of the Wallal Sandstone in the south east corner (near Shay Gap) and increases towards the coast (Fig. 5b). However we do not see an increasing trend in SiO2or Sr concentrations (Figs. 4a and5c) as would be expected if silicate weathering is occurring. This trend does not initially support silicate mineral weathering processes.
However, the loss of SiO2from aqueous phase commonly occurs in clay mineral transformation reactions (Langmuir, 1997), which could account for the decrease in SiO2concentration (Fig. 4a), as we see in our data.
The [DIC] and d13CDIC values of the groundwaters in the Wallal Sandstone increase with distance from Shay Gap (Fig. 6a and b) and conversely, the 14CDIC values decrease (Fig. 6c). The only groundwater sample to contain
detectable3H (i.e. above the quantification limit of 0.2 TU, Table 2) was WCB15 (0.7 TU). Except for this sample which is located inland near the unconfined section of the Wallal Sandstone, these results suggest groundwater has not experienced any recent recharge (i.e. in contact with the atmosphere in the past approximately 50 years).
5. DISCUSSION 5.1. Groundwater recharge source
Cyclonic rainfall events are likely to be the dominant source of groundwater recharge to the system, which can be identified in the rainfall records for the region but also from the lowd2H andd18O values in groundwater (average d18O and d2H values of 7.87‰ and 56.9‰ (n = 33), respectively). The negative values of the groundwater can be explained by studies such as Zwart et al. (2016) who undertook isotopic measurement of rainfall during two monsoonal events in northern Australia. They observed that the isotopic composition of precipitation (range129 to 6‰ for d2H and 17 to +1‰ for d18O) was related Table 2
Environmental isotope results for groundwaters samples (Uncert. =3H uncertainty, QL =3H quantification limit).
ID Date sampled d13CDIC d87Sr 3H Uncert. QL d2H d18O 14CDIC DIC SIcal
‰ ‰ TU TU TU ‰ ‰ pmc mmol L1
Cooragoora 7/06/2012 10.3 13.7 0.32 0.05 0.27 56.8 7.94 27.3 3.6 1.3
MMCA03E 25/05/2013 11.0 9.1 0.02 0.03 0.19 57.1 7.94 75.3 3.4 1.6
Pardoo 7/06/2012 9.4 12.9 53.7 7.40 16.0 3.9 1.2
Pardoo 2014 56.9 7.76
PB1 12/02/2008 13.1 60.95 8.35 77.3 2.7 1.5
PB1 10/12/2009 0.04 0.06 0.3
PB3 12/02/2008 15.5 61.18 8.2 81.7 2.3 2.1
PB3 10/12/2009 12.2 0.06 0.06 0.3 57.4 8.43 1.7 1.8
WCB04 8/06/2012 12.3 12.4 0.35 0.03 0.14 56.4 7.79 0.7 2.3 1.5
WCB04 12/04/2008 15.4 59.32 7.71 0.3 2.0 1.7
WCB04 2014 56.3 7.76
WCB08 12/03/2008 12.4 56 7.13 3.8 4.8 1.2
WCB08 2014 53.2 7.25
WCB09 12/03/2008 17.8 58.33 7.95 0.5 1.8 1.7
WCB09 2014 57.3 7.89
WCB10 12/04/2008 14.6 55.25 7.59 0.5 2.3 1.5
WCB10 2014 55.6 7.47
WCB15Y 5/06/2012 12.2 7.3 0.65 0.05 0.16 58.6 8.32 69.7 2.5 1.0
WCB17 12/03/2008 10.6 56.15 7.65 22.5 6.2 1.2
WCB17 14/10/2009 9.9 0.06 0.06 0.3 55 7.77 3.6 1.0
WCB17 2014 56.1 7.95
WCB18 23/05/2013 10.0 7.2 0.03 0.03 0.19 57.0 8.15 89.0 3.7 1.2
WCB19 23/05/2013 12.9 7.1 0.01 0.03 0.19 57.8 8.19 61.6 3.4 1.2
WCB20Y 24/05/2013 11.0 8.4 0.01 0.03 0.19 58.4 7.98 54.8 3.2 0.9
WCB21Y 22/05/2013 10.5 9.6 0.07 0.02 0.15 57.7 7.98 45.0 3.4 1.5
WCB21Y 2014 56.9 8.09
WCB22 12/03/2008 10.8 54.81 7.58 16.8 5.9 1.2
WCB22 2014 54.2 7.54
WCB23B 25/05/2013 12.3 6.7 0.02 0.03 0.19 57.8 8.02 49.6 3.8 1.4
WCB24Y 21/05/2013 12.4 9.2 0.01 0.03 0.15 57.2 8.02 44.6 3.2 1.5
WCB24Y 2014 57.0 8.07
WCB25X 6/06/2012 11.1 11.8 0.01 0.02 0.14 56.7 8.08 32.2 3.0 1.0
WCB25Y 6/06/2012 10.9 13.5 0.04 0.04 0.14 56.2 7.95 29.9 3.1 1.3
WCB25Y 2014 56.6 7.96
to the size and activity of the convective envelope, with the most negative values occurring when eastward and west- ward moving precipitation systems merged over the mea- surement site. The isotopic composition of precipitation from tropical cyclones, similar to those experienced across the study site, have not been studied for this area but are suggested to be typically more depleted in18O and2H than any other precipitation (e.g. Lawrence et al., 2002;
Fudeyasu et al., 2008; Munksgaard et al., 2015).
Long-term isotopic data for precipitation was not avail- able for this site therefore, to investigate the impact of trop- ical cyclones, we use the Global Network of Isotopes in Precipitation (GNIP) data from Darwin (1500 km north- east from the study area). Even though this is a large dis- tance from our study site it is used because it represents the only Australian rainfall dataset to potentially contain such precipitation events resulting from a cyclone. Using all the available data from Darwin, the precipitation weighted averaged18O and d2H values were calculated to be5.51‰ and32.6‰, respectively. This value is much higher than groundwaters from the study site and they also, plot to the right of the Darwin LMWL (Fig. 3b), suggesting they are not the source waters for groundwater for the region. But when we take a subset of data from Darwin for those months during which a cyclone was recorded, the precipitation weighted average d18O and d2H values were more depleted than the average value (6.42‰ and 40.9‰, respectively). However these values are still less depleted in heavy isotopes when compared to groundwaters from our study site suggesting these types of rainfall events do not represent recharge. The difference in these values can be explained by the studies undertaken by Lawrence and Gedzelman (1996)andGedzelman et al. (2003)that the iso- topic composition of precipitation (d18O and d2H) while depleted relative to other rainfall was still varied and not consistent within cyclone systems.
Interestingly, when we add an Evaporation Line (EL) derived from surface water sampling in a similar climatic environment (Dogramaci et al., 2012) (Fig. 3b) we see a much closer relationship with groundwater samples. The slight increase ind2H andd18O along the evaporation line does suggest minor evaporation of the recharge water but not open-water evaporation which would display much higher values (i.e. towards 0‰). Based on these results we suggest that groundwater recharges fairly quickly through the unsaturated zone because thed2H and d18O cyclonic signal is retained in the groundwater, even under extreme temperatures. Therefore, we suggest that the observed sig- nal in the groundwater is produced by the mixing of evap- orated soil water contained in the unsaturated zone and the incoming recharge water before it reaches the water table.
Further unsaturated zone soil profile sampling would be required to confirm this.
5.2. Evolution of DIC
Groundwater flow direction follows a general west- north-west direction (as shown with arrows inFig. 2) and the dominant recharge area is to the south-east of the study site. However, within the study area an unconfined section
of the Wallal Sandstone occurs at the south eastern bound- ary (Figs. 1b and 2) providing potential pathways for focussed groundwater recharge from cyclonic events.
Understanding this recharge pathway is important for trac- ing the evolution of DIC into the Wallal Sandstone. Fur- thermore, the Wallal Sandstone has been found to contain rare siltstones and gravel beds but also traces of carbonate minerals and pyrite (Leech, 1979).
The presence of carbonate minerals suggests that car- bonate dissolution has the potential to influence the hydro- chemical evolution of groundwater DIC. Interestingly, there is no firm evidence for this process in the saturation indices of carbonate minerals such as calcite and dolomite in the groundwater samples because they are under- saturated with respect to these minerals. But when we look at the relationship between 14CDIC, d13CDIC, and [DIC]
(Fig. 7), the various geochemical processes that may influence the carbon chemistry of the groundwater system (i.e. [DIC]), and the carbon isotopic composition of DIC (14CDIC,d13CDIC) can be identified.
Firstly, we see the changes ind13CDICand [DIC] caused by geochemical processes that involve the addition or removal of DIC with respect to enrichment or depletion of 13C (Fig. 7I).Fig. 7I does not show14C decay, this is because it does not affect d13CDIC and [DIC], therefore, any changes in these parameters is the results of other phys- ical/chemical process(es) that may alter 14CDIC in the absence of14C decay (e.g. mixing of waters with different d13CDICand [DIC] values or addition or removal of carbon caused by physical/chemical processes) (Han et al., 2012;
Han and Plummer, 2016). FromFig. 7I we see the general trend is an increase ind13CDICwith increasing [DIC].
Here we suggest that the main reason for the increase of DIC in groundwater is carbonate dissolution according to the following(1):
CO2ðaqÞþMeCO3ðsÞþH2O¼2HCO3ðaqÞþMe2þðaqÞ ð1Þ where Me is generally Ca or Mg. The subscripts aq and s represent dissolved and solid states, respectively. The con- centration of CO2(aq)and HCO3
(aq)are equal at pH of 6.4 in dilute aqueous solutions at 25°C. Thus, the [DIC] may increase if the CO2(aq)reacts further with carbonate miner- als (Eq.(1)). If soil CO2in the recharge area were the only source for CO2(aq) in Eq.(1), we would see an increase in pH value with increasing [DIC] and decreasing [CO2(aq)], and from the material balance the maximum final concen- tration of DIC would not exceed 1.5 times of the initial [DIC] value. At maximum conversion of CO2(aq) (i.e.
[DIC] approaches 1.5 times of its initial value), the resultant pH value would be greater than 8. But in fact, we do not see significant changes in pH with changing [DIC] (Table 1) and [DIC] has increased more than 1.5 times of the initial value (Table 1 and Fig. 6a). Therefore, the increase in [DIC] is most likely to be caused by the reaction of carbon- ate minerals with CO2(aq)that was added to the system dur- ing DIC evolution, and in addition to soil CO2.
To eliminate all the possible processes influencing DIC evolution, first we assume that the addition of CO2 has resulted from the oxidation of fossil organic matter in the aquifer and carbonate dissolution (with a d13C value of 492 K.T. Meredith et al. / Geochimica et Cosmochimica Acta 220 (2018) 483–498
0‰). Under these assumptions, the added DIC should have ad13C value of approximately half of the soil CO2(i.e. 0.5 d13C(CO2)). The soil CO2 was calculated to be between 18.4 and16.9‰ford13C(CO2)(see Appendix B). There- fore, the added DIC should have ad13CDIC between9.2 and8.5‰, as marked by letter S’’ inFig. 7I. This range is more negative compared to the extrapolated value from the regression line for all the data (i.e.6.6‰at point S’).
The weathering of silicate minerals is a dominant hydro- geochemical process in groundwaters with distance from Shay Gap as identified by increasing Na/Cl ratios and d87Sr values (Fig. 5a and b). But because we see also see a decrease in SiO2 we know that other processes such as clay mineral transformation reactions (Langmuir, 1997) must be occuring. The presence of weathering products such as kaolin minerals in the core material from the Wallal Sandstone examined at WCB04 (Drake, 1979) further sug- gest these processes are possible. Clay mineral processes would also explain the loss of Sr after silicate weathering reactions via ion exchange reactions (Fig. 5c). If ion exchange is occurring we would see increased carbonate dis- solution adding further dead carbon to the system (Han et al., 2012). However, if carbonate dissolution was the only process leading to the [DIC] increase in groundwater then the linear extrapolation of the regression line would extend to point S inFig. 7I. This is because the carbonate minerals in the aquifer should have d13C value close to 0‰, espe- cially for the Wallal Sandstone which forms the basal unit of a marine transgression (Leech, 1979). Higher carbonate values are confirmed with the only published study to examined13C values for carbonates of the Canning Basin, which found baseline values of +1 to +2‰ (Stephens and Summer, 2003).
Thus, we see that the DIC is the result of at least two geochemical processes. These could be carbonate dissolu- tion caused by ion exchange on clay minerals and the addi- tion of CO2from oxidation of fossil organic matter in the aquifer. Both of these processes would add dead carbon to the system and dilute14CDIC. The net effect of these pro- cesses is the addition of DIC which has ad13CDICof6.6‰ (Fig. 7I).
5.3. Calculation of14C ages for groundwater
When we compare 14CDIC and d13CDIC values in Fig. 7III, we see two data groups emerge that also corre- spond with the cluster groupings derived from the hydro- chemistry. Groundwaters from Groups 1 and 2 have very low14CDICvalues (3.8–0.3 pmc). These waters when com- pared to others contain significantly higher Cl concentra- tions. The Cl/Br ratios are higher (800–1200) than other groundwaters from the study site (Fig. 3a) suggesting that the primary salinity is too low to be directly related to the dissolution of evaporites but that it may be contributed from a mixture of waters. The source of salinity to these groundwaters has been hypothesised to be from mixing or intrusion of higher salinity water associated with basement or structural changes in the western boundary of the Basin (Leech, 1979). Due to low14CDICvalues no age calculations could be completed for these waters using this isotope.
With the exception of two samples (sample WCB18 and PB3), groundwaters from cluster groups 4b and 4c form a curved relationship between 14CDIC and d13CDIC (Fig. 7III). Therefore, the statistical modelling approach (Gonfiantini and Zuppi, 2003; Han et al., 2014) is used to estimate14C ages based on the following equation Fig. 7. The14CDIC-d13CDIC-[DIC] diagrams (Han and Plummer diagrams;Han and Plummer, 2013) of the data. See text for explanation of end members (A and S).
lnð14CDICÞ ¼ 1þk14
k13
lnðd13CEd13CDICÞ þlnð14CiÞ 1þk14
k13
lnðd13CEd13CiÞ ð2Þ wherek14is the14C decay constant (=1.21104a1),k13
is the apparent rate constant of the reaction(s) that affect the d13CDIC value. 14Ci and d13Ci are the initial 14CDIC andd13CDICat the start of radioactive decay and any geo- chemical processes that alter 14CDIC and d13CDIC along with decay of14C in the aquifer.d13CEis the d13C value of DIC in the aquifer at time?1(i.e.6.6‰;Fig. 7I).
If the kinetics of the geochemical/physical processes that affect 14CDIC and d13CDIC can be represented by a pseudo-first-order process, a plot of ln(14CDIC) vs. ln (d13CEd13CDIC) should form a straight line, and the slope of the straight line (1þk14=k13) can be used to calculatek13, the rate constant, of the geochemical reactions. Thus, the overall rate constant for14CDIC decrease is k14þk13 (e.g.
decay of14C and dead carbon dilution of14C) and ground- water ages can be estimated by using the equation
14CDIC¼14Cieðk14þk13Þt ð3Þ
The definition of14Ci is different to the initial14CDIC
value (14C0) that is used in single-sample-based correction models (Han and Plummer, 2016). In the single-sample- based models, 14C0 is the initial14CDIC value used in the decay equation that considers only14C decay rate. In the statistical approach, the14Ciis the initial14CDICvalue used in the equation (Eq.(3)) for both14C decay and geochem- ical processes that cause decrease in14CDICalong with14C decay (e.g. dilution by dead carbon).
Fig. 8shows a plot of ln(14CDIC) vs. ln(d13CE-d13CDIC) for the groundwater samples. All samples from Groups 1 and 2 plot well below the curve (also refer toFig. 7III for shaded box) and are excluded from these regression calcu- lations. Two samples (WCB18 and PB3) from cluster group 4a were also excluded from regression calculations (also
refer toFig. 7III). From the regression line inFig. 8, a slope of 1.692 was obtained. From the slope, the values ofk13and the 14C ‘‘apparent decay constant” (k14þk13) can be obtained using (Eq.(2)).
Assuming that 14Ci is 100 pmc (for a system evolved under open-system conditions in the unsaturated zone ie open to soil gas CO2 influx, see Appendix B), using the regression line, by extrapolation to 14CDIC= 100 pmc (Fig. 8), we obtain d13CDICof15‰. On the other hand, assuming that14Ci= 75 pmc (for a system evolved under closed-system conditions in the unsaturated zone, also see Appendix B), by extrapolation to 14CDIC= 75 pmc, we obtain d13CDIC of 13.5‰. These two values are shown on the regression line (Fig. 8) and are represented as open and closed, respectively. These points represent the range of carbon isotopic composition of DIC values before14C decay and increase of d13CDIC. They also represent end member A inFig. 7.
Because we do not know the exact value of14Ci, but we know it is between 100–75 pmc (see Appendix B), we use a mean value of 87.5 pmc for age calculations (as represented as partially open inFig. 8). Assuming that14Ci= 87.5 pmc, using the regression line, by extrapolation to14CDIC= 87.5 pmc, we obtain ad13CDICof14.1‰. Using this initial14Ci
value (87.5 pmc), the calculated ages by using Eq.(3)are given inTable 3.
By knowing k14 (=1.21104a1), k13
(=1.75104a1), d13CE (=6.6‰), 14Ci (=87.5 pmc), and d13Ci (=14.1‰), the curve in Fig. 7III (i.e. depen- dence of 14CDIC on d13CDIC) is calculated using Eq. (2).
Groundwaters that plot on or close to the calculated curve are likely to have moved under piston-flow conditions, which relates to a traverse from Group 3 to group 4a to 4b to 4c groundwaters. This indicates that the geochemical environment is relatively homogenous and that along the groundwater flow path, the geochemical processes occurred slowly in the aquifer at a reaction rate comparable to14C decay timescales (Han et al., 2014).
Fig. 8. Plot of ln(14CDIC) vs. ln(d13CE-d13CDIC) for the samples. The linear relationship indicates that the processes that affect the13C carbon isotopic composition takes place in parallel with14C decay.
494 K.T. Meredith et al. / Geochimica et Cosmochimica Acta 220 (2018) 483–498
