Available online at www.sciencedirect.com ScienceDirect Geochimica et Cosmochimica Acta 220 (2018) 483–498 www.elsevier.com/locate/gca Evolution of dissolved inorganic carbon in groundwater recharged by cyclones and groundwater age estimations using the 14C 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 a a The Environment, Australian Nuclear Science and Technology Organisation, New Illawarra Road, Lucas Heights, NSW 2234, Australia b Hydrology 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 14 of dissolved inorganic carbon (DIC) and provide groundwater age estimations using radiocarbon ( CDIC). To do this we combine hydrochemical and isotopic techniques to investigate the type of precipitation that recharge the aquifer and identify 14 13 the carbon processes influencing CDIC, d CDIC, 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 average d2H and d18O values in groundwater (À56.9‰ and À7.87‰, 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 14 14 14 14 the C statistical approach. In the single-sample-based C models, C0 is the initial CDIC value used in the decay equation that considers only 14C decay rate. A major advantage of using the statistical approach is that both 14C decay and geochemical 14 14 processes that cause the decrease in CDIC are accounted for in the calculation. The CDIC values 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 a 15% 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 Copyright Ó 2017 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 ⇑ Corresponding author. located in Western Australia, is remote and one of the E-mail address: [email protected] (K.T. Meredith). https://doi.org/10.1016/j.gca.2017.09.011 0016-7037/Crown Copyright Ó 2017 Published by Elsevier Ltd. All rights reserved. 484 K.T. Meredith et al. / Geochimica et Cosmochimica Acta 220 (2018) 483–498 hottest and driest areas of Australia, experiencing extreme niques that measure the water sample directly (Fontes, temperatures (up to 49 °C) but it is also one of the most 1980) may prove more robust, and will provide a greater economically important. The prediction that average tem- understanding of the hydrological response of large peratures will rise by 1.5 °C by 2030 (McFarlane, 2015) will groundwater basins. directly increase the evaporation rates that greatly exceed Estimating groundwater age using radio-isotope tracers the often seasonal and highly variable rainfall and thus such as radiocarbon (14C) is important for any groundwater reduce the surface water availability for the region. resource assessment because they can offer guidance on the Although the direct impact of these changes on surface sustainability of a groundwater resource and can also be water resources is well known, the impact of longer drought used to calibrate groundwater flow models that can be used periods and higher intensity rainfall on groundwater stor- as tools to give an understanding of groundwater resources. age remains largely unknown. It has been identified that cli- But in order to use the 14C content of dissolved inorganic 14 mate extremes will influence groundwater recharge (Taylor carbon ( CDIC) to calculate groundwater ages, the initial 14 14 14 et al., 2013) and that there is a need to better understand CDIC value ( C0) must be known. The initial CDIC available groundwater storage as a result of rainfall intensi- value can be estimated provided that the carbon sources fication (Taylor, 2014). and reactions that affect carbon mass transfer are known The Canning Basin is the largest sedimentary basin in or there is strong geochemical evidence for the assumptions Western Australia and is second in size to the Great Arte- used (e.g. Ingerson and Pearson, 1964; Gonfiantini, 1972; sian Basin in Australia. Gravity Recovery and Climate Geyh, 2000; Han and Plummer, 2013; Plummer and Experiment (GRACE) satellite mission data showed that Glynn, 2013). These single-sample-based models rely on the Canning Basin is a variably stressed aquifer system with mass balances of major carbon species or carbon isotopes a positive value of recharge and negative value of use (14C and 13C) of DIC in a single water sample. Therefore, (Richey et al., 2015). The high stress ratio given to the Basin an understanding of evolution of DIC in groundwater is suggests that about 150% more water is being depleted than important for the successful application of 14C in estimating is naturally available. This assessment contradicts other groundwater age. The graphical analysis method Han et al. global studies that suggest that the aquifers of the Canning (2012) can be used to aid in revealing the complexity of the Basin are renewable groundwater resources (Gleeson et al., geochemical environment, conceptualising the processes 14 2012). that affect CDIC in aquifers, and, determining which The conflicting global scale studies highlight our limited approach is most appropriate for the estimation of the ini- 14 knowledge of the hydrological balance of the Canning tial CDIC. Basin. Rainfall sources to the Canning Basin are well The main aims of this paper are to provide a detailed known, particularly in the coastal areas, as being influenced understanding of the source of groundwater recharge, the by both tropical maritime air from the Indian Ocean and chemical evolution of DIC and provide the first groundwa- continental air from inland. These weather patterns result ter age estimations for the Canning Basin. To do this we in extreme rainfall conditions ranging from severe droughts combine hydrochemical and isotopic techniques to investi- to cyclonic rainfall events. The rainfall in summer can be gate the type of precipitation that recharge the aquifer 14 13 widespread, associated with cyclonic weather from tropical and identify the carbon processes influencing CDIC, d - low-pressure systems with centres formed to the north-west CDIC, and [DIC]. This enables us to select an appropriate of the coast (Sturman and Tapper, 1999; Dogramaci et al., model for radiocarbon dating of DIC in groundwater. In 2012). The Pilbara coast, where the Canning Basin is addition, this paper not only presents the first hydrochem- located, experiences more cyclones than any other part of ical and isotope dataset for groundwater in the Canning Australia. Since 1910, there have been 48 cyclones (i.e. on Basin but is also the first study to use the statistical mod- average about one every two years (BOM, 2016)). The win- elling approach for calculating a 14C age for groundwaters ter rainfall for the region is relatively low, with the majority that were recharged by cyclone sourced rainfall. Further- of which (about 70–90%) resulting from north-west cloud more, this is the first study to test the sensitivity of the sta- bands (Wright, 1997; Sturman and Tapper, 1999). Based tistical approach and provide meaningful error calculations on the climatic data, it seems reasonable to suggest that for this groundwater dating method. cyclone events are the dominant groundwater recharge sources but understanding how this water is recharged 2. ENVIRONMENTAL SETTING and the volume being recharged is still poorly understood for the region. 2.1. Study site Sustainable groundwater use requires quantification of fluxes and groundwater storage (Gleeson et al., 2016). A The West Canning Basin (WCB) covers an area of groundwater flow model was created for the site 10,000 km2 and is a subset of the Canning Basin, Western (Aquaterra, 2009), which can be used to give an indication Australia. The study site is situated between Pardoo Station of the groundwater storage volume. However, estimating in the north-western corner and Shay Gap in the south- groundwater storage is challenging in most arid zone sys- eastern corner (Fig. 1). The nearest high quality long-term tems (Scanlon, 2000) and models require data for valida- climate monitoring site is Port Hedland (Bureau of Meteo- tion. Monitoring and sampling groundwater recharge rology site number: 004032) where a mean annual rainfall from high volume rainfall events is difficult especially in of 318.5 mm yrÀ1 (1942–2013) and potential evaporation remote arid regions.