Using 14C and 3H to understand groundwater flow and recharge in an aquifer window

Hydrol. Earth Syst. Sci., 18, 4951–4964, 2014 www.hydrol-earth-syst-sci.net/18/4951/2014/ doi:10.5194/hess-18-4951-2014 © Author(s) 2014. CC Attribution 3.0 License. Using 14C and 3H to understand groundwater flow and recharge in an aquifer window A. P. Atkinson1,2 , I. Cartwright1,2 , B. S. Gilfedder3 , D. I. Cendón4,5 , N. P. Unland1,2 , and H. Hofmann6 1 School of Earth, Atmosphere & Environment, Monash University, Clayton, VIC, 3800, Australia 2 National Centre for Groundwater Research and Training, Flinders University, Adelaide, SA 5001, Australia 3 Department of Hydrology, University of Bayreuth, Bayreuth, Germany 4 Australian Nuclear Science and Technology Organisation, Menai, NSW 2232, Australia 5 School of Biological Earth and Environmental Sciences, The University of New South Wales, Sydney, NSW 2052, Australia 6 School of Earth Sciences, The University of Queensland, Brisbane, QLD 4072, Australia Correspondence to: A. P. Atkinson () Received: 24 April 2014 – Published in Hydrol. Earth Syst. Sci. Discuss.: 6 June 2014 Revised: 24 October 2014 – Accepted: 9 November 2014 – Published: 9 December 2014 Abstract. Knowledge of groundwater residence times and recharge locations is vital to the sustainable management of groundwater resources. Here we investigate groundwater residence times and patterns of recharge in the Gellibrand Valley, southeast Australia, where outcropping aquifer sediments of the Eastern View Formation form an “aquifer window” that may receive diffuse recharge from rainfall and recharge from the Gellibrand River. To determine recharge patterns and groundwater flow paths, environmental isotopes (3 H, 14 C, δ 13 C, δ 18 O, δ 2 H) are used in conjunction with groundwater geochemistry and continuous monitoring of groundwater elevation and electrical conductivity. The water table fluctuates by 0.9 to 3.7 m annually, implying recharge rates of 90 and 372 mm yr−1 . However, residence times of shallow (11 to 29 m) groundwater determined by 14 C are between 100 and 10 000 years, 3 H activities are negligible in most of the groundwater, and groundwater electrical conductivity remains constant over the period of study. Deeper groundwater with older 14 C ages has lower δ 18 O values than younger, shallower groundwater, which is consistent with it being derived from greater altitudes. The combined geochemistry data indicate that local recharge from precipitation within the valley occurs through the aquifer window, however much of the groundwater in the Gellibrand Valley predominantly originates from the regional recharge zone, the Barongarook High. The Gellibrand Valley is a regional discharge zone with upward head gradients that limits local recharge to the upper 10 m of the aquifer. Additionally, the groundwater head gradients adjacent to the Gellibrand River are generally upwards, implying that it does not recharge the surrounding groundwater and has limited bank storage. 14 C ages and Cl concentrations are well correlated and Cl concentrations may be used to provide a first-order estimate of groundwater residence times. Progressively lower chloride concentrations from 10 000 years BP to the present day are interpreted to indicate an increase in recharge rates on the Barongarook High. 1 Introduction Groundwater residence time can be defined as the period of time elapsed since the infiltration of a given volume of water (Campana and Simpson, 1984), or perhaps more accurately, the mean time that a mixture of waters of different ages have resided in an aquifer (Bethke and Johnson, 2008). The residence time of water within an aquifer is a key parameter in describing catchment storage and may be used to estimate historical recharge rates (Le Gal La Salle et al., 2001; Cook and Robinson, 2002; Cartwright and Morgenstern, 2012; Zhai et al., 2013), elucidate groundwater flow paths (Gardner et al., 2011; Smerdon et al., 2012), calibrate hydraulic models (Mazor and Nativ, 1992; Reilly et al., 1994; Post et al., 2013) and characterize the rate of contaminant Published by Copernicus Publications on behalf of the European Geosciences Union. A. P. Atkinson et al.: Using 14 C and 3 H 4952 spreading (Böhlke and Denver 1995; Tesoriero et al., 2005). From a water resource perspective, information on groundwater residence times is required for sustainable aquifer management by identifying the risk posed to groundwater reserves by over-exploitation (Foster and Chilton, 2003), climate change (Manning et al., 2012) and contamination (Böhlke, 2002). Unconfined aquifers may be recharged over broad regions, leading to young groundwater at shallow depths over broad areas (Cendón et al., 2014). On the other hand, the residence time of groundwater in confined aquifers generally increases away from discrete recharge areas. The geology of catchments is often complex and heterogeneous, and outcrops of aquifers in more than one location may provide “windows” for groundwater recharge (Meredith et al., 2012). It is important to document groundwater flow from such aquifer windows. If they act as recharge areas, changes in land-use such as agricultural development may introduce contaminants to the deeper regional groundwater systems. By contrast, if they are local discharge areas, use of regional groundwater from these areas may impact rivers, lakes or wetlands that are receiving groundwater. Rivers may also recharge shallow groundwater if the hydraulic gradient between the river and the groundwater is reversed during high flows (Doble et al., 2012). Episodic recharge of aquifers by large over-bank floods is also locally important (Moench and Barlow, 2000; Cendón et al., 2010; Doble et al., 2012), particularly in arid areas (Shentsis and Rosenthal, 2003); however, the potential for over-bank events to recharge aquifers in temperate areas is still poorly understood. Additionally, during high flow, water from rivers is likely stored temporarily in the banks (McCallum et al., 2010; Unland et al., 2014); however, the depth and lateral extent to which bank exchange water infiltrates the aquifer is not well documented. Lastly, knowledge of residence times of groundwater in close proximity to the river can provide important information on groundwater–river interactions (Gardner et al., 2011). Local groundwater flow paths in connection with rivers are often underlain by deeper regional flow paths (Tóth, 1963), but the role these flow paths play in contributing to river baseflow remains unclear (Sklash and Farvolden, 1979; McDonnell et al., 2010; Frisbee et al., 2013; Goderniaux et al., 2013). This may be elucidated from understanding residence times of near-river groundwater (Smerdon et al., 2012). Radioactive environmental isotopes, in particular 14 C and 3 H, have proved useful tools for determining groundwater residence times (Vogel et al., 1974; Wigley, 1975). Produced in the atmosphere via the interaction of N2 with cosmic rays, 14 C has a half life of 5730 years and can be used t (...truncated)