Residence Times and Mixing of Water in River Banks: Implications for Recharge and Groundwater–Surface Water Exchange

Hydrol. Earth Syst. Sci., 18, 5109–5124, 2014 www.hydrol-earth-syst-sci.net/18/5109/2014/ doi:10.5194/hess-18-5109-2014 © Author(s) 2014. CC Attribution 3.0 License.

Residence times and mixing of water in river banks: implications for recharge and groundwater–surface water exchange

N. P. Unland1,2, I. Cartwright1,2, D. I. Cendón3,4, and R. Chisari3 1School of Earth, Atmosphere and Environment, Monash University, Clayton, Vic 3800, Australia 2National Centre for Groundwater Research and Training, School of Environment, Flinders University, Adelaide, SA 5001, Australia 3Australian Nuclear Science and Technology Organisation, Kirrawee DC, NSW 2232, Australia 4School of Biology, Earth and Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia

Correspondence to: N. P. Unland ([email protected])

Received: 15 January 2014 – Published in Hydrol. Earth Syst. Sci. Discuss.: 7 February 2014 Revised: 18 September 2014 – Accepted: 20 September 2014 – Published: 12 December 2014

Abstract. Bank exchange processes within 50 m of the 1 Introduction Tambo River, southeast Australia, have been investigated through the combined use of 3H and 14C. Groundwater residence times increase towards the Tambo River, which sug- Documenting water balances in river systems is vitally im- gests the absence of significant bank storage. Major ion con- portant to understanding hydrological processes and protect- centrations and δ2H and δ18O values of bank water also ing and managing water resources. While surface run-off indicate that bank infiltration does not significantly impact and regional groundwater inflows are the two main compo- groundwater chemistry under baseflow and post-flood con- nents of river flow, river banks may act as sites of transient ditions, suggesting that the gaining nature of the river may water storage. Bank storage represents water that infiltrates be driving the return of bank storage water back into the into alluvial aquifers at high river stage and subsequently re- Tambo River within days of peak flood conditions. The co- turns to the river as the river stage declines (e.g. Chen and variance between 3H and 14C indicates the leakage and mix- Chen, 2003; McCallum et al., 2010; Singh, 1968; Winter et ing between old (∼ 17 200 years) groundwater from a semi- al., 1998). Bank storage is an important hydrological pro- confined aquifer and younger groundwater (< 100 years) cess that may considerably reduce peak river discharge dur- near the river, where confining layers are less prevalent. It ing floods and maintain river discharge during periods of de- is likely that the upward infiltration of deeper groundwater creased rainfall (Pinder and Sauer, 1971). In addition, bank from the semi-confined aquifer during flooding limits bank waters may represent a source of nutrients or contaminants infiltration. Furthermore, the more saline deeper groundwa- derived from the river that are gradually released following ter likely controls the geochemistry of water in the river bank, diminishing of the flood peak. The volume and duration of minimising the chemical impact that bank infiltration has in bank storage for a given river stretch will depend on the flood this setting. These processes, coupled with the strongly gain- peak height and the flood duration (Cooper and Rorabaugh, ing nature of the Tambo River are likely to be the factors 1963), as well as the hydraulic conductivity of the alluvial reducing the chemical impact of bank storage in this setting. aquifer and the hydraulic gradient between the aquifer and This study illustrates the complex nature of river groundwa- river (Cartwright et al., 2014; Chen et al., 2006; McCal- ter interactions and the potential downfall in assuming sim- lum et al., 2010). Whiting and Pomeranets (1997) showed ple or idealised conditions when conducting hydrogeological that deeply incised narrow rivers with wider floodplains and studies. coarse alluvial material have greater bank storage potential. The potential for significant storage beneath the streambed was identified by Chen and Chen (2003), while Chen et al. (2006) showed that bank storage will return more rapidly

Published by Copernicus Publications on behalf of the European Geosciences Union. 5110 N. P. Unland et al.: Residence times and mixing of water in river banks in gaining river sections. Bank slope has also been shown to extraction systems for water supply (Hiscock and Grischek, impact bank storage, with shallower bank slope providing a 2002). greater potential for bank storage (Doble et al., 2012). It is important to carry out studies of bank storage in a The concentrations of solutes in river water are commonly range of environments as variations in climate and river form lower than those in groundwater, and mixing between in- translate into variations in river regime (e.g. frequency and filtrating river water and groundwater may produce zones duration of floods) that may cause differences in bank infil- of lower solute concentrations in river banks. McCallum et tration. Field studies focussed on bank storage and the dat- al. (2010) showed that solute concentrations of bank water ing of bank water in Australian catchments has been lim- may take months to return to their original concentration. ited. Lamontagne et al. (2011) and Cendón et al. (2010) indi- During that time period, the water that flows from the banks cated the presence of relatively young (< 50 years) ground- into the river is a mixture of regional groundwater and bank water in river banks, and Cartwright et al. (2010) showed that storage waters. This suggests that there may be a component preferential floodplain recharge is likely to occur near rivers of bank storage waters in river banks with a residence time of during flooding. In contrast, groundwater in upland catch- months to years. Recognising that bank storage waters may ments in Australia has been shown to have relatively long contribute to rivers over long time frames is important for es- residence times (Atkinson et al., 2013). This study investi- timating groundwater discharge by chemical mass balance. gates bank storage processes in the Tambo River catchment, If the bank storage waters are chemically similar to surface Victoria, Australia. The objectives of the study are to use water rather than regional groundwater, using the composi- the geochemistry of groundwater in the banks of the Tambo tion of regional groundwater as an endmember will result in River at different discharges in order to (1) define the major underestimation of the groundwater flux (Cartwright et al., processes controlling the chemistry of water stored in river 2014; McCallum et al., 2010; Unland et al., 2013). banks, (2) determine the age and likely sources water stored While the concept of bank storage is well understood, ac- in river banks and (3) identify the factors controlling bank curately quantifying the volume of water that infiltrates the storage and the distance over which bank storage is occur- banks and the duration of bank return flows remains difficult. ring. While this study uses data from specific field area, the Many studies have focused on using analytical and numer- Tambo River is similar to many others globally, and the re- ical solutions to understand bank storage from variations in sults will help in understanding bank storage processes in the river hydrograph and groundwater heads. Most of these general. studies have concluded that bank storage periods will significantly exceed the duration of flood events. Typically bank 1.1 Study area storage return to the river is proposed to decrease exponen- tially after flood events, and in the case of sandy river banks Investigations took place on the middle reaches of the Tambo with wide floodplains, residence times can be of the order River in southeast Australia. The Tambo River is perennial of years (Doble et al., 2012; McCallum et al., 2010; Whit- and flows through forest and woodland, with cattle grazing ing and Pomeranets, 1997). These studies commonly assume on the river floodplains (Department of Agriculture, Fish- ideal or generalised conditions such as aquifer homogeneity, eries and Forestry, 2006). It discharges into the saline Lake vertical river banks and saturated conditions (Doble et al., King, and the lower ∼ 15 km of the river is estuarine. Av- 2012), making them difficult to apply to many natural set- erage annual precipitation in the catchment increases from tings. Therefore, there is a need to document the extent and 655 mm in the upper reaches to 777 mm in the middle and timescales of bank storage in specific catchments. lower reaches (Bureau of Meteorology, 2013). During the Geochemical processes occurring within river banks, such majority of the study period, the discharge of the Tambo as the bacterial degradation of organic matter or the weath- River ranged from 1010 to 1011 m3 s−1 (Victorian Water Re- ering of minerals, can influence the concentrations of DOC, sources Data Warehouse, 2013); however significant rainfall O2, NO3, Na, K and other major ions in near-river groundwa- during August 2011 and March 2012 resulted in discharge ter (Bourg and Bertin, 1993). Fukada et al. (2003) identified events that peaked at greater than 5 × 1012 m3 s−1 (Fig. 2). the continuing denitrification of river water as it infiltrated The upper catchment of the Tambo River drains indurated an alluvial aquifer and demonstrated that the chemistry of Ordovician and Devonian turbidites and granites of the East- infiltrating water is likely to vary according to its residence ern Victorian Uplands, while the lower and middle catchment time within the alluvial aquifer. Understanding the source is in the Gippsland Basin (Birch, 2003). The near-river sed- and load of nutrients in rivers is fundamental in understand- iments in the lower and middle catchment comprise coarse ing their ecology (Boulton, 1993, 2005), while determining Quaternary alluvial gravels and sands. These recent alluvial the different sources of water in the riparian zone is crucial sediments overlie the Plio–Pleistocene Haunted Hill Grav- to effective vegetation management (Cey et al., 1999; Lambs, els, which represents the shallowest regional-scale aquifer in 2004; Lamontagne et al., 2005; Woessner, 2000). Similarly, the Gippsland Basin. Clay layers throughout the Quaternary the impact of infiltrating river water on water quality in al- alluvium and Haunted Hill Gravels act as aquitards, separat- luvial aquifers is important when developing groundwater ing a number of aquifer horizons that range from unconfined

Hydrol. Earth Syst. Sci., 18, 5109–5124, 2014 www.hydrol-earth-syst-sci.net/18/5109/2014/ N. P. Unland et al.: Residence times and mixing of water in river banks 5111

Figure 1. Location of ﬁeld area and schematic cross sections of bore transects at Bruthen (a), Tambo Upper (b) and Kelly Creek (c). Screened sections indicated by open boxes. Dashed line represents Tambo River basin boundary (transects orientated facing upstream).

(Southern Rural Water, 2013). Overall, the lower and middle reaches of the Tambo River are gaining, especially at low river discharge when hydraulic gradients in the shallow alluvial sediments are towards the river (Unland et al., 2013). The regional groundwater from the deeper aquifers is artesian and head gradients around Lake King (Fig. 1) are upwards (Victorian Water Resources Data Warehouse, 2013; South- ern Rural Water, 2013). The regional groundwater also has a high salinity (total dissolved solids up to 20 000 mg L−1), is Figure 2. Surface and groundwater sampling frequency super- generally anoxic and has a significantly higher temperature imposed on Tambo River hydrograph (Battens Landing, station (up to 40 ◦C) than that of the shallow groundwater. Springs 223209 – Water Resources Data Warehouse, 2013) and rainfall fed by this deeper groundwater are recorded around Lake (Bairnsdale Airport, station 85279 – Bureau of Meteorology, 2013). King; however, these are rare elsewhere (although, as discussed below, the regional groundwater may mix with the shallow groundwater and river water in the banks at Tambo Upper, Fig. 1). Most groundwater that discharges into the to fully confined (Hocking, 1976). Deeper aquifer systems in middle reaches of the Tambo River is most likely derived the lower Tambo catchment include the late Miocene to early from the shallower aquifers (Southern Rural Water, 2013), Pliocene Boisdale Formation and the Oligocene to Pliocene and the flow is part of a local rather than regional system Jemmy’s Point, Tambo River and Lake Wellington forma- (cf. Tóth, 1963). This is consistent with the presence of clays tions (Leonard, 1992; Birch, 2003; Hofmann and Cartwright, in the Gippsland Basin sediments that produce a compart- 2013). These deeper aquifer systems are separated from mentalised aquifer system. the near-surface sediments by clay layers that locally form aquitards. Regional groundwater flow in all aquifers is from the margins of the Gippsland Basin towards Lake King www.hydrol-earth-syst-sci.net/18/5109/2014/ Hydrol. Earth Syst. Sci., 18, 5109–5124, 2014 5112 N. P. Unland et al.: Residence times and mixing of water in river banks oeln seSc.4.3). Sect. (see modelling al 1. Table

a 021683 7668 .349 2.27 4.91 2.43 6.87 1.2 Studysites 6.35 8.93 10.4 7.31 11.5 5.28 8.47 2.51 22.0 4.09 BD 2.68 BD 16.5 0.00 12.2 4.12 27.3 12.1 0.03 0.07 10.1 2.03 0.77 183 6.07 0.08 0.06 23.4 30.2 2.72 16.9 0.03 36.2 11.4 0.00 0.21 17.8 5.36 0.16 1.48 0.05 67.1 17.5 5.02 0.03 0.30 0.05 78.2 0.05 0.20 18.3 0.04 14.6 5.83 0.09 17.6 2.17 8.38 0.08 0.17 11.1 0.90 0.05 5.56 8.36 0.03 18.8 11.5 287 0.08 0.37 0.00 8.36 0.02 12.7 23.5 10.3 136 0.07 0.11 18.3 7.47 5.66 6.05 293 0.06 0.02 17.6 0.10 7.94 2012 17.6 1.94 Mar 0.12 0.05 156 0.15 5.56 0.01 7.88 2012 Mar 607 0.06 0.04 18.3 0.07 7.91 6.75 2012 Mar 3 215 0.04 0.24 17.6 0.02 17.6 8.89 3.63 2012 2 229 Mar 5.56 0.05 8.84 13.3 2011 0.04 1 Nov 293 0.00 0.03 8.84 5.61 2011 R Nov 173 0.05 9.60 8.81 2011 0.12 3 Nov 179 5.56 9.63 2011 0.06 2 Aug 145 0.00 7.66 2011 0.04 1 Aug 18.3 7.61 2011 200 3 0.08 Aug 17.6 7.90 2011 2 109 5.56 Aug 7.88 2011 261 Apr 1 0.00 7.87 2011 191 Apr R 7.84 2011 146 2 Apr 2011 Feb 1 121 2011 Feb R 2011 Feb 3 2011 2 Feb 1 Date R Site

Transects of groundwater monitoring bores were set up at umr aafrBuhntasc.HCO transect. Bruthen for data Summary three locations on the river banks of the middle Tambo River (Fig. 1, Tables 1, 2 and 3). The transect at Bruthen is 28.5 km

upstream of Lake King and consists of three bores installed at cm µS 5.5, 17.6 and 18.3 m distance from the river and 8.0, 5.4 and CLvlDs.FC rNO Br Cl F Dist. Level EC −

7.1 m depth below ground surface, respectively (Fig. 1). The 1 transect at Tambo Upper, 20.2 km upstream of Lake King,

consists of ﬁve bores installed at 8.8, 15.0, 22.3, 23.8 and L mg m m 37.9 m distance from the Tambo River and 6.7, 6.2, 23.1, 6.7 and 9.8 m depth below ground surface, respectively. The ﬁnal transect at Kelly Creek, 13.8 km upstream of Lake King, consists of four bores installed at 7.0, 17.9, 24.9 and 26.8 m from − the Tambo River at depths of 8.1, 7.8, 28 and 7.9 m depth, re- 1 3

spectively. Bores at Tambo Upper have 1.5 m screens starting L mg aai tlc acltdvacag aac.MR engonwtrrsdnetm scluae i xoeta itnﬂow piston exponential via calculated as time residence groundwater mean – MGRT balance. charge via calculated – italics in data 1 m from the borehole bottom, while all other installations − have a 3 m screened section set at the bottom of the borehole. 1

Sediment samples taken during installation of the shallower L mg (< 15 m deep) bores via auger drilling indicate that the allu- − vial aquifer at all transects is dominated by coarse sands with 1

10 to 20 cm thick clay layers dispersed throughout. Deeper L mg

bores were constructed via mud rotary drilling, which tends − to preferentially return coarser sediment fractions. While 1 3 clay layers are harder to identify, the groundwater at > 20 m L mg SO − 1 depth is artesian (see below), which suggests the presence of 4 a clay-rich conﬁning layer. All bores are screened in the allu- gL mg HCO 44.7 38.7 vial sands/gravels and (except for the deeper bore at Tambo 62.1 45.9 63.1 49.5 75.1 52.5 85.0 41.3 50.9 46.4 − 1 Upper) probably sample the local shallow groundwater rather 3

than the deeper regional groundwater. L mg 9116 .36.87 6.38 8.43 5.08 14.4 17.8 1.66 4.58 8.17 5.37 15.8 1.72 19.1 8.20 1.53 10.4 3.32 9.41 4.91 3.91 11.1 4.85 2.10 33.1 11.3 13.7 6.44 15.2 2.44 7.33 1.68 5.14 2.52 3.05 10.3 11.7 3.95 10.4 8.33 2.52 7.77 3.00 1.65 19.2 1.38 7.16 12.1 7.65 aKC gOH O Mg Ca K Na − 1

2 Methods L mg − Bore and river elevations were determined to ±1 cm rela- 1 tive to the Australian Height Datum (AHD) using a Trim- L mg ble digital global positioning system (DGPS). Bores were − 1 sampled using an impeller pump set at the screened section, gL mg and at least three bores’ volumes were pumped before sam- − ple collection. Five sets of groundwater samples and four 1 − − − − sets of river samples were collected between February 2011 − − − − − − − − − − − − δ 6.66 5.38 5.93 5.37 4.67 5.13 7.56 7.40 6.10 4.81 4.34 7.61 5.42 7.53 4.97 5.69 and March 2012 at each transect. Sampling during Febru- 18 O ary 2011, April 2011 and November 2011 took place at con- − − − − − − − − − − − − − − − − 36.5 29.2 99.33 98.04 2.84 2.65 37.4 34.7 22.2 28.7 46.2 44.4 34.6 31.6 24.7 46.6 36.5 47.9 33.6 39.1 ditions close to baseflow, while sampling during August 2011 δ 2 and March 2012 took place ∼ 1 week after significant flood- Years ‰ pMC TU H ing in the catchment (Fig. 2). Rising head slug tests were 3 conducted by pumping bores for ∼ 10 min with an impeller H pump at a rate of 4 L min−1 and then allowing groundwa- 14 ter heads to recover. Changes to groundwater levels were C

recorded using a Rugged TROLL 200 logger logging at 1 s − − δ 542.2 3.4 15.4 13.9 13

intervals. Hydraulic conductivity was calculated using the MGRT C Hvorslev method outlined by Fetter (1994). The anisotropy ratio of the sediments was not taken into account as it is not explicitly known and the aim was to provide a general characterisation of the hydraulic properties of the sediments.

Hydrol. Earth Syst. Sci., 18, 5109–5124, 2014 www.hydrol-earth-syst-sci.net/18/5109/2014/ N. P. Unland et al.: Residence times and mixing of water in river banks 5113 C MGRT 10 27 9.3 68/52 7.25.7 100/80 17 200 8.7 99/78 13 − − − − − δ C 14 H 3 0.03 10.6 < H TU pMC ‰ Years 2 δ 45.2 32.9 31.7 33.4 27.7 29.4 35.0 37.9 39.1 35.2 38.1 46.4 28.3 26.6 40.8 36.9 1.21 79.2 34.9 26.7 38.3 34.9 1.55 94.5 38.2 0.36 35.4 38.3 38.4 39.6 39.1 37.3 38.8 61.9 37.0 0.40 38.0 − − − − − − − − − − − − − − − − − − − − − − − − − − − − − H activity. 3 O 18 7.66 6.01 5.92 6.03 5.38 5.34 6.07 5.93 6.23 5.37 5.84 7.64 5.65 5.88 6.12 5.74 5.84 6.18 5.35 5.98 5.63 6.06 5.97 6.32 5.43 5.69 7.88 5.98 δ − − − − − − − − − − − − − − − − − − − − − − − − − − − − 1 − mg L 1 − mg L 1 − mg L 1 − Na K Ca Mg O H 554 41.2 63.1484 38.7 35.2 58.7 33.3 339280600 27.3 21.3115 48.4 105303 78.0241 3.31 80.9 38.2 15.5 31.7 14.2 36.5 45.2 158 79.4 91.6 20.9 141 6.21 26.5 190 54.4 19.4 68.4 11.7 68.5 21.7 70.4 24.2 35.7 148 6.38 63.3 21.2 279280374 23.0 19.2 35.4 99.6 62.4 64.6 30.3 33.9 29.8 11.6 1.8598.3 5.87 3.44 4.78 32.6 17.6 98.812.0 4.00 1.76 36.088.7 8.69 20.1 4.48 5.47 38.0 19.7 n/a 7.83 1.7472.8 8.24 4.65 3.86 33.9 17.7 mg L 3 1 − BD BD 760 279 799 118 383 294 170 212 406 316 157 207 36.5 89.8 48.1 48.5 73.0 57.6 27.7 60.7 HCO mg L H activities calculated by mixing and for the measured 4 1 3 − SO mg L 3 1 − mg L 1 − mg L 1 data in italics – calculated via charge balance. MGRT – mean groundwater residence time as calculated via exponential piston − 3 mg L 1 − m m mg L 5.04 22.34 0.19 6395.04 22.34 2.28 0.24 0.21 958 46.8 2.46 0.05 41.0 > > 1 − EC Level Dist. F Cl Br NO µS cm Summary data for Tambo Upper transect. HCO R1 Aug2 2011 Aug3D 2011 Aug4 2011 Aug 2011 1485 2682 Aug1 2011 2207 4.74 3250 Aug2 2011 4.77 Nov3D 2011 0.00 4.75 5.04 8.82 774 Nov4 2011 Nov 15.03 1039 2011 22.345 0.06 2018 4.70 NovR 2011 0.11 4.80 2168 0.19 23.73 2938 Nov1 2011 0.26 3.74 37.87 18.1 Mar2 2012 3.70 599 8.82 864 Mar3D 2012 0.08 501 15.03 1337 753 0.14 Mar 0.024 2012 Mar 2012 3.61 165 1.63 0.30 3.70 1.42 1350 207 0.07 23.73 2.09 Mar 2012 227 2.46 37.87 1763 3.32 3210 0.53 4.30 439 0.27 0.41 0.18 0.11 0.00 531 4.26 0.43 4.21 0.08 8.82 881 12.6 15.03 1.42 22.2 0.12 34.2 0.09 1.21 178 0.08 4.22 0.07 276 0.07 23.73 0.56 5.59 19.6 0.12 0.56 11.6 0.83 385 0.10 544 49.7 0.04 4.46 0.21 0.11 0.93 1.11 221 0.54 1.24 48.3 0.13 0.07 0.53 4.21 33.5 29.7 0.09 1.45 5 Apr 2011 1043 3.39 37.87 0.11 203 0.61 0.075 Mar 2012 50.3 80.5 1320 4.32 122 37.87 7.10 0.09 64.9 344 20.7 0.76 0.30 40.4 3D4 Apr 2011 Apr 2011 2488 4.69 717 22.34 3.30 23.73 0.34 0.11 534 140 1.72 0.42 0.33 0.47 10.9 15.9 439 100 468 98.1 33.8 4.42 71.8 37.1 41.2 21.2 2 Apr 2011 2180 3.36 15.03 0.11 533 1.76 1.24 25.9 272 333 23.1 86.8 52.8 12 Feb3D 2011 Feb4 2011 Feb 20115 2395 FebR 2011 2155 Feb1 2656 2011 3.90 Apr 2011 3.86 Apr 4.85 8.82 764 2011 15.03 1023 22.34 112 3.78 0.14 3.89 2210 0.12 23.73 0.36 37.87 3.43 3.38 572 0.00 0.12 555 607 8.82 1.88 0.04 1.77 176 1.98 0.14 0.33 1.58 0.52 2.54 460 25.2 14.7 0.07 22.8 1.43 0.01 21.3 0.22 0.08 36.8 0.92 291 280 16.0 26.9 4.34 111 10.0 40.7 15.1 Site Date R Feb 2011 120 3.58 0.00 0.08 9.78 0.04 0.11 2.04 Table 2. ﬂow modelling (see Sect. 4.3). MGRTs at TU1, TU2 and TU5 have been given for the www.hydrol-earth-syst-sci.net/18/5109/2014/ Hydrol. Earth Syst. Sci., 18, 5109–5124, 2014 5114 N. P. Unland et al.: Residence times and mixing of water in river banks o oeln seSc.4.3). Sect. (see modelling ﬂow 3. Table e 011 4 .5003 322. .0825 0.30 22.0 6322 0.38 0 3.05 340 18 2011 Feb R Date Site e 0120 .470 . 1 .403 0.43 0.35 1.54 518 0.3 7.02 3.14 2004 2011 Feb 1 e 0124 .71. .35717 .72.01 0.77 1.77 597 0.43 17.9 3.17 2349 2011 Feb 2 DFb21 .22. 6 142. 9.31 23.6 11.4 162 1.14 0.56 1.92 637 0.38 26.8 24.9 3.2 3.82 2364 2011 Feb 2011 Feb 4 3D p 0129 .52. .76018 .707 9 1 .230147.8 48.7 42.3 340.1 43.6 123.3 3.12 288.9 206.9 17.8 2.83 313 3.29 4.82 372 252 591 204 2.24 445 488 0.78 446 0.03 68.9 29.1 1.00 1.87 0.28 21.6 0.68 0.13 0.43 1.83 0.27 0.05 1.30 0.93 1.57 630 1.36 0 413 255 558 0.57 474 0.39 0.21 0.33 26.8 0.3 3.15 170 24.9 0 17.9 2099 3.95 7.02 3.07 3.15 2669 2011 Aug 2455 2011 Apr 2145 R 2011 4210 Apr 2011 4 Apr 2011 3D Apr 2011 2 Apr 1 R u 0126 .370 .45014 .40.38 0.84 1.40 590 0.24 7.02 3.63 2568 2011 Aug 1 u 0127 .31. .66514 .81.56 0.18 1.44 655 0.26 17.9 3.63 2777 2011 Aug 2 DAg21 4843 4902 0 .021 61.4 2.12 1.30 608 0.23 24.9 4.30 2438 2011 Aug 3D o 0124 .204 3 .027 0.68 2.70 0.52 1.70 0.01 533 1.26 513 0.45 7.02 0.26 26.8 3.68 2742 2717 2011 Nov 2011 Aug 1 4 o 0124 .61. .25316 .351.4 0.13 1.62 563 0.32 17.9 3.36 2542 2011 Nov 2 DNv21 7843 4904 3 .400 0.57 0.04 1.44 436 0.41 24.9 4.33 2738 2011 Nov 3D o 0121 .12. .55217 .80.54 2.48 1.71 532 0.35 26.8 3.41 2218 2011 Nov 4 a 0227 .2035814 .42.44 7.25 0.04 0.21 1.48 0.09 598 33.4 0.3 0.08 0 7.02 2770 195 2012 Mar 2012 Mar 1 R a 0229 7904 0 .301 1.11 0.10 1.53 605 0.41 17.9 2495 2012 Mar 2 DMr21 352. .75014 .60.46 0.16 1.45 560 0.37 24.9 2345 2012 Mar 3D a 0221 6803 1 .401 43.4 0.13 1.64 713 0.32 26.8 2715 2012 Mar 4 umr aafrKlyCektasc.HCO transect. Creek Kelly for data Summary Scm µS CLvlDs.FC rNO Br Cl F Dist. Level EC − 1 gL mg m m − 1 gL mg 3 aai tlc acltdvacag aac.MR engonwtrrsdnetm scluae i xoeta piston exponential via calculated as time residence groundwater mean – MGRT balance. charge via calculated – italics in data − 1 gL mg − 1 gL mg − 1 3 gL mg SO − 1 4 gL mg HCO 8,690 24.3 39.0 397 637 256 899 294 374 271 355 292 848 291 711 998 552 465 430 − 1 3 gL mg 5811120521 182.0 151 3558 6313437116 403.7 153 2683 3422 6. 35.5 262.8 2.28 73.4 211. 7430.1 47.4 11.9 82.1 3823 . 5.03 6.3 2.38 13.8 7 .218033.6 178.0 3.82 178 0 .321032.4 231.0 3.93 202 9 .929029.6 229.0 2.59 197 4 .728942.8 218.9 4.27 146 3 .326349.8 286.3 3.73 135 1 2115751 185.7 12.1 218 8 .429348.7 249.3 6.14 286 6 .627542.9 267.5 3.26 269 3 1012050.7 192.0 11.0 338 2 .929534.4 229.5 2.49 226 1 .925237.9 215.2 4.39 218 1 .227832.8 237.8 3.72 219 9 .519441.6 199.4 8.75 292 8 .227527.6 217.5 3.42 189 aKC gOH O Mg Ca K Na − 1 gL mg − 1 gL mg − 1 gL mg − 1 − − − − − − − − − − − − − − − − − − − − − − − − δ 3.38 5.63 5.61 5.81 6.68 6.17 5.58 5.63 5.58 7.69 7.48 5.27 5.28 5.55 5.42 5.56 5.69 5.60 5.48 7.58 5.52 5.45 5.65 5.62 18 O − − − − − − − − − − − − − − − − − − − − − − − − 24.6 37.0 36.9 37.8 41.1 8505 84.2 0.51 83.6 38.5 80.4 0.37 36.3 0.40 36.2 35.5 45.5 46.0 29.5 29.6 29.2 30.3 35.5 36.9 36.3 36.2 45.5 33.1 32.8 34.2 30.7 δ 2 UpC‰Years ‰ pMC TU H 3 H 14 C δ − − − 13 . 96 100 3.7 99 2.9 2.8 MGRT C

Hydrol. Earth Syst. Sci., 18, 5109–5124, 2014 www.hydrol-earth-syst-sci.net/18/5109/2014/ N. P. Unland et al.: Residence times and mixing of water in river banks 5115

Commonly, the recovery time was < 10 s, and the estimated hydraulic conductivities are likely to be minimum values. Electrical conductivity (EC) was measured in field to ±1 % using a calibrated TPS pH/EC meter, and groundwater levels were measured using an electronic water level tape. Water samples were preserved by refrigeration in airtight polyethylene bottles. HCO3 and dissolved CO2 were measured within 48 h of sample collection by titration using a Hach digital titrator and reagents with a precision of ±5 %. Anion concentrations were determined on filtered (0.45 µ cellulose nitrate filters) samples using a Metrohm ion chro- matograph at Monash University, Clayton, with a precision of ±2 % estimated by replicate analysis. Cation concentrations were determined on samples that were filtered and acid- ified to pH < 2 using twice-distilled 16 M nitric acid by the Varian Vista ICP-AES at the Australian National University or the ThermoFinnigan X series II, quadrupole ICP-MS at Monash University. Drift during ICP-MS analysis was cor- rected using internal Sc, Y, In and Bi standards, with replicate Figure 3. Groundwater elevations during February 2011 and Au- analysis returning a precision of ±5 %. Stable isotope ratios gust 2011 at Bruthen (a), Tambo Upper (b) and Kelly Creek (c). were measured at Monash University using ThermoFinni- White rectangles – measured elevation; dashed lines – interpolated gan MAT 252 and DeltaPlus Advantage mass spectrome- elevations. ters. δ18O values of water were measured via equilibration ◦ with He–CO2 at 32 C for 24–48 h in a ThermoFinnigan Gas Bench. δ2H values of water were measured via reac- following Stuiver and Polach (1977). The average error asso- tion with Cr at 850 ◦C using a Finnigan MAT H/Device. ciated with radiocarbon measurements is 0.3 %. δ18O and δ2H values were measured relative to internal standards that were calibrated using International Atomic En- 3 Results ergy Agency (IAEA) SMOW (Standard mean ocean water), GISP (Greenland ice sheet precipitation) and SLAP (Stan- 3.1 Groundwater elevations and hydraulic dard light antarctic precipitation) standards. Data were nor- conductivities malised following Coplen (1988) and are expressed relative to V-SMOW, where δ18O and δ2H values of SLAP are −55.5 Groundwater elevation at Bruthen varied between and −428 ‰, respectively. The precision (1σ ) of the analy- 7.45 m AHD in April 2011 and 8.89 m AHD in August 2011. ses based on replicate analyses is δ18O = ±0.2 ‰ and δ2H There was less than 6 cm difference in elevations across the = ±1 ‰. transect during any given sampling period. Groundwater Samples for 14C and 3H analysis were collected during elevations in B1 and B2 were within 3 cm of each other the April 2011 sampling period (Fig. 2). 3H water samples during all sampling periods, while elevations in B3 were were distilled and electrolytically enriched prior to analy- 2 to 6 cm higher than in B1 and B2 (Fig. 3). Groundwater sis by liquid scintillation (Morgenstern and Taylor, 2009). elevations at Bruthen were 3 to 4 cm higher than river eleva- The 3H concentrations were expressed in tritium units (TU), tions during all sampling periods except during March 2012, with uncertainties ranging from ∼ 25 % at the quantifica- when groundwater levels were approximately 90 cm higher tion limit (0.13 TU) to < 6 % for 3H concentrations above than the river elevation. Rising head slug tests at this transect 1.5 TU. For 14C analysis, the total DIC (dissolved organic indicate a hydraulic conductivity of ∼ 8.5 × 10−3 m s−1. carbon) was converted to CO2 by acidifying the samples with Groundwater elevation in the shallow bores at Tambo Up- H3PO4 and extracting the liberated CO2 gas using a custom- per ranged from 3.30 m AHD in April 2011 to 4.80 m AHD built extraction line. The CO2 sample was then heated in in August 2011. Groundwater elevations in TU5, TU2 and a sealed glass tube containing baked CuO and Ag and Cu TU1 in individual campaigns were within 3 to 5 cm of each wire at 600 ◦C for 2 h to remove any sulfur compounds that other. Groundwater elevations in TU4 were the lowest in may have been liberated and subsequently graphitised. The the transect, averaging 3.92 m AHD over the study, approx- graphite targets were analysed using the STAR (Small Tan- imately 9 cm lower than the average levels in TU1, TU2 dem for Applied Research) AMS (accelerator mass spec- and TU5 (Fig. 3). The deeper bore (TU3D) was artesian trometry) at ANSTO (Australian Nuclear Science and Tech- during all sampling periods; this bore samples a deeper, nology Organisation) following Fink et al. (2004). The ac- semi-confined aquifer within the alluvial gravel that has tivity of 14C is expressed as percent modern carbon (pMC) higher heads than the surficial aquifer. During February www.hydrol-earth-syst-sci.net/18/5109/2014/ Hydrol. Earth Syst. Sci., 18, 5109–5124, 2014 5116 N. P. Unland et al.: Residence times and mixing of water in river banks and April 2011, groundwater elevations in this bore were 4.85 m and 4.69 m AHD, respectively, while in all other sampling periods the elevation exceeded that of the cas- ing (5.04 m AHD). Groundwater elevations at Tambo Up- per were higher than the river elevation during all periods except April 2011. Groundwater elevation closest to the river (TU1) was 32 cm higher than river elevation in Febru- ary 2011, 5 cm lower then river water during April 2011, 3 cm higher than river elevation during August 2011 and 99 cm greater than river elevation during March 2012. Slug tests at this transect yielded hydraulic conductivities ranging × −4 × −5 −1 from 5.1 10 to 8.6 10 m s in the surficial aquifer 18 2 and 1.9 × 10−5 m s−1 in the semi-confined aquifer. Figure 4. δ O and δ H values of bank water and river water from the Tambo River. LMWL defined by Melbourne meteoric water line At Kelly Creek, groundwater levels in the shallower bores in Hughes and Crawford (2012). Dashed lines indicate typical river ranged from 3.07 m AHD in April 2011 to 3.68 m AHD in water and bank water values. August 2011 (Fig. 3). Groundwater levels in these bores generally decreased with proximity to the river during all sample periods except April 2011. Groundwater levels in the from −4.3 to −7.5 ‰ and were generally higher closer to the deeper bore at Kelly Creek (KC3D) were higher than the river at B1 (average = −4.8 ± 0.4 ‰) than further from the shallow bores, ranging from 3.82 m AHD in February 2011 river at B2 and B3 (average = −5.3 ± 2.2 ‰). Stable isotope to 4.33 m AHD in November 2011. Slug tests at this tran- values were less variable at Tambo Upper, with δ18O values sect indicate hydraulic conductivities ranging from 2.4 to ranging from −5.3 to −6.3 ‰. Groundwater at TU3D, TU1 3.4 × 10−5 m s−1. and TU2 has lower δ18O values (average = −6.0 ± 0.2 ‰) than at TU4 and TU5 (average = −5.6 ± 0.2 ‰). Shal- 3.2 Electrical conductivity low groundwater at Kelly Creek showed little variability, with δ18O values ranging from −5.3 to −5.8 ‰ dur- Groundwater EC values at Bruthen ranged from 136 to ing the study. The deeper groundwater at Kelly Creek had 607 µS cm−1. Groundwater at B3 was generally the most slightly lower δ18O values (average = −5.9 ± 0.5 ‰) than saline, ranging from 261 to 607 µS cm−1, while that from the shallow groundwater. River water had lower δ18O values B1 ranged from 136 to 293 µS cm−1. Shallow groundwater than groundwater during all sampling periods except Febru- at Tambo Upper was more saline than that from Bruthen, ary 2011. During this period, δ18O values of river water in- ranging from 717 to 2682 µS cm−1. Shallow groundwater at creased from −5.7 ‰ at Bruthen to −3.4 ‰ at Kelly Creek. Tambo Upper was also generally more saline closer to the There was less variation in the river water at other times river than further from the river, averaging 2110 µS cm−1 during the study, with δ18O values ranging from −7.9 to at TU1 and TU2 over the study period, compared to −7.5 ‰. 980 µS cm−1 at TU4 and TU5. Deeper groundwater at Tambo Upper was consistently the most saline in the transect, rang- 3.4 3H and 14C ing from 2490 µS cm−1 in April 2011 to 3250 µS cm−1 in 3 14 August 2011. Groundwater at Kelly Creek was generally H and C activities in April 2011 were the highest in more saline than Tambo Upper, with EC values ranging from groundwater from Bruthen, ranging from 2.7 to 2.8 tritium 3 2000 to 2777 µS cm−1 over the study period. Groundwater units and 98.0 to 99.3 pMC, respectively. H activities were EC values were less variable at Kelly Creek and did not higher in groundwater further from the river at Tambo Up- 3 generally increase or decrease with proximity to the Tambo per at TU4 and TU5 ( H activities of 1.6 and 1.2 tritium River. units, respectively) compared to groundwater closer to the river at TU1 and TU2 (3H activities of 0.40 and 0.36 tritium 3 3.3 Stable isotopes units, respectively). H activities in the deeper groundwater at TU3D were below detection. 14C activities show a similar δ18O and δ2H values of groundwater and river water gener- variation, with higher activities at TU4 and TU5 (94.5 and ally plot close to the both local (LMWL) and global meteoric 79.2 pMC) compared to groundwater at TU1 and TU2 (35.4 14 water lines (GMWL); however river water at Kelly Creek and 38.0 pMC). Deeper groundwater at TU3D had lower C 3 from February 2011 plots to the right of the GMWL (Fig. 4). activities (10.6 pMC). H activities in groundwater at Kelly Groundwater and surface water samples from February 2011 Creek decreased from 0.51 tritium units at KC4 to 0.40 and 14 define an array with a slightly shallower trend than the 0.36 tritium units at KC1 and KC2, respectively. C activi- LMWL, while those from March 2012 define an array with a ties follow a similar trend, decreasing from 84.2 pMC at KC4 steeper trend than the LMWL. δ18O values at Bruthen ranged to 80.4 pMC at KC1.

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Figure 7. Trends in major ion chemistry at Kelly Creek indicating Cl inputs during increased rainfall.

Figure 8. Trends in major ion chemistry at Tambo Upper indicating mixing between groundwater in the shallow, unconﬁned aquifer and Figure 5. Piper plot of bank water from the Tambo River. Black groundwater from the deeper, semi-conﬁned aquifer. markers – HCO3 measured; grey and white markers – HCO3 calculated via charge balance. at Tambo Upper, with relative standard deviations of between 15 and 21 % of the mean values. At Tambo Upper Na and K concentrations increase and Ca and Mg concentrations decrease with increasing Cl concentrations (Fig. 8). Groundwa- ter further from the river at Tambo Upper (TU4 and TU5) has Cl concentrations below 10 mmol L−1, K concentrations below 0.2 mmol L−1 and Na concentrations below 7 mmol L−1 Figure 6. Trends in major ion chemistry at Bruthen indicating min- (Table 1). Deeper groundwater from Tambo Upper (TU3D) eral dissolution, the input of Cl into groundwater and the input of K −1 into groundwater. has Cl concentrations greater than 15 mmol L , K concentrations greater than 0.8 mmol L−1 and Na concentrations greater than 16 mmol L−1. Groundwater closer to the river at 3.5 Major ions Tambo Upper (TU1 and TU2) has concentrations of Na, K, Mg and Ca that are intermediate between those of groundwa- Despite sampling groundwater from similar aquifers, there ter at TU3D and groundwater at TU4 and TU5. are considerable differences in the geochemistry of ground- Shallow groundwater at Kelly Creek is Cl–Ca–Na type. water from the three locations. SO4 is the most variable anion at Kelly Creek, with rela- Groundwater from Bruthen is HCO3–Ca–Na type (Fig. 5). tive standard deviations of between 108 and 206 % of mean NO3, Br and Cl are the most variable anions over time, values at each bore. Na is the most variable cation at Kelly with relative standard deviations of 120, 91 and 82 % of Creek, with relative standard deviations of between 22 and the mean concentrations, respectively. Na is the most vari- 43 % of mean values. At Kelly Creek, shallow groundwater − able cation at the site, with relative standard deviations of has Cl concentrations that range from 11.6 to 20.1 mmol L 1 − 53 % of the mean concentrations. The concentration of most and Ca concentrations that range from 3.1 to 8.5 mmol L 1 major cations at Bruthen decrease with increasing Cl con- (Fig. 7). Ca, Na, K and Mg concentrations generally in- centrations; however K has a weak positive correlation with crease with Cl concentrations. Deeper groundwater from Cl (Fig. 6). Molar Na : Cl ratios at Bruthen generally range Kelly Creek shows similar trends in major ion concentrations from 2 to 4 during periods of lower rainfall in the catchment to shallower groundwater; however, the relative proportion (February 2011, April 2011 and November 2011) and are of Na and Mg is higher and the relative proportion of Ca is generally below 1 during periods of increased rainfall (Au- lower. Molar Cl : Br ratios in groundwater at Kelly Creek in- gust 2011 and March 2012). Molar Cl : Br ratios at Bruthen crease from ∼ 650 to ∼ 1000, while Na : Cl ratios decrease increase from 140 to over 1000 with increasing Cl concen- from 1.4 to 0.4 as Cl concentrations increase. trations (Fig. 6). Groundwater from Tambo Upper is Cl–Na–Ca type (Fig. 5). NO3 is the most temporally variable anion at Tambo Upper, with a relative standard deviation of 97 % of the mean. In contrast, temporal variations in cations are relatively low www.hydrol-earth-syst-sci.net/18/5109/2014/ Hydrol. Earth Syst. Sci., 18, 5109–5124, 2014 5118 N. P. Unland et al.: Residence times and mixing of water in river banks

4 Discussion the bank waters at Tambo Upper increases during the wetter periods in August 2011 and March 2012, suggesting that hy- The following section focusses on identifying the source draulic loading of the deeper, semi-confined aquifer is driv- of water stored in the banks of the Tambo River. Through ing increased flow of deeper groundwater into the overlying groundwater dating, the prevalence of bank storage is evalu- alluvial aquifer at these times. As indicated in Sect. 3.5, high ated and patterns in groundwater recharge and flow are iden- variations in NO3 concentrations were observed both tem- tified. These evaluations are further coupled with major ion porally and spatially across the transects. These variations and stable isotope analysis under changing hydrological con- were not systematic with changing hydrological conditions ditions, in order to identify processes controlling the chem- or other major ions, suggesting that perhaps changing redox istry of bank water and the potential impacts to river and conditions have impacted the observed NO3 concentrations. groundwater quality. However, as redox conditions were not recorded and multi- species analysis of N were not undertaken, these processes 4.1 Hydrogeochemical processes remain unresolved.

Na : Cl ratios were higher in the groundwater at Bruthen and 4.2 Aquifer interactions Kelly Creek during periods of lower rainfall compared to pe- 14 3 riods of higher rainfall (Figs. 6 and 7). This suggests that the The C and H activities in groundwater may be predicted groundwater present in the banks during periods of low rain- from their atmospheric concentrations and groundwater res- fall has longer residence times that facilitate water–rock in- idence times. The activities of these isotopes in the at- teraction, specifically the dissolution of Na-bearing minerals mosphere were elevated due to nuclear tests that occurred such as plagioclase (Edmunds, 2009; Herczeg et al., 2001). mainly in the 1960s (the so-called “bomb pulse”). For this 3 The same trend is also apparent in the deeper groundwater study, present-day H activities are taken to be 3.2 tritium 3 at Tambo Upper (Fig. 8). Cl : Br ratios at Bruthen and Kelly units, which is the weighted average rainfall H activity for Creek increase with increasing Cl concentrations (Figs. 6 and July 2005 to June 2011 in the Melbourne area (Tadros et al., 7). As evapotranspiration, which is the dominant process that 2014), and we assume that pre-bomb-pulse tritium activities controls groundwater salinity in southeast Australia (Herczeg were similar to these (Allison and Hughes, 1977). For inter- 3 et al., 2001; Cartwright et al., 2007, 2010), does not impact vening years, the mean weighted average of H concentra- Cl : Br ratios, this implies the addition of Cl from an exter- tion of precipitation in Melbourne was taken as that of local nal source. Halite dissolution is a potential Cl source; how- precipitation with the record extrapolated for years with no 3 14 ever, there are no obvious stores of halite in the catchment. data (Cartwright et al., 2013). Unlike H, C activities of An alternative source of Cl is KCl fertilisers that are used atmospheric CO2 were similar in the Northern and Southern locally (Department of Environment and Primary Industries, hemispheres (Fontes, 1983). The data of Hua et al. (2013) 14 2013). K : Cl ratios decrease with increasing Cl concentra- were used for C activities of precipitation from 1950 to 14 tions at Bruthen (Fig. 6), which would be not be expected 2011. Pre-1950, C activities are assumed to have decreased for a KCl source; however, K may be removed from wa- from 100 pMC in 1905 to 97.5 pMC in 1950 due to fossil fuel ters recharging through the soils by vegetation (e.g. Schacht- burning (Suess, 1971). man and Schroder, 1994) or sorption onto clay minerals such Lumped-parameter models are commonly used to describe as illite (Griffioen, 2001). In any case, the observation that groundwater flow in shallow unconfined and semi-confined increased Cl concentrations coincide with increased rainfall aquifers (Małoszewski and Zuber, 1991, 1982; Morgenstern suggests that infiltration facilitates the transport of Cl from et al., 2010; Zuber et al., 2005). Piston flow models assume the land surface and/or the soil profile into shallow ground- that no mixing takes place between recharge and water in the water (cf. Panno et al., 2006). aquifer and are suitable for settings where dispersion is low. Mixing between deeper groundwater (TU3D) and shal- Conversely, exponential flow models assume a vertical strat- low groundwater (TU4) appears to dominate the chemical ification of groundwater ages in an aquifer and are suitable variability of groundwater throughout the rest of the tran- for the sampling of fully penetrating wells or surface water sect at Tambo Upper (Fig. 8). The deeper groundwater has bodies fed by aquifers receiving homogeneous recharge. This elevated Na : Cl and K : Cl ratios, likely to be attributed study uses the exponential piston flow model (EPFM) which to greater residence times and Na and K mineral disso- combines a portion of piston flow followed by a portion of lution. Deeper groundwater is also relatively saline (17.11 exponential flow and is appropriate for bores in unconfined to 27.03 mmol L−1) compared to shallower groundwater at to semi-confined aquifers screened below the water table that TU4 (3.94 to 6.24 mmol L−1). Groundwater throughout the do not sample the shallowest groundwater that has very short rest of the transect contains intermediate Cl concentrations residence times (Morgenstern et al., 2010; Cartwright and and cation-to-chloride ratios, which is consistent with mix- Morgenstern, 2012). ing between the two endmembers. It is also apparent that the relative proportion of the deeper groundwater endmember in

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displace water compositions to the left of the predicted 14C vs. 3H trends. Closed-system calcite dissolution that lowers 14C but which does not impact 3H activities produces a similar displacement. The co-variance between 3H and 14C for groundwater samples is shown in Fig. 9. Groundwater from Bruthen and Kelly Creek has 3H and 14C activities lie close to the predicted co-variance curves. Groundwater in aquifers dominated by siliciclastic sediments typically undergo up to 20 % closed-system calcite dissolution (Vogel, 1970; Clark and Fritz, 1997), and, if that is the case for this groundwater, it would explain the slightly lower than expected 14C activities. Regardless, there can be very limited mixing between Figure 9. Co-variance of 3H and 14C in groundwater and that older and younger groundwater at these localities. By con- predicted by Eq. (1) (solid lines) for 0 and 20 % DIC input from closed-system calcite dissolution. Dashed lines are the predicted co- trast, groundwater from TU1, TU2 and TU5 at Tambo Upper variance of 3H and 14C from the renewal rate model of Le Gal La follow a trend consistent with the mixing between younger Salle et al. (2001). groundwater in the shallow aquifer (TU4) and older groundwater in the deeper semi-conﬁned aquifer (TU3D) (Fig. 9). The trend indicates increased leakage from the deeper aquifer 3 14 For the EPFM the activity of H or C at time t (Ct ) is into the surface aquifer closer to the river at TU1 and TU2. given by This is consistent with higher groundwater levels and electri-

∞ cal conductivities at TU1 and TU2 (Fig. 3) that would result Z from increased connectivity with artesian groundwater in the (−λτ) Ct = Ci (t − τ)g(τ)e dτ, (1) deeper, semi-confined aquifer. This connection may have re- 0 sulted from erosion of the clay layers closer to the Tambo River during periodic flooding. 3 14 where Ci is the initial H or C activity, λ is the decay con- −3 −1 3 −4 −1 14 stant (5.63 × 10 yr for H, 1.21 × 10 yr for C), τ 4.3 Groundwater residence times and mixing is the transit time and g(τ) is the system response function. The system response function is given by Groundwater residence times were calculated using the 3H activities and the EPFM with f = 0.8. Groundwater from g(τ) = 0 for τ < T (1 − f ) (2a) Bruthen has relatively short residence times of 2 to 4 years. and Groundwater from Kelly Creek has longer residence times (96 to 100 years), which is consistent with the higher degrees g(τ) = (f T )−1e(−τ/(f tt)+1/f −1) for τ > T (1 − f ), (2b) of mineral dissolution at Kelly Creek discussed previously. Groundwater from TU4 at Tambo Upper has an intermediate where T is the mean residence time and f is the ratio of ex- residence time of 27 years. To assess the sensitivity of these ponential flow to piston flow for the total flow volume (Zu- results, f values in this study were varied between 0.6 and ber et al., 2005). f was estimated at 0.8 for shallow bores 1.0. This results in variations of < 0.1 years at Bruthen and neighbouring the Tambo River on the basis of bore depth, < 15 years at Kelly Creek. Uncertainties in groundwater age screen length and aquifer lithology (cf. Cartwright and Mor- based on the uncertainty of 3H activities were < 1 year at genstern, 2012; Cartwright et al., 2013). Bruthen (based on an uncertainty of 0.14 tritium units) and While there are some differences in the estimated ground- < 1.5 years at Kelly Creek (based on an uncertainty of 0.04 water residence times between different types of flow mod- tritium units). As deeper groundwater from Tambo Upper site els, the predicted variation in 14C and 3H activities are sim- is 3H-free, residence times were calculated from 14C activi- ilar in all flow models that involve attenuation of the bomb- ties. Making the assumption of 15 % calcite dissolution, age pulse peak of 3H and 14C during flow (e.g. as discussed by estimates based on Clark and Fritz (1997, their Eq. 2, p. 206) Cartwright et al., 2013). A similar covariance of 14C and 3H are ∼ 17 200 years. activities would be obtained using a dispersion model (Zu- The relatively young groundwater residence times from ber et al., 2005) or the renewal rate model of Le Gal La the shallow aquifers implies that groundwater recharge in Salle et al. (2001; Fig. 9). The 14C and 3H activities also the area is dominantly local, probably within a few hundred constrain mixing within the groundwater system (Le Gal metres of the Tambo River. Mean groundwater residence La Salle et al., 2001; Cartwright et al., 2007, 2010, 2013). times from the Bruthen bores are similar and within analyti- Mixing between recently recharged groundwater and older cal uncertainty, preventing calculation of horizontal flow ve- groundwater with low 14C and negligible 3H activities will locities. Mean groundwater residence times at Kelly Creek www.hydrol-earth-syst-sci.net/18/5109/2014/ Hydrol. Earth Syst. Sci., 18, 5109–5124, 2014 5120 N. P. Unland et al.: Residence times and mixing of water in river banks

Figure 10. 3H and 14C activities of groundwater at Tambo Up- per and predicted trend for mixing between deeper groundwater (TU3D) and shallow groundwater (TU4). Curve based on DIC, 3H and 14C activities from Table 2.

increase from 96 years at KC4 to 100 years at KC2. The age of groundwater at KC1 is 99 years and within the analytical uncertainty of groundwater at KC2. Based on these data, groundwater at Kelly Creek has a horizontal flow velocity of between 1.3 and 6.5 m year−1 towards the river. The 3H and 14C activities predicted by the mixing between groundwater from TU4 and deeper groundwater are shown in Fig. 10. While it is possible that groundwater from TU4 has already undergone some mixing with deeper groundwater (and C inputs from the aquifer are less than 10 % op- posed to the 10–20 % indicated), this remains difficult to define. As such, mixing estimates at Tambo Upper will be conservative with respect to the input of deeper groundwater. Groundwater from TU1, TU2 and TU5 plot below the mixing trend in Fig. 10. While there are uncertainties in these calculations, it is possible that 3H activities are lower than Figure 11. Predicted mixing curves between river water and expected due to the decay of 3H in shallow groundwater. Ex- groundwater at Bruthen (a, b), Tambo Upper (c, d) and Kelly ponential piston flow modelling of water at TU1 and TU2 Creek (e, f) constructed using the endmember compositions from indicates that a residence time of ∼ 20 years would be re- Tables 1 to 3. Yellow data points – February 2011; blue data points – August 2011. quired to cause the observed deviation in 3H activities from the mixing trend shown in Fig. 10. This suggests a horizontal flow rate of 1.8 ± 0.6 m year−1 towards the Tambo River groundwater a Bruthen was approximately the same age at at the Tambo Upper transect. This is consistent with shallow 18 and 6 m distance from the Tambo River. groundwater recharge on the floodplains of the Tambo River As the 3H and 14C activities were analysed for ground- and groundwater flow towards the river, which is expected water sampled in April 2011, these data can only be used given the gaining nature of this section of the river (Unland to evaluate bank storage for the hydrological conditions at et al., 2013). and immediately prior to sampling. This included a discharge event that increased river height by 0.5 m approxi- 4.4 Implications for groundwater–surface water mately 2 weeks prior to sampling. As such, these data in- interaction dicate that an increase in river height of 0.5 m is not large enough to produce bank storage 5 to 10 m distance from of The distribution of groundwater residence times does not the river for a period greater than 2 weeks. Major ions and support increased bank storage in the area immediately stable isotopes were analysed several times, including after (within tens of metres) neighbouring the Tambo River. If this flood events which increased river height by ∼ 5 m. Again were so, groundwater closer to the Tambo River would con- there is little evidence of river water infiltrating into the river tain a higher proportion of younger water than groundwater banks following these events. The curves expected for the further from the river and groundwater ages would decline mixing between shallow groundwater furthest from the river, towards the river. Instead, groundwater ages increase towards deeper groundwater and river water at each transect with re- the Tambo River at Kelly Creek and Tambo Upper, while spect to Cl : Br, Na : Cl and K : Cl ratios are shown in Fig. 11.

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Data are shown for February 2011 and August 2011 to represent baseflow conditions, when bank infiltration is likely to have the least impact on groundwater chemistry, and post- flood conditions, when bank infiltration is most likely to impact groundwater chemistry. The composition of groundwater from the two bores closest to the Tambo River at each transect are not consistent with the trends expected for mixing between river water and deeper or shallow groundwater further form the river. For ex- ample, during both February and August at Bruthen, Cl : Br, Na : Cl and K : Cl ratios from groundwater at B1 and B2 plot Figure 12. Schematic representation of the Tambo River and major to the left the curves expected for mixing between river wa- hydrogeochemical processes at baseflow (solid line) and high-flow ter and groundwater further from the river at B3 (Fig. 11a (dashed line) conditions. The deeper groundwater is from 15 to 20 m and b). This is partly due to the higher Cl concentrations below ground surface within the alluvial aquifer and is confined in river water and groundwater from B3 (which range from beneath the clay layer. 0.3 to 0.5 mmol L−1) compared to groundwater from B1 and B2 (which range from 0.1 to 0.35 mmol L−1). If mixing between river water and groundwater at B3 were occurring, Cl the gaining nature of the Tambo River near these transects concentrations at B1 and B2 would be intermediate between is driving the return of bank water back into the river be- those in the river and at B3. fore sampling has taken place (Fig. 12). If this is the case, The same is true at Tambo Upper, where Cl concentrations the storage period (∼ 1 week after the flood peak) is signif- in groundwater at TU1 and TU2 are higher (∼ 15 mmol L−1) icantly shorter than the several weeks to months predicted than in river water or groundwater further from the river at by modelling (e.g. Cooper and Rorabaugh, 1963; Doble et TU4 (< 10 mmol L−1) during both baseflow and flood condi- al., 2012; McCallum et al., 2010; Whiting and Pomeranets, tions (Fig. 11c and d). The same is true for EC values which 1997). It is likely that the apparent absence of bank storage are lower in river water and at TU4 (ranging from 120 to near the Tambo River is being driven by a combination of 881 µS cm−1 over the study) compared to EC values at TU1 such factors, including (1) upward flow of groundwater from and TU2, which ranged from 1350 to 2682 µS cm−1 over the deeper aquifer into the river bank due to pressure loading the study. Again, if mixing between river water and water on the floodplain; (2) the return of any bank waters back into at TU4 had a significant impact on groundwater chemistry the river under strongly gaining conditions; and (3) the high- at TU1 and TU2, Cl concentrations and EC values would TDS (total dissolved solids) water from the confined system be expected to be intermediate between the river water and masking the chemical impact of infiltrating river water. groundwater from TU4. Instead, groundwater in TU1 and TU2 has a geochemistry that is similar to that which would be expected for mixing between TU4 and TU3D (Fig. 11c 5 Conclusions and d). As asserted in Sect. 4.1, such mixing is implied by the 3H and 14C data. Similarly, δ2H and δ18O values of ground- This study was able to determine the absence of signifi- water close to the Tambo River do not decline after signif- cant bank storage near the Tambo River by dating near-river icant flooding, as would be expected for the infiltration of groundwater and characterising its major ion chemistry be- river water with the lower δ2H and δ18O values observed dur- fore and after flooding. The combined use of 3H and 14C ac- ing flooding. tivities indicate that recharge in the alluvial aquifer is domi- These observations indicate that river water penetrates nantly local (within hundreds of metres of the Tambo River). < 5 m into the banks during flooding, suggesting limited The covariance between 3H and 14C activities show that mix- bank infiltration. The absence of significant bank infiltration ing between relatively old groundwater from a deeper semi- is consistent with results of Vekerdy and Meijerink (1998) confined aquifer and younger groundwater from the uncon- and Wett et al. (2002), who found bank infiltration to be fined alluvial aquifer is occurring in parts of the Tambo River minimal in confined and semi-confined aquifers, where pres- bank. It is further shown that, by coupling 3H and 14C to de- sure loading from the flood wave propagated rapidly into fine a mixing trend, deviations in the activity of 3H from the the neighbouring aquifers, limiting bank infiltration. While trend can be used to estimate the likely age of groundwater most bores near the Tambo River are screened in the alluvial along its flow path. Na : Cl ratios > 1 in groundwater sam- aquifer which is unconfined, leakage of the underlying semi- pled during baseflow conditions and in older groundwater confined aquifer into the alluvial aquifer does occur (Fig. 9). from the area indicate the dissolution of Na-bearing minerals This upward leakage occurs close rivers where erosion has and are consistent with the weathering of silicic sands in the removed some of the confining layers (Rinaldi and Darby, aquifer. Increasing Cl : Br ratios and increasing Cl concentra- 2007). It is possible that bank storage is occurring but that tions during periods of increased rainfall indicate an input of www.hydrol-earth-syst-sci.net/18/5109/2014/ Hydrol. Earth Syst. Sci., 18, 5109–5124, 2014 5122 N. P. Unland et al.: Residence times and mixing of water in river banks

Cl, which is consistent with the mobilisation of Cl accumu- Boulton, A. J.: Stream ecology and surface-hyporheic hydrologic lated in the soil profile through the use of fertilisers. Increas- exchange: Implications, techniques and limitations, Mar. Freshw. ing groundwater age with proximity to the Tambo River is Res., 44, 553–564, 1993. consistent with the gaining nature of the Tambo River but Boulton, A. J.: Chances and challenges in the conservation of does not suggest that exchange between groundwater and groundwaters and their dependent ecosystems. Aquatic Conser- surface water increases with increasing proximity to the river. vation, Mar. Freshw. Ecosyst., 15, 319–323, 2005. Bourg, A. C. M. and Bertin, C.: Biogeochemical processes during Major ions, δ2H and δ18O values support this and do not the infiltration of river water into an alluvial aquifer, Environ. show trends consistent with an increased input of river water Sci. Technol., 27, 661–666, 1993. to the groundwater closer to the river. These results suggest Bureau of Meteorology: Commonwealth of Australia, Bureau of either that the strongly gaining nature of the Tambo River at Meteorology, available at: http://www.bom.gov.au, last access: the study locations is preventing significant lateral infiltration 28 August 2013. of river water into the bank or that the rapid propagation of Cartwright, I. and Morgenstern, U.: Constraining groundwater pressure into the underlying semi-confined aquifer, followed recharge and the rate of geochemical processes using tritium and by leakage into the above unconfined aquifer is preventing major ion geochemistry: Ovens catchment, southeast Australia, significant bank infiltration. J. Hydrol., 475, 137–149, 2012. Cartwright, I., Weaver, T. R., Stone, D., and Reid, M.: Constrain- These results are indicative of the highly complex nature 3 of groundwater and surface water processes that may be oc- ing modern and historical recharge from bore hydrographs, H, 14C, and chloride concentrations: Applications to dual-porosity curring within river banks and illustrates that, while mod- aquifers in dryland salinity areas, Murray Basin, Australia, J. Hy- els can significantly help in conceptualising our understand- drol., 332, 69–92, 2007. ing of groundwater–surface water interactions, field studies Cartwright, I., Weaver, T. R., Simmons, C. T., Fifield, L. K., can offer complementary information that may otherwise be Lawrence, C. R., and Chisari, R.: Physical hydrogeology and overlooked. In this setting, the assumption of typical bank environmental isotopes to constrain the age, origins, and stabil- storage processes and the use of a regional groundwater end- ity of a low-salinity groundwater lens formed by periodic river member during mass balance calculations would lead to poor recharge: Murray Basin, Australia, J. Hydrol., 380, 203–221, groundwater flux estimates. In this context, the importance 2010. 3 of nested piezometers near rivers to more accurately charac- Cartwright, I., Fifield, L. K., and Morgenstern, U.: Using H and 14 terise near-river groundwater processes has been highlighted. C to constrain the degree of closed-system dissolution of cal- Even the monitoring of relatively inexpensive parameters cite in groundwater, Appl. Geochem., 32, 118–128, 2013. Cartwright, I., Gilfedder, B., and Hofmann, H.: Contrasts between such as groundwater levels, EC and Cl can provide signif- estimates of baseflow help discern multiple sources of wa- icant information to researchers and groundwater managers ter contributing to rivers, Hydrol. Earth Syst. Sci., 18, 15–30, when conducting such studies. doi:10.5194/hess-18-15-2014, 2014. Cendón, D. I., Larsen, J. R., Jones, B. G., Nanson, G. C., Rickle- man, D., Hankin, S. I., Pueyo, J. J., and Maroulis, J.: Freshwa- Acknowledgements. The authors thank Massimo Raveggi and ter recharge into a shallow saline groundwater system, Cooper 3 Rachelle Pierson for their help with major ion analyses. H and Creek floodplain, Queensland, Australia, J. Hydrol., 392, 150– 14 C analysis at ANSTO was conducted with the help of AINSE 163, 2010. funding. Further funding for this research was provided by the Cey, E. E., Rudolph, D. L., Aravena, R., and Parkin, G.: Role of the National Centre for Groundwater Research and Training, an riparian zone in controlling the distribution and fate of agricul- Australian Government initiative, supported by the Australian tural nitrogen near a small stream in southern Ontario, J. Con- Research Council and the National Water Commission. tamin. Hydrol., 37, 45–67, 1999. Chen, X. and Chen, X.: Stream water infiltration, bank storage, and Edited by: M. Giudici storage zone changes due to stream-stage fluctuations, J. Hydrol., 280, 246–264, 2003. Chen, X., Chen, D. Y., and Chen, X.-H.: Simulation of baseflow accounting for the effect of bank storage and its implication in References baseflow separation, J. Hydrol., 327, 539–549, 2006. Clark, I. D. and Fritz, P.: Environmental Isotopes in Hydrogeology, Allison, G. B. and Hughes, M. W.: The history of tritium fallout in Lewis, New York, USA, 206 pp., 1997. southern Australia inferred from rainfall and wine samples, Earth Cooper, H. H. and Rorabaugh, M. I.: Ground-water movements and Planet. Sci. Lett., 36, 334–340, 1977. bank storage due to flood stages in surface streams, United States Atkinson, A. P., Cartwirght, I., Gilfedder, B. S., Hofmann, H., Un- Department of the Interior, Geological Survey Water-Supply Pa- land, N. P., Cendón, D. I., and Chisari, R.: A multi-tracer ap- per, Washington, 1963. proach to quantifying groundwater inflows to an upland river; Coplen, T. B.: Normalisation of oxygen and hydrogen isotope data, assessing the influence of variable groundwater chemistry, Hy- Chem. Geol.: Isotope Geoscience section, 72, 293–297, 1988. drol. Process., doi:10.1002/hyp.10122, in press, 2013. Birch, W. D.: The Geology of Victoria. Geological Society of Aus- tralia (Victorian Division), Sydney, 2003.

Hydrol. Earth Syst. Sci., 18, 5109–5124, 2014 www.hydrol-earth-syst-sci.net/18/5109/2014/ N. P. Unland et al.: Residence times and mixing of water in river banks 5123

Department of Agriculture, Fisheries and Forestry: Gippsland http://www.clw.csiro.au/publications/waterforahealthycountry/ Catchment, available at: http://adl.brs.gov.au/water2010/pdf/ 2011/wfhc-interconnection-Gwydir-River.pdf, 2011. catchment2230summary.pdf (last access: 19 November 2012), Le Gal La Salle, C., Marlin, C., Leduc, C., Taupin, J. D., Mas- 2006. sault, M., and Favreau, G.: Renewal rate estimation of ground- Department of Environment and Primary Industries: Victorian Re- water based on radioactive tracers (3H, 14C) in an unconfined sources Online, Groundwater, available at: http://vro.dpi.vic.gov. aquifer in a semi-arid area, Iullemeden Basin, Niger, J. Hydrol., au/dpi/vro/vrosite.nsf/pages/water-gw-quality-quantity, last ac- 254, 145–156, 2001. cess: 5 July 2013. Leonard, J. G.: An overview of Victoria’s groundwater resources. Doble, R., Brunner, P., McCallum, J., and Cook, P. G.: An analysis Background report S3, short term planning guidelines. Drought of river bank slope and unsaturated flow effects on bank stor- management plan for Victoria’s water resources, Department of age,Ground Water, 50, 77–86, 2012. Water Resources, Melbourne, Victoria, 1992. Edmunds, W. M.: Geochemistry’s vital contribution to solving wa- Małoszewski, P. and Zuber, A.: Determining the turnover time of ter resource problems, Appl. Geochem., 24, 1058–14073, 2009. groundwater systems with the aid of environmental tracers: 1. Fetter, C. W.: Applied hydrogeology, Merrill, New Jersey, 1994. Models and their applicability, J. Hydrol., 57, 207–231, 1982. Fink, D., Hotchkis, M., Hua, Q., Jacbosen, G., Smith, A. M., Zoppi, Małoszewski, P. and Zuber, A.: Influence of Matrix Diffusion and U., Child, D., Mifsud, C., Vender Gaast, H., Williams, A., and Exchange Reactions on Radiocarbon Ages in Fissured Carbonate Williams, M.: The Antares AMS facility at ANSTO, Nuclear In- Aquifers, Water Resour. Res., 27, 1937–1945, 1991. strum. Methods Phys. Res., 223, 109–115, 2004. McCallum, J. L., Cook, P. G., Brunner, P., and Berhane, D.: So- Fontes, J.-Ch.: Dating of Groundwater. Guidebook on Nuclear lute dynamics during bank storage flows and implications for Techniques in Hydrology, Technical Reports Series No. 91, chemical base flow separation, Water Resour. Res., 46, W07541, IAEA, Vienna, 285–317, 1983. doi:10.1029/2009WR008539, 2010. Fukada, T., Hiscock, K. M., Dennis, P. F., and Grischek, T.: A dual Morgenstern, U. and Taylor, C. B.: Ultra low-level tritium measure- isotope approach to identify denitrification in groundwater at a ment using electrolytic enrichment and LSC, Isotopes Environ. river-bank infiltration site, Water Res., 37, 3070–3078, 2003. Health, 45, 96–117, 2009. Griffioen, J.: Potassium adsorption ratios as an indicator for the fate Morgenstern, U., Stewart, M. K., and Stenger, R.: Dating of of agricultural potassium in groundwater, J. Hydrol., 254, 244– streamwater using tritium in a post nuclear bomb pulse world: 254, 2001. continuous variation of mean transit time with streamflow, Hy- Herczeg, A. L., Dogramaci, S. S., and Leany, F. W.: Origin of dis- drol. Earth Syst. Sci., 14, 2289–2301, doi:10.5194/hess-14-2289- solved salts in a large, semi-arid groundwater system: Murray 2010, 2010. Basin, Australia, Mar. Freshw. Res., 52, 41–52, 2001. Panno, S. V., Hackley, K. C., Hwang, H. H., Greenberg, S. E., Kra- Hiscock, K. M. and Grischek, T.: Attenuation of groundwater pol- pac, I. G., Landsberger, S., and O’Kelly, D. J.: Source identi- lution by bank filtration, J. Hydrol., 266, 139–144, 2002. fication of sodium and chloride in natural waters: Preliminary Hocking, J. B.: Definition and revision of Tertiary stratigraphic results, Ground Water, 44, 176–187, 2006. units, onshore Gippsland Basin, Geological Survey of Victoria, Pinder, G. F. and Sauer, S. P.: Numerical simulation of flood wave Department of minerals and energy, Victoria, 1976. modification due to bank storage effects, Water Resour.Res., 7, Hofmann, H. and Cartwright, I.: Using hydrogeochemistry to un- 63–70, 1971. derstand inter-aquifer mixing in the on-shore part of the Gipp- Rinaldi, M. and Darby, S. E.: 9 Modelling river-bank-erosion pro- sland Basin, southeast Australia, Appl. Geochem., 33, 84–103, cesses and mass failure mechanisms: progress towards fully cou- 2013. pled simulations, Develop. Earth Surf. Proc., 11, 213–239, 2007. Hua, Q., Barbetti, M., and Rakowski, A. Z.: Atmospheric Radio- Schachtman, D. P. and Schroeder, J. I.: Structure and transport carbon for the period 1950–2010, Radiocarbon, 55, 2059–2072, mechanism of a high-affinity potassium uptake transporter from 2013. higher plants, Nature, 370, 655–658, 1994. Hughes, C. E. and Crawford, J.: A new precipitation weighted Singh, K. P.: Some Factors Affecting Baseflow, Water Resour. Res., method for determining the meteoric water line for hydrologi- 4, 985–999, 1968. cal applications demonstrated using Australian and global GNIP Southern Rural Water. Gippsland Groundwater Atlas, available at: data, J. Hydrol., 464–465, 344–351, 2012. http://www.srw.com.au/gippsland (last access: 25 June 2014), Lambs, L.: Interactions between groundwater and surface water at 2013 river banks and the confluence of rivers, J. Hydrol., 288, 312– Stuiver, M. and Polach, H. A.: Reporting of 14C data, Radiocarbon, 326, 2004. 19, 355–363, 1977. Lamontagne, S., Leaney, F. W., and Herczeg, A. L.: Groundwater- Suess, H.: Climatic changes and atmospheric radiocarbon, Palaeo- surface water interactions in a large semi-arid floodplain: im- geogr. Palaeocli. Palaeoecol., 10, 199–202, 1971. plications for salinity management, Hydrol. Process., 19, 3063– Tadros, C. V., Hughes, C. E., Crawford, J. C., Hollins, S. E., and 3080, 2005. Chisari, R.: Tritium in Australian precipitation: a 50 year record, Lamontagne, S., Taylor, A. R., Cook, P. G., and Bar- J. Hydrol., 513, 262–273, 2014. rett, C.: Interconnection of Groundwater Systems Tóth, J.: A theoretical analysis of groundwater flow in small – River Losses from Losing/Disconnected Streams. drainage basins, J. Geophys. Res., 68, 4795–4812, 1963. Gwydir River Site Report. CSIRO: Water for a Healthy Country National Research Flagship, available at:

www.hydrol-earth-syst-sci.net/18/5109/2014/ Hydrol. Earth Syst. Sci., 18, 5109–5124, 2014 5124 N. P. Unland et al.: Residence times and mixing of water in river banks

Unland, N. P., Cartwright, I., Andersen, M. S., Rau, G. C., Reed, J., Whiting, P. J. and Pomeranets, M.: A numerical study of bank stor- Gilfedder, B. S., Atkinson, A. P., and Hofmann, H.: Investigating age and its contribution to streamflow, J. Hydrol., 202, 121–136, the spatio-temporal variability in groundwater and surface wa- 1997. ter interactions: a multi-technique approach, Hydrol. Earth Syst. Winter, T. C., Harvey, J. W., Franke, O. L., and Alley, W. M.: Sci., 17, 3437–3453, doi:10.5194/hess-17-3437-2013, 2013. Ground water and surface water a single resource, US Geolog- Vekerdy, Z. and Meijerink, A. M. J.: Statistical and analytical study ical Survey Circuar, Denver, Colorado, 1998. of the propagation of flood-induced groundwater rise in an allu- Woessner, W. W.: Stream and fluvial plain ground water interac- vial aquifer, J. Hydrol., 205, 112–125, 1998. tions: Rescaling hydrogeologic thought, Ground Water, 38, 423– Victorian Water Resources Data Warehouse, available at: http:// 429, 2000. www.vicwaterdata.net/vicwaterdata/home.aspx, last access: 28 Zuber, A., Witczak, S., Rozaski, K., liwka, I., Opoka, M., Mochal- August 2013. ski, P., Kuc, T., Karlikowska, J., Kania, J., Jackowicz-Korczyski, 3 Vogel, J. C.: Groningen radiocarbon dates IX, Radiocarbon, 12, and Duliski, M.: Groundwater dating with H and SF6 in rela- 444–471, 1970. tion to mixing patterns, transport modelling and hydrochemistry, Wett, B., Jarosch, H., and Ingerle, K.: Flood induced infiltration af- Hydrol. Process., 19, 2247–2275, 2005. fecting a bank filtrate well at the River Enns, Austria, J. Hydrol., 266, 222–234, 2002.

Hydrol. Earth Syst. Sci., 18, 5109–5124, 2014 www.hydrol-earth-syst-sci.net/18/5109/2014/