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 res- idence 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 signif- icantly 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).