Physical hydrogeology and environmental isotopes to constrain the age, origins, and stability of a low-salinity groundwater lens formed by periodic river recharge: Murray Basin, Australia

Abstract. An understanding of hydrological processes is vital for the sustainable management of groundwater resources, especially in areas where an aquifer interacts with surface water systems or where aquifer interconnectivity occurs. This is particularly important in areas that are subjected to frequent drought/flood cycles, such as the Cressbrook Creek catchment in Southeast Queensland, Australia. In order to understand the hydrological response to flooding and to identify inter-aquifer connectivity, multiple isotopes (δ 2 H, δ 18 O, 87 Sr / 86 Sr, 3 H and 14 C) were used in this study in conjunction with a comprehensive hydrochemical assessment, based on data collected 6 months after severe flooding in 2011. The relatively depleted stable isotope signatures of the flood-generating rainfall (δ 2 H: −30.2 to −27.8 ‰, δ 18 O: −5.34 to −5.13 ‰ VSMOW) were evident in surface water samples (δ 2 H: −25.2 to −23.2 ‰, δ 18 O: −3.9 to −3.6 ‰ VSMOW), indicating that these extreme events were a major source of recharge to the dam in the catchment headwaters. Furthermore, stable isotopes confirmed that the flood generated significant recharge to the alluvium in the lower part of the catchment, particularly in areas where interactions between surface waters and groundwater were identified and where diffuse aquifer recharge is normally limited by a thick (approximately 10 m) and relatively impermeable unsaturated zone. However, in the upper parts of the catchment where recharge generally occurs more rapidly due to the dominance of coarse-grained sediments in the unsaturated zone, the stable isotope signature of groundwater resembles the longer-term average rainfall values (δ 2 H: −12.6, δ 18 O: −3.4 ‰ VSMOW), highlighting that recharge was sourced from smaller rainfall events that occurred subsequent to the flooding. Interactions between the bedrock aquifers and the alluvium were identified at several sites in the lower part of the catchment based on 87 Sr / 86 Sr ratios; this was also supported by the hydrochemical assessment, which included the modelling of evaporation trends and saturation indices. The integrated approach used in this study facilitated the identification of hydrological processes over different spatial and temporal scales, and the method can be applied to other complex geological settings with variable climatic conditions.

Section: A C King: Identifying Flood Recharge and Inter-aquifer Conmentioning

confidence: 99%

Identifying flood recharge and inter-aquifer connectivity using multiple isotopes in subtropical Australia

King

Raiber

Cendón

et al. 2015

“…Geochemical tracers including major ions, stable isotopes and radiogenic isotopes have been used to estimate groundwater fluxes in gaining rivers (Cartwright et al, 2008(Cartwright et al, , 2010Cook, 2012;Cook et al, 2003Cook et al, , 2006Durand et al, 1993;Genereux et al, 1993;Genereux and Hemond, 1990;Lamontagne et al, 2005Lamontagne et al, , 2008Lamontagne and Cook, 2007;Mullinger et al, 2007Mullinger et al, , 2009Négrel et al, 2003;Rhode, 1981;Ribolzi et al, 2000;Stellato et al, 2008). The utility of each of these tracers depends on a variety of factors including the difference between the concentration of the tracer in groundwater compared to surface water, its spatial and temporal variability, the accurate characterisation of its sources and sinks, and the potential for it to change by processes such as evaporation, precipitation, radioactive decay, degassing, or biogeochemical reactions.…”

Section: N P Unland Et Al: Investigating the Spatio-temporal Variamentioning

confidence: 99%

Investigating the spatio-temporal variability in groundwater and surface water interactions: a multi-technique approach

Unland

Cartwright

Andersen

et al. 2013

Abstract. The interaction between groundwater and surface water along the Tambo and Nicholson rivers, southeast Australia, was investigated using 222 Rn, Cl, differential flow gauging, head gradients, electrical conductivity (EC) and temperature profiles. Head gradients, temperature profiles, Cl concentrations and 222 Rn activities all indicate higher groundwater fluxes to the Tambo River in areas of increased topographic variation where the potential to form large groundwater-surface water gradients is greater. Groundwater discharge to the Tambo River calculated by Cl mass balance was significantly lower (1.48 × 10 4 to 1.41 × 10 3 m 3 day −1 ) than discharge estimated by 222 Rn mass balance (5.35 × 10 5 to 9.56 × 10 3 m 3 day −1 ) and differential flow gauging (5.41 × 10 5 to 6.30 × 10 3 m 3 day −1 ) due to bank return waters. While groundwater sampling from the bank of the Tambo River was intended to account for changes in groundwater chemistry associated with bank infiltration, variations in bank infiltration between sample sites remain unaccounted for, limiting the use of Cl as an effective tracer. Groundwater discharge to both the Tambo and Nicholson rivers was the highest under high-flow conditions in the days to weeks following significant rainfall, indicating that the rivers are well connected to a groundwater system that is responsive to rainfall. Groundwater constituted the lowest proportion of river discharge during times of increased rainfall that followed dry periods, while groundwater constituted the highest proportion of river discharge under baseflow conditions (21.4 % of the Tambo in April 2010 and 18.9 % of the Nicholson in September 2010).

“…Field studies focussed on bank storage and the dating of bank water in Australian catchments has been limited. Lamontagne et al (2011) and Cendón et al (2010) indicated the presence of relatively young (< 50 years) groundwater in river banks, and Cartwright et al (2010) showed that preferential floodplain recharge is likely to occur near rivers during flooding. In contrast, groundwater in upland catchments in Australia has been shown to have relatively long residence times .…”

Section: N P Unland Et Al: Residence Times and Mixing Of Water Inmentioning

confidence: 99%

“…6 and 7). As evapotranspiration, which is the dominant process that controls groundwater salinity in southeast Australia (Herczeg et al, 2001;Cartwright et al, 2007Cartwright et al, , 2010, does not impact Cl : Br ratios, this implies the addition of Cl from an external source. Halite dissolution is a potential Cl source; however, there are no obvious stores of halite in the catchment.…”

Section: Hydrogeochemical Processesmentioning

confidence: 99%

“…9). The 14 C and 3 H activities also constrain mixing within the groundwater system (Le Gal La Salle et al, 2001;Cartwright et al, 2007Cartwright et al, , 2010Cartwright et al, , 2013. Mixing between recently recharged groundwater and older groundwater with low 14 C and negligible 3 H activities will displace water compositions to the left of the predicted 14 C vs. 3 H trends.…”

Section: Aquifer Interactionsmentioning

confidence: 99%

See 1 more Smart Citation

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

Unland

Cartwright

Cendón

et al. 2014

Abstract. Bank exchange processes within 50 m of the Tambo River, southeast Australia, have been investigated through the combined use of 3 H and 14 C. Groundwater residence times increase towards the Tambo River, which suggests the absence of significant bank storage. Major ion concentrations and δ 2 H and δ 18 O values of bank water also indicate that bank infiltration does not significantly impact groundwater chemistry under baseflow and post-flood conditions, suggesting that the gaining nature of the river may be driving the return of bank storage water back into the Tambo River within days of peak flood conditions. The covariance between 3 H and 14 C indicates the leakage and mixing between old (∼ 17 200 years) groundwater from a semiconfined aquifer and younger groundwater (< 100 years) near the river, where confining layers are less prevalent. It is likely that the upward infiltration of deeper groundwater from the semi-confined aquifer during flooding limits bank infiltration. Furthermore, the more saline deeper groundwater likely controls the geochemistry of water in the river bank, minimising the chemical impact that bank infiltration has in this setting. These processes, coupled with the strongly gaining nature of the Tambo River are likely to be the factors reducing the chemical impact of bank storage in this setting. This study illustrates the complex nature of river groundwater interactions and the potential downfall in assuming simple or idealised conditions when conducting hydrogeological studies.