Decoupling of solutes and water in regional groundwater systems: the Murray Basin, Australia

Cartwright, Ian, Hofmann, Harald, Currell, Matthew and Fifield, L. Kieth (2017) Decoupling of solutes and water in regional groundwater systems: the Murray Basin, Australia. Chemical Geology, 466 466-478. doi:10.1016/j.chemgeo.2017.06.035

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Author Cartwright, Ian
Hofmann, Harald
Currell, Matthew
Fifield, L. Kieth
Title Decoupling of solutes and water in regional groundwater systems: the Murray Basin, Australia
Journal name Chemical Geology   Check publisher's open access policy
ISSN 0009-2541
1872-6836
Publication date 2017-06-27
Year available 2017
Sub-type Article (original research)
DOI 10.1016/j.chemgeo.2017.06.035
Open Access Status File (Author Post-print)
Volume 466
Start page 466
End page 478
Total pages 13
Place of publication Amsterdam, Netherlands
Publisher Elsevier
Language eng
Formatted abstract
Documenting the origins, residence times, and movement of groundwater and the solutes that it contains is critical to understanding hydrogeological systems. This study uses Cl mass balance to determine the Cl accession time (i.e. the time required for Cl to accumulate) and 36Cl to estimate the residence times of Cl in the Victorian portion of the Murray Basin, southeast Australia. Much of the Murray Basin contains saline groundwater with total dissolved solids (TDS) concentrations commonly > 14,000 mg/L and locally up to 300,000 mg/L. The total mass of Cl stored in the Victorian portion of the basin is estimated as between 12,400 and 47,100 MT. Using present day rainfall totals and Cl concentrations in rainfall, the Cl accession time is 170 to 650 ka. Aquifer thicknesses and groundwater salinity both increase westwards in this part of the Murray Basin. Consequently, the Cl accession times increase westward from 0.1–0.6 ka to 286–1080 ka. By contrast, 14C activities of the majority of the groundwater are > 2 pMC, and commonly much higher. Notwithstanding the difficulty in correcting 14C residence times, the widespread occurrence of groundwater with above background 14C activities implies that groundwater residence times are generally < 30 ka, which is substantially shorter than the Cl accession times. R36Cl (the ratio of 36Cl to total Cl × 1015) values of the groundwater are between 20 and 230, and are uncorrelated with Cl concentrations. While it is difficult to determine precise Cl residence times, the observation that the R36Cl values are significantly higher than those that represent secular equilibrium with the aquifer matrix (R36Cl of 5 to 10) indicates that they are up to a few hundred thousand years and similar to the Cl accession times. The R36Cl values together with the geology of the aquifers, stable isotope ratios, and major ion geochemistry precludes halite dissolution, incorporation of connate water, or the long-term diffusion of Cl from clays as mechanisms of producing the elevated Cl concentrations. Rather, it is most likely that the high Cl concentrations result from recycling of solutes in saline lakes and playas or repeated cycles of evapotranspiration in the unsaturated zone.
Keyword Groundwater
Salinity
Accession times
Residence times
36Cl
Q-Index Code C1
Q-Index Status Provisional Code
Institutional Status UQ

Document type: Journal Article
Sub-type: Article (original research)
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