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

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.