Strontium and carbon isotope constraints on carbonate-solution interactions and inter-aquifer mixing in groundwaters of the semi-arid Murray Basin, Australia

Strontium and carbon isotopes are used to study the effect of carbonate mineral dissolution, and inter-aquifer mixing on the chemical evolution of groundwater from the Murray Group Limestone Aquifer in semi-arid SE Australia. The ⁸⁷Sr/⁸⁶Sr ratio of groundwater evolves from 0.7097 at the basin margin to a less radiogenic value of 0.7084 about 250 km down-gradient, which is similar to the ⁸⁷Sr/⁸⁶Sr ratio of the aquifer carbonate matrix. The concomitant increase in δ¹³C_DIC, Sr/Ca and Mg/Ca ratios in the groundwater along a 250 km transect suggests that incongruent dissolution of high Mg-calcite controls the carbonate geochemistry in this aquifer. Further down-gradient, the groundwater is characterized by a relatively more radiogenic ⁸⁷Sr/⁸⁶Sr ratio caused by upward leakage and mixing with more radiogenic groundwater from the Renmark Group Sand Aquifer. A mixing model using ⁸⁷Sr/⁸⁶Sr ratio suggests that the fraction of water that contribute to the Murray Group Aquifer through upward leakage from underlying Renmark Group Aquifer ranges from 15 to 85%.