Carbon dioxide Capture and Storage (CCS) is crucial for mitigating greenhouse gas emissions and stabilising atmospheric CO₂. Australia’s Surat Basin, particularly its lower Jurassic aquifers, exhibits significant potential for geological CO₂ storage. The Hutton and Gubberamunda Sandstone aquifers, overlying the reservoir, contain minerals including illite, anorthite, portions of calcite, and siderite, which could react with leaked CO₂, potentially impacting groundwater. This study investigates the geochemical responses of these formations by leaching ten representative samples (five from each formation) with carbonated water for 15 days. The leaching process caused release of Ba, Ca, Sr, Mg, Na, S and Fe, with each element dissolving by more than 20% from the rock. Especially Ca and Sr, dissolving by over 80%, some released more than 95%, significantly impacted the water chemistry. The pH decreased to 4.38 - 5.50 and alkalinity dropped from 130 – 220 to 85 – 120 mg/L. Strontium isotopes in the rock samples shifted from 0.708789 - 0.720234 to 0.707031 - 0.707207. One of water samples from the Gubberamunda sandstone showed 62 ppb of Volatile Organic Acids (VOCs), exceeding drinking water standards. Additionally, toxic elements including Sb, As, Ba, Cd, Pb, Se and Ni exceeded guideline levels, raising concerns about potential contamination. These findings highlight the need for further research to fully understand the long-term geochemical impacts of CO₂ sequestration in these formations. Future studies should prioritise extended laboratory experiments and field tests to accurately assess the risks and ensure the safety of underground water systems.