Stable Cl isotope ratios ( 37Cl/ 35Cl) were measured in groundwater samples from the southwestern flow system of the Great Artesian Basin, Australia to gain a better understanding of the Cl − sources and transport mechanisms. δ 37Cl values range from 0‰ to −2.5‰ (SMOC), and are inversely correlated with Cl − concentration along the inferred flow direction. The Cl isotopic compositions, in conjunction with other geochemical parameters, suggest that Cl − in groundwaters is not derived from salt dissolution. Mixing of the recharge water with saline groundwater cannot explain the relationship between δ 37Cl and Cl − concentration measured. Marine aerosols deposited via rainfall and subsequent evapotranspiration appear to be responsible for the Cl − concentrations observed in wells that are close to the recharge area, and in groundwaters sampled along the southern transect. δ 37Cl values measured in the leachate of the Bulldog shale suggest that the aquitard is the subsurface source of Cl − for the majority of groundwater samples studied. Diffusion is likely the mechanism through which Cl − is transported from the pore water of the Bulldog shale to the aquifer. However, a more detailed study of the aquitard rocks is required to verify this hypothesis.