While B1 and B2 alluvial water types are similar, they represent two different hydrochemical pathways: an example of these pathways is most obvious in the Oakey Creek alluvium. The lower CCR index values for alluvial B1 and B2 water samples shows an WH-4-023 exchange process occurs within the alluvium, where basalt-derived Ca–Mg–HCO3 and Ca–MgH–CO3–Cl water types evolve to have higher sodium composition (Na–Ca–MgCl–HCO3 water types) (Fig. 10a). However, B1 water samples with a positive CCR index are maintained as EC increases and bicarbonate is depleted, leading to Ca–Mg–Cl water types (Fig. 10b). This process is spatially restricted to the upper zones of the Oakey Creek alluvium, where adjacent basalt outcrops must provide a consistent source of calcium and magnesium (Fig. 11a). In contrast, negative CCR index values for B2 suggest atomic number cation exchange processes may occur simultaneously in the Oakey Creek alluvium as evapotranspiration proceeds, particularly in areas around Oakey.