Topographic controls on the hydrochemical and isotopic evolution of groundwater in semiarid landscapes

Sustainable groundwater management in water-scarce semiarid environments is challenging because it relies on understanding the intricate coupling between episodic recharge and hydrochemical evolution. In this study, conducted in the Pilbara region of Western Australia, ~6000 water analyses from ~1,800 groundwater boreholes and ~300 surface-water sites collected over a decade of monitoring (2015–2024) were used to develop a conceptual model of groundwater hydrochemical evolution.

Stable hydrogen and oxygen isotope compositions indicate that groundwater recharge is not seasonal but instead occurs during sporadic tropical cyclones, which deliver precipitation with distinctly lower δ²H and δ¹⁸O values than local groundwater. Self-Organizing Maps (SOM), applied to stable isotope and hydrochemical data, identified five distinct categories that capture hydrochemical transition from recharge sources to endorheic basins. The evolutionary pathway begins in freshwater headwaters, where sulfide oxidation generates acidity that enhances carbonate dissolution and silicate weathering. As groundwater moves to alluvial plains, geochemical control shifts towards cation exchange, ultimately cumulating in low-lying zones where evaporative concentration dominates, and brine formation occurs. Structural Equation Modelling (SEM) confirms a fundamental spatial regime shift between inland and coastal systems. Inland chemistry is primarily controlled by topography and, at times, by internal rock-water interactions. Conversely, coastal water hydrochemistry is related to distance to the coast.

Hydrochemical categories serve as effective proxies for hydraulic behaviour. Hydrochemically "young" recharge freshwaters exhibit dynamic water level responses to cyclonic events, whereas evolved, saline waters in the alluvial plains maintain comparatively stable water tables. These patterns demonstrate that hydrochemical evolution and hydraulic dynamics are tightly coupled, reflecting systematic differences in water retention times across the landscape. Together, they reveal a clear transition from lithological controls in recharge zones to salinity-driven physical controls in terminal areas.

These findings indicate that sustainable yield assessments should distinguish between the rapid-response behaviour of headwater systems and the storage-dominated dynamics of downstream alluvial basins.