Stable isotope and hydrochemical evolution of groundwater in the semi-arid Hamersley Basin of subtropical northwest Australia

SummaryThe Hamersley Basin, in the semi-arid Pilbara region of northwest Australia, is currently subject to increasing pressure from altered hydrology associated with mining activities as well as water abstraction for regional development. Sustainable water management across the region must be underpinned by an understanding of the factors that constrain water supply in arid zones. We measured the amount and isotopic signature of individual rainfall events over three consecutive years (2009–2011) to determine the likely processes that control surface water pools in streams and groundwater recharge across the Hamersley Basin. We also measured concentrations of ions (in particular bromide and chloride) to define and quantify sources of major recharge. Stable isotope composition of precipitation across the basin forms a Local Meteoric Water Line (LMWL) defined by the equation: δ2H = 7.03 ± 0.17 × δ18O + 4.78 ± 1.45. Thus, the slope of the LMWL was similar to the Global Meteoric Water Line (GMWL). However, the intercept of the LMWL was significantly different to the GMWL, which is attributable to the amount or “rainout” effect. The stable isotope composition of rainfall events was highly variable and dependent on event size. However, the δ2H and δ18O values of fresh groundwater from the alluvium and fractured aquifers were similar and characterised by a very narrow range (alluvium aquifer δ18O −8.02 ± 0.83‰, δ2H −55.6 ± 6.0‰, n = 65; fractured aquifer δ18O −8.22 ± 0.70‰, δ2H −56.9 ± 5.0‰, n = 207). Our findings suggest that intense rainfall events of >20 mm with limited evaporation prior to infiltration contribute most to recharge. In contrast, the δ2H and δ18O values and chemical composition of the relatively saline groundwater in the terminal Fortescue Marsh suggest a combination of evaporation and cyclic drying and wetting of the marsh surface prior to recharge. Saline groundwater samples were more 18O enriched than fresh groundwater; δ2H and δ18O values shifted to the right of the LMWL, forming a straight line with a slope of 3.58 ± 0.20 and an intercept of −25.55 ± 0.71 (R2 = 0.95, p < 0.001, n = 18). The stable isotope mass balance for most of the surface water pools in the basin show significant evaporation and are highly enriched compared to underlying groundwater. We conclude that significant seepage from highly evaporated pools to groundwater is very limited.

► Isotope composition of rain is governed by evaporation of droplets and rainout effect.
► Only infrequent large-volume rainfalls in the wet season recharge aquifers.
► Hydrochemistry of groundwater is controlled by evaporation and carbonate equilibrium.
► Most of the recharge occurs through alluvium sediments.