Surface-groundwater hydrological interaction: an isotope geochemical case study on the Bird in Hand Mine South Australia.

Abstract

Isotopes of Hydrogen (δD), Oxygen (δ18O) and Strontium (87Sr/86Sr) were used as geochemical tracers to build a better understanding of the groundwater system impacting the Bird in Hand mine and its interactions with seasonal rainfall variation and surface processes. These isotopes were used to investigate the dependency of surface vegetation on aquifer waters and rain/soil water, particularly local Eucalyptus species. Isotopes were used to build a better understanding of the interconnectivity of this groundwater system, and the relationships between water sources at different depths and geographical locations. Groundwater from seven bore locations, rainwater sampled over two collection periods, and soil and tree samples were taken. It was found that the aquifer accessed by BHMO3 bore is a slow recharging, deep water source with little connection to surface rainfall and nearby bores. The mineralogy at this site changes little from the groundwater level to the surface, and the trees access a Sr source with no input from atmospheric sources. Waters accessed by BH36 bore showed greater influence isotopically from seasonal shifts in rainfall isotopic composition, indicating a greater interaction with surface water/rainfall. Eucalypts at BH36 may access groundwater sources through the extensive fracture system around this site, but do not access Sr from the surface soil layers and receive little input from atmospheric Sr sources. All shallow bores sampled access waters that show seasonal trends in isotopic composition following rainfall. Eucalypts at shallow groundwater sites appear to be accessing a mix of sources for Sr, indicating changes in depth of nutrient uptake which may be seasonal and indicate groundwater dependence, although this requires further investigation. The waters accessed by the shallow bores are isotopically distinct from those of the deeper bores, while BH36 and BHMO3 show differing water-isotope patterns, indicating that there is little to no connectivity between the two sites.