Groundwater residence time in a dissected and weathered sandstone plateau: Kulnura–Mangrove Mountain aquifer, NSW, Australia

Groundwater residence time in the Kulnura–Mangrove Mountain aquifer was assessed during a multi-year sampling programme using general hydrogeochemistry and isotopic tracers (H2O stable isotopes, δ13CDIC, 3H, 14C and 87Sr/86Sr). The study included whole-rock analysis from samples recovered during well construction at four sites to better characterise water–rock interactions. Based on hydrogeochemistry, isotopic tracers and mineral phase distribution from whole-rock XRD analysis, two main groundwater zones were differentiated (shallow and deep). The shallow zone contains oxidising Na–Cl-type waters, low pH, low SC and containing 3H and 14C activities consistent with modern groundwater and bomb pulse signatures (up to 116.9 pMC). In this shallow zone, the original Hawkesbury Sandstone has been deeply weathered, enhancing its storage capacity down to ∼50 m below ground surface in most areas and ∼90 m in the Peats Ridge area. The deeper groundwater zone was also relatively oxidised with a tendency towards Ca–HCO3-type waters, although with higher pH and SC, and no 3H and low 14C activities consistent with corrected residence times ranging from 11.8 to 0.9 ka BP. The original sandstone was found to be less weathered with depth, favouring the dissolution of dispersed carbonates and the transition from a semi-porous groundwater media flow in the shallow zone to fracture flow at depth, with both chemical and physical processes impacting on groundwater mean residence times. Detailed temporal and spatial sampling of groundwater revealed important inter-annual variations driven by groundwater extraction showing a progressive influx of modern groundwater found at >100 m in the Peats Ridge area. The progressive modernisation has exposed deeper parts of the aquifer to increased NO3− concentrations and evaporated irrigation waters. The change in chemistry of the groundwater, particularly the lowering of groundwater pH, has accelerated the dissolution of mineral phases that would generally be inactive within this sandstone aquifer triggering the mobilisation of elements such as aluminium in the aqueous phase. © 2020 Informa UK Limited

Keywords

Aquifers, Isotope effects, Rocks, Water, Oxidation, Carbonates

Citation

Cendon, D. I., Hankin, S. I., Williams, J. P., Van der Ley, M., Peterson, M., Hughes, C. E., Meredith, K., Graham, I. T., Hollins, S. E., Levchenko, V., & Chisari, R. (2014). Groundwater residence time in a dissected and weathered sandstone plateau: Kulnura-Mangrove Mountain aquifer, NSW, Australia. Australian Journal of Earth Sciences, 61(3), 475-499. doi:10.1080/08120099.2014.893628