With the majority (80%) of the Australian’s solid geology overlain by regolith, groundwater is increasingly being investigated as an exploration tool to see through cover material, though few investigations have surveyed for regional hydrogeochemical trends and environmental baselines. In this study, as part of the Australian Government ‘Exploring for the Future’ Program and in collaboration with the Northern Territory Geological Survey, groundwater chemistry was utilised as a tool for passive sampling of underlying geology to detect hidden mineralisation and hydrocarbons as well as to build a regional baseline to inform environmental monitoring and decision making.
Groundwater samples were collected from private bores in the Lake Woods (Caritat et al., 2019), Tennant Creek and McArthur River regions of the Northern Territory for a comprehensive suite of hydrogeochemical analyses and fingerprinting including: field physicochemical parameters (EC, pH, Eh, DO and T); major and trace elements; isotopes of δ18O and δD, DIC (δ13C) and dissolved strontium (87Sr/86Sr); dissolved hydrocarbon VFAs, BTEX, methane concentrations and methane isotopes (δ13C and δD).
Results of the current study show groundwater chemistry matches regional geological trends, demonstrating the capacity of hydrogeochemistry to reveal the chemistry of rocks under cover. Anomalous trace element concentrations from different regions are associated with different physicochemical parameter groupings indicating predictable patterns of trace element mobility related to the geological environment. Additionally, elevated Cu concentrations in groundwater are associated with elevated Cu concentrations in independent datasets of soil geochemistry (Bastrakov et al., 2018) and in machine learning Cu concentration predictions (Wilford et al., 2018).
These initial results show regional hydrogeochemical sampling surveys can highlight trace element anomalies as well as provide an environmental background. By identifying the underlying chemical reactions, the results of this survey allow an interpretation of groundwater processes that can inform decision making with respect to groundwater use, mineralisation, hydrocarbons and the environment.