Hydrogeochemistry of basalt aquifers: Multi-isotope analysis for better understanding of groundwater recharge, flow-paths and surface water interactions of the McBride and Nulla Basalt Provinces - northern Queensland, Australia

Basalt aquifers of the Upper Burdekin region are utilised for stock and domestic purposes and farming as well as contributing to springs, wetlands and rivers. However, the underlying dynamics of the groundwater system are poorly understood. In this study, as part of the Australian Government ‘Exploring for the Future’ Program, targeted hydrogeochemical sampling was undertaken to identify aquifer system processes controlling groundwater recharge, flowpaths and surface water interactions in the McBride and Nulla Basalt Provinces. Groundwater was sampled from existing and new monitoring bores, and surface water and springs were sampled from selected sites for a wide suite of hydrogeochemical parameters to fingerprint and trace groundwater dynamics. Analytes included: physicochemical parameters (EC, pH, Eh, DO and T); major anions and cations; and isotopes of water (δ18O and δD), DIC (δ13C), and dissolved strontium (87Sr/86Sr) as a tool. Hydrogeochemical signatures identified in this study support groundwater recharge and groundwater flowpaths being largely restricted to within the basalt provinces. Groundwater typically occurs in a main basal groundwater system and multiple shallow groundwater systems. Groundwater flowpaths of the main basal system were traced from higher elevations in the mountains through to the Burdekin River, showing interaction between the basal and shallow systems. Groundwater recharge to the basal system is indicated from δ18O and δD as being largely from higher elevation and higher rain-out in the mountains. In contrast groundwater recharge to the shallow systems is indicated as being from localised rainfall, as well as local input from rivers and the basal aquifer. Springs across the study area are largely sourced from the main basal groundwater system and form the tributaries that discharge into the Burdekin River. Likewise, some springs contribute to tributary rivers along flowpath. Groundwater signatures also suggest the basalt aquifers discharge directly to the Burdekin River linking groundwater to surface waters in places. This study demonstrates how hydrogeochemistry, including isotope analysis, can be used as a tool to reveal fundamental controls on groundwater system processes. The results of this study identify the principal processes of groundwater recharge processes, flowpaths and surface water interactions, which can be used to better understand the basalt aquifer systems in the Upper Burdekin region. Presented at the 2019 Australasian Groundwater Conference (AGC 2019)