Event-based groundwater recharge: drip observations reach new depths in mine tunnels

Drip sensors in underground spaces such as mine tunnels can quantify vertical water flux through the vadose zone during rainfall events. Australia’s National Groundwater Recharge Observing System (NGROS), established in 2022, provides the first dedicated sensor network for observing the recharge of groundwater at an event-scale across a wide range of geologies, environments, and climate types. As part of this program, differences in recharge fluxes and patterns through fractured metamorphic rock were analysed for hourly data in a shallow mine tunnel (14 sensors at ~30-50 m below ground) and a deep mine tunnel (12 sensors at ~115 m below ground). Average annual rainfall varied from <500 to >1200 mm per annum at the deep and shallow tunnel sites respectively. Recharge was evident despite a thick (>100 m) unsaturated zone above the tunnel. In the shallow mine tunnel, distinct recharge pathways including fracture zones around quartzite intrusions were evident, and time-lags were relatively short. Data analysis will include patterns of rainfall thresholds at which vertical fluxes occur, and spatial patterns within tunnels and between different sites in the NGROS network. The drip data can be useful for managing mine water related risks, and to complement recharge estimates from other methods for sustainable groundwater management.