Multiple isotope tracers reveal the sources of water sustaining ecologically and culturally significant springs, and their vulnerability to mining development

• Use of multiple isotope tracers to determine sources of spring water discharge. • Spring discharge derived from a mixture of multiple groundwater sources. • Groundwater component over 500 ka old supports multiple springs. • Springs close to mining development sustained by local-scale flow paths. • Mine dewatering likely to divert flow away from these springs and reduce discharge. The development of Australia’s largest approved coal mine in the Galilee Basin of northeast Australia will dramatically alter the region’s water resources. The Carmichael Coal Mine is located within eight kilometres of the Doongmabulla Springs Complex, which support endemic flora and fauna and hold crucial cultural significance to the Wangan and Jagalingou Traditional Owners. Uncertainties regarding the source(s) of water to the springs have so far prevented a clear understanding of how mining will affect their long-term health and hydrology. Environmental tracers (including 3 H, 14 C, 36 Cl, 2 H, 18 O, 13 C and 87 Sr/ 86 Sr) from 8 springs and 15 surrounding water bores, indicate that multiple sources of water, with vastly different residence times, sustain the Doongmabulla springs. R 36 Cl (79.1–80.8 x10 -15 ), a 14 C (47.2–91.1 pMC) and the presence of 3 H (up to 0.28 TU) suggest that relatively young water recharged during the Holocene and Late Pleistocene sustains three eastern springs close to the Carmichael mine. Significant R 36 Cl decay (to between 19.46 x10 -15 and 56.94 x10 -15 ) indicates that much older water from the Early-Mid Pleistocene (residence time > 500 ka) sustains springs in the west of the complex, alongside intermediate-aged water fractions (a 14 C 0.99–––95.73 pMC) and minor modern components ( 3 H up to 0.518 TU). Regional groundwater flow in the Clematis Sandstone (sampled R 36 Cl values: 22.92 x10 -15 ––109.9 x10 -15 ) is likely the predominant source of very old water in the western springs, although contribution from deeper aquifers (e.g., Dunda Beds or Permian coal measures) cannot be ruled out. Aquifer geometry and hydraulic gradients suggest local-scale flow paths in the Clematis Sandstone and Dunda Beds are the likely source of Holocene water that is critical to the eastern springs. The revised understanding of the sources of groundwater implies that both the western and eastern springs are more vulnerable to mining impacts than has been previously assumed. Protection of ecologically and culturally significant springs worldwide from drawdown associated with resource extraction requires robust methods to characterise their water sources and this study presents a multi-isotope tracer approach that may be widely applicable.