Quick Groundwater Flow to Tropical Savanna Springs (Mataranka, Northern Territory, Australia)

Abstract

The Mataranka Springs Complex is a regional groundwater discharge for the Cambrian Limestone Aquifer (CLA) of the Northern Territory (Australia) and forms the headwaters for the environmentally sensitive Roper River. Whilst a regional groundwater contribution to the spring flow is well established, the absence of cover over the CLA in the vicinity of the springs and the prevalence of karst suggest that a component of quick flow during the wet season is possible. A quick flow contribution to the springs was evaluated using a biweekly monitoring programme for several environmental tracers (major ions, stable isotopes of water, and 222Rn) at two large springs (Rainbow Spring and Bitter Spring) and at one minor spring (Fig Tree Spring) over a two-year period that included a relatively dry (2019–2020) and a relatively wet (2020–2021) rainy season. There were limited variations in all tracers at Rainbow and Bitter springs throughout the monitoring programme, indicating an absence or a minimal contribution from quick flow. In contrast, all tracers responded to large rainfall events at a scale of days to weeks in Fig Tree Spring, consistent with a component of quick flow. However, the tracer response at Fig Tree was complex and possibly involved a combination of quick flow, unsaturated zone processes, and changes in the geochemical environment in the aquifer. Quick flow may be favoured in the parts of the Mataranka Springs Complex, where flow paths flow through the karstic tufa layer overlying the CLA.