The Nulla Basalt Province, a 7300 km2 lava-field province resulting from numerous effusive volcanic eruptions over the last 5 Ma, hosts groundwater supplies that support stock and domestic water use, sustain springs and wetlands, and contribute baseflow to the Burdekin River. The province comprises stacked lava flows with concentrated flow-top vesiculation. Weathering has expanded vesicular pore space, and fractures have enhanced connectivity, leading to numerous discrete permeable zones and heterogeneous hydrogeological properties. Possible increased demand for water from agricultural development would impact on water availability for natural ecosystems. Managing water resources requires an understanding of interactions between groundwater and surface water systems, to enable the protection of groundwater-dependent ecosystems and define critical components of the water balance. Characterising springs will aid this understanding.
For this project, springs have been identified, mapped, and categorised using existing spring databases together with new and existing geological, hydrogeological, hydrochemical, and geomorphic data, including recently-acquired 1 m resolution LiDAR elevation data, and Landsat earth observation data from Digital Earth Australia. Springs vary from local discharge features occurring as small seeps to regionally significant discharge features exhibiting high flow rates. Different temporal dynamics and hydrochemical signatures are present, which may reflect various groundwater sources and positions in the landscape.
This work shows that contacts between lava flows; the presence and distribution of Cenozoic sediments that exist beneath, between, and above basalts; variation in the paleotopographic surface and basement geology; modern topography; and the potentiometric surface combine to determine the position of springs in the Nulla Basalt Province. Conceptual models are presented for each spring type identified. These models describe the factors that affect the location and source of each spring type. This work informs understanding of the hydrogeological system, thus improving conceptualisation of the water cycle processes in the region.