The Lamington and Main Range volcanics form a major regional fractured rock aquifer system (more than 12,000 km2) within the Clarence-Moreton Basin in south-east Qld and north-east NSW. Although widely recognised as an important hydrological feature, there has been no systematic or comprehensive basin-wide assessment of their significance as a major preferential recharge area.
Within the Clarence-Moreton bioregional assessment, an integrated hydrogeological assessment of basin-scale aquifer systems involved developing a 3D geological model, groundwater recharge and regional stream flow assessments, and characterising aquifer hydrochemistry (within the volcanics and neighbouring aquifer systems). The 3D geological model yielded a median thickness of approximately 130 m for these volcanic aquifers, and a maximum thickness of ~800 m near the crest of the volcanics in elevated areas. However, borehole lithological data confirmed that these volcanic rocks do not consist of a single homogeneous basalt flow nor form a single aquifer, but instead consist of many thinner overlapping basalt flows (up to ~10 m thick). The groundwater recharge assessment (using chloride mass balance) indicated recharge rates that can locally be more than 1000 mm/yr, with recharge rates to the volcanic aquifers at least one order of magnitude greater than recharge rates for most sedimentary bedrock units. However, a large proportion of groundwater recharge to the Lamington Volcanics discharges locally with short transit times (days to months) into the alluvium or streams, whereas only a small proportion percolates to deeper aquifers.
Based on this integrated approach, we have developed conceptual models that describe the relationship of the volcanic aquifers with alluvial aquifers and streams as a primary driver of alluvial groundwater and surface water quality in the Clarence-Moreton Basin. This conceptual understanding provides important knowledge for assessments of any future coal seam gas or agricultural development activities within the Clarence-Moreton Basin.