Influence of mechanical loading and unloading in interpreting vertical connectivity between two aquifers – an example from Condamine alluvium and Walloon coal measures connectivity study

Full text: The groundwater resources of the Condamine River Alluvial Aquifer (CA) in Queensland have been extensively developed over the past 60 years, primarily for irrigation water supply. Current and proposed coal seam gas development in the Surat Basin extends beneath the western edge of the CA footprint and involves depressurisation of the Walloon Coal Measures (WCM), which form the basement for the alluvium. This could potentially impact CA groundwater resources, depending on the hydraulic connectivity between the alluvium and the underlying WCM. An initial impact assessment was undertaken in 2012 by the Queensland Office of Groundwater Impact Assessment (OGIA) in the Surat Underground Water Impact Report (UWIR). OGIA has since then been leading an extensive study, supported by Arrow Energy through drilling and test pumping. This study comprises multiple lines of investigation to improve understanding of connectivity in terms of geological, hydrogeological and hydrochemical characteristics of the interface between the CA and the WCM. As part of this study, two detailed pumping tests were carried out in the CA footprint to assess the hydraulic connection between the CA and WCM, involving pumping from the CA while monitoring the response at monitoring bores in both formations. During one of the pumping tests, groundwater levels in the confined WCM were observed to vary by up to a metre, but there appeared to be little or no correlation between these fluctuations and the more substantial groundwater level changes observed in the overlying CA. A detailed assessment of these data indicates that mechanical loading and unloading effects related to changes in surface water storage and groundwater storage in the overlying CA is likely to have caused much of the observed fluctuations in the WCM. Mechanical loading effects are not routinely considered in most groundwater investigations, but these results indicate that for a confined aquifer like the WCM, exclusion of these effects could lead to an overestimation of vertical hydraulic conductivity. (author)