Decoupling of solutes and water in regional groundwater systems: the Murray Basin, Australia

Documenting the origins, residence times, and movement of groundwater and the solutes that it contains is critical to understanding hydrogeological systems. This study uses Cl mass balance to determine the Cl accession time (i.e. the time required for Cl to accumulate) and Cl to estimate the residence times of Cl in the Victorian portion of the Murray Basin, southeast Australia. Much of the Murray Basin contains saline groundwater with total dissolved solids (TDS) concentrations commonly > 14,000 mg/L and locally up to 300,000 mg/L. The total mass of Cl stored in the Victorian portion of the basin is estimated as between 12,400 and 47,100 MT. Using present day rainfall totals and Cl concentrations in rainfall, the Cl accession time is 170 to 650 ka. Aquifer thicknesses and groundwater salinity both increase westwards in this part of the Murray Basin. Consequently, the Cl accession times increase westward from 0.1–0.6 ka to 286–1080 ka. By contrast, C activities of the majority of the groundwater are > 2 pMC, and commonly much higher. Notwithstanding the difficulty in correcting C residence times, the widespread occurrence of groundwater with above background C activities implies that groundwater residence times are generally < 30 ka, which is substantially shorter than the Cl accession times. RCl (the ratio of Cl to total Cl × 10) values of the groundwater are between 20 and 230, and are uncorrelated with Cl concentrations. While it is difficult to determine precise Cl residence times, the observation that the RCl values are significantly higher than those that represent secular equilibrium with the aquifer matrix (RCl of 5 to 10) indicates that they are up to a few hundred thousand years and similar to the Cl accession times. The RCl values together with the geology of the aquifers, stable isotope ratios, and major ion geochemistry precludes halite dissolution, incorporation of connate water, or the long-term diffusion of Cl from clays as mechanisms of producing the elevated Cl concentrations. Rather, it is most likely that the high Cl concentrations result from recycling of solutes in saline lakes and playas or repeated cycles of evapotranspiration in the unsaturated zone

Problems with the application of hydrogeological science to regulation of Australian mining projects: Carmichael Mine and Doongmabulla Springs

Understanding and managing impacts from mining on groundwater-dependent ecosystems (GDEs) and other groundwater users requires development of defensible science supported by adequate field data. This usually leads to the creation of predictive models and analysis of the likely impacts of mining and their accompanying uncertainties. The identification, monitoring and management of impacts on GDEs are often a key component of mine approvals, which need to consider and attempt to minimise the risks that negative impacts may arise. Here we examine a case study where approval for a large mining project in Australia (Carmichael Coal Mine) was challenged in court on the basis that it may result in more extensive impacts on a GDE (Doongmabulla Springs) of high ecological and cultural significance than predicted by the proponent. We show that throughout the environmental assessment and approval process, significant data gaps and scientific uncertainties remained unresolved. Evidence shows that the assumed conceptual hydrogeological model for the springs could be incorrect, and that at least one alternative conceptualisation (that the springs are dependent on a deep fault) is consistent with the available field data. Assumptions made about changes to spring flow as a consequence of mine-induced drawdown also appear problematic, with significant implications for the spring-fed wetlands. Despite the large scale of the project, it appears that critical scientific data required to resolve uncertainties and construct robust models of the springs’ relationship to the groundwater system were lacking at the time of approval, contributing to uncertainty and conflict. For this reason, we recommend changes to the approval process that would require a higher standard of scientific information to be collected and reviewed, particularly in relation to key environmental assets during the environmental impact assessment process in future projects

Deficiencies in the scientific assessment of the Carmichael Mine impacts to the Doongmabulla Springs

This work is made available with the Creative Commons, Attribution License CC-BY https://creativecommons.org/licenses/by/4.0/ Copyright (2019) Flinders University.Key points: (1) Adani appears likely to have significantly under-estimated future impacts to the Doongmabulla Springs Complex (DSC) arising from the Carmichael Mine. (2) Should the Carmichael Mine cause springs within the DSC to cease flowing, this impact may be irreversible. (3) The safeguard against DSC impacts proposed by Adani, namely Adaptive Management, is unsuitable and unlikely to protect the DSC from severe degradation or cessation of flow. (4) Possible cumulative impacts to the DSC from other mining activities in the Galilee Basin have not been adequately considered. We conclude that the DSC face a legitimate threat of extinction due to the Carmichael Mine project

Sustainable management of groundwater extraction: An Australian perspective on current challenges

Study focus: Our incomplete knowledge of groundwater systems and processes imposes barriers in attempting to manage groundwater sustainably. Challenges also arise through complex institutional arrangements and decision-making processes, and the difficulty in involving stakeholders. In some areas, these difficulties have led to water table decline and impacts on groundwater users and groundwater-dependent ecosystems. However, there is potential to improve the sustainable use of groundwater resources through improvements in management practices. We discuss some of the challenges, and present survey results of research, government, and industry professionals across the groundwater sector in Australia. New hydrological insights for the region: The highest-ranked challenge identified in the survey was the difficulty in determining regional-scale volumetric water extraction limits. This is surprising given the criticism in the international literature of volumetric based approaches for groundwater management, and the decreased reliance on this approach in Australia and elsewhere in recent years. Other major challenges are the difficulty in determining and implementing maximum drawdown criteria for groundwater levels, determining water needs of ecosystems, and managing groundwater impacts on surface water. Notwithstanding these gaps in technical understanding and tools and a lack of resources for groundwater studies, improvements in stakeholder communication should enable more effective decision-making and improve compliance with regulations designed to protect groundwater and dependent ecosystems