Oral Presentation NCGRT/IAH Australasian Groundwater Conference 2019

To connect or not to connect: the importance of hydrogeological conceptualisation for groundwater impact assessments (509)

Angela L. Bush 1
  1. AGE Consultants, Townsville, QLD, Australia

The groundwater regulatory environment for Queensland is moving from a reference site approach towards a site-specific approach. Complex dual-porosity groundwater flow systems are often hosted in mineralised rocks, causing naturally elevated solute levels. Water-rock interaction in fractured rock terrains can result in highly variable groundwater quality. Thus, a site-specific approach to groundwater compliance lends itself well to such systems. This approach detects a departure from background water quality. However, it is then crucial to determine the cause of that departure, which requires a robust conceptual model of the hydrogeology. This presentation interrogates the influence of conceptual interpretation on such assessments.  

Interpretation of hydraulic connectivity forms an important basis of the conceptual hydrogeological model, influencing the outcome of assessments and predictions. Conceptual models, like numerical models, are non-unique solutions, designed to best fit the available data. However, the possible solutions are narrowed during the concept development, as assumptions drive the outcomes of interpretation and constrain predictions. Therefore, careful analysis of primary data to form the conceptual model of connectivity is vital. Using the precautionary principle, connectivity is the most conservative assumption, as a more connected system typically allows pathways to develop more readily between causes and effects. Therefore, the interpretation of a system as “disconnected” should be supported by clear and convincing data trends. However, classical concepts of fracture-hosted groundwater flow often involve disconnected systems, comprising isolated blocks of groundwater, and limited connectivity across faults and contacts. Therefore, assessment of dual porosity systems for beneficial management is hampered by the subjectivity of data interpretation.

Hydraulic head distribution trends, both temporal and spatial, are used to assess the degree of connectivity at a range of metalliferous mine sites with fractured rock settings. Geological structure and hydraulic properties are interpreted to build a framework within which the flow system is analysed. The concept is also deepened through characterisation of groundwater quality, via both primary (sample analysis) and secondary (conductivity survey) methods. Contrasting conceptual models of both connected and disconnected systems are presented and discussed.   

The benefits of understanding connectivity, or lack thereof, are improved efficiency and greater efficacy in managing and protecting groundwater quality. Furthermore, the repercussions for ignoring the conceptual question of connectivity are significant, especially as the approaches to groundwater compliance change. This presentation questions our assumptions and promotes the case that one size does not fit all.