Understanding, detecting and conceptualising hydrogeologic barriers in groundwater modelling studies

Author: Sarah Marshall

Marshall, Sarah, 2021 Understanding, detecting and conceptualising hydrogeologic barriers in groundwater modelling studies, Flinders University, College of Science and Engineering

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Understanding and predicting groundwater flow can be challenging in regions with complex geology. Geological structures that inhibit flow, known as hydrogeologic barriers, can impact groundwater velocities, chemistry, and discharge dynamics, amongst other things. Examples of hydrogeologic barriers include some faults and dykes. Although not all faults and dykes are hydrogeologic barriers, this thesis is

focused on the role of geological structures as hydrogeologic barriers, rather than as conduits. Barriers are dicult to detect and characterise. As such, they are often excluded from a conceptual model of a site. This could have implications for groundwater model predictions, and subsequent groundwater management, if the geological reality of a site is misrepresented. The thesis is part of a larger project investigating groundwater resources in the Pilbara region of Western Australia. The scope and research questions initially stemmed from this broader project. In the Pilbara region, hydrogeological barriers, including faults and dolerite dykes, are known to impact groundwater flow. However, the contents of the research is intrinsically universal and not based on one field area specifically.

This thesis addresses key issues on understanding, detecting and conceptualising groundwater barriers in groundwater modelling studies. On understanding barriers, by describing scenarios and quantifying conditions under which they can significantly impact groundwater level recovery associated with groundwater extraction. On detecting barriers, by showing how two commonly-collected hydrogeological datasets, hydraulic head and groundwater age, can assist in locating and characterising barriers depending on their configuration and the recharge setting. And on conceptualising barriers, by demonstrating a new method for inverse modelling that uses hydraulic head and groundwater age data to conceptualise models with no prior knowledge of barriers properties.

Specifically, the new contributions to research address:

1. During pumping, a hydrogeological barrier may be undetected if it is located beyond the maximum extent of the cone of depression; yet it may still control drawdown during the recovery phase. Non-dimensional solutions are developed to show the conditions under which a barrier may be undetected during pumping but still significantly impact groundwater level recovery. The magnitude of the impact from an undetected barrier will increase as the ratio of pumping rate to aquifer transmissivity increases.

2. The joint role of hydraulic head and groundwater age data in detecting and characterising hydrogeologic barriers, such as faults and dykes is studied. Numerical flow and transport models were developed with different hydrogeologic barrier configurations in a hypothetical aquifer. Computed hydraulic head and groundwater age distributions were compared to those without a barrier and two forms of recharge were compared.

3. Sharp barriers are included in groundwater model inversion, even where their presence is uncertain. A new method is introduced utilising 'phantom structures' - randomly located, linear groups of model cells assigned a unique hydraulic conductivity value - to improve identifiability of barriers. Automated parameter estimation using PEST is implemented to determine model structures that best match the hydraulic head and groundwater age observation data from a hypothetical aquifer. The results are compared to model inversion using traditional pilot points.

As a whole, the thesis contributes to the study of structural geological heterogeneity and how it can be better represented in groundwater models that aid decision making. It aims to improve best practice for investigating and modelling aquifers with fault-like barriers. This has implications for projects where barriers play a significant role in the compartmentalisation of groundwater flow, spring dynamics, and drawdown and recovery associated with groundwater extraction.

Keywords: groundwater, numerical modelling, mining, resource management, analytical solution, hydrogeology, groundwater age, hydraulic head, geological fault, geological dyke, hydrogeologic barrier

Subject: Marine Geology and Geophysics thesis

Thesis type: Doctor of Philosophy
Completed: 2021
School: College of Science and Engineering
Supervisor: Peter Cook