Inter-aquifer connectivity: Investigating groundwater movement through a regional-scale aquitard using a multi-scale and multi-tracer approach

Author: Stacey Priestley

  • Thesis download: available for open access on 11 Jan 2020.

Priestley, Stacey, 2018 Inter-aquifer connectivity: Investigating groundwater movement through a regional-scale aquitard using a multi-scale and multi-tracer approach, Flinders University, College of Science and Engineering

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Aquitards play an important role in the physical and chemical evolution of groundwater, however, the location and amount of inter-aquifer leakage through the aquitard is often unknown. ‘Diffuse inter-aquifer leakage’ through aquitard pores can be a significant component of the water balance over large areas. ‘Enhanced inter-aquifer leakage’ can occur through the aquitard via preferential pathways causing an increased rate of groundwater movement through the aquitard relative to diffuse inter-aquifer leakage. Despite the potential importance of inter-aquifer leakage, it is not generally studied in groundwater investigations, in part because of the inherent difficulty in obtaining the necessary data to determine the magnitude of the groundwater flux. Quantifying inter-aquifer leakage at a regional-scale is complicated by diffuse inter-aquifer leakage rates that are often so small that they are very hard to detect and quantify, although they can be a significant component of the water balance over large areas. Localised enhanced inter-aquifer leakage fluxes can be missed at the spatial resolution of regional surveys. Additionally, there remain issues with incorporating scale and heterogeneity in the few methods that are available to investigate inter-aquifer leakage.

This thesis investigated inter-aquifer leakage at multiple scales in an arid zone regional groundwater basin using environmental tracers in an attempt to account for heterogeneity and the different scales of leakage. Diffuse inter-aquifer leakage of the aquitard pore water and enhanced inter-aquifer leakage through preferential pathways were included in this approach. This thesis focussed on the connectivity between the Great Artesian Basin (GAB) and underlying Arckaringa Basin, located in central Australia. A sparse observation network in central Australia and hence data scarcity is an additional challenge encountered in this study, as well as limited hydrogeological investigations elsewhere.

The first part of this thesis determined inter-aquifer leakage at a regional-scale by the implementation of a comprehensive multidisciplinary approach. Diffuse inter-aquifer leakage direction and flow rates through the aquitard pores were estimated by calculating the aquitard’s hydraulic resistance. The locations of enhanced inter-aquifer leakage were identified through the interpretation of hydrological data, and environmental tracers including a range of isotopes. While the exact nature of the enhanced inter-aquifer leakage mechanism remains unknown in detail, the proportion of enhanced inter-aquifer leakage was calculated using strontium isotopes and radiogenic helium. The protocol of well-couplet sampling and analysis methodologies undertaken in this study provided a comprehensive approach to investigate inter-aquifer leakage on a regional-scale where only limited wells were available.

The second part of this study investigated uranium isotope distributions and the processes that control it in groundwater in the GAB and underlying Arckaringa Basin. Through interpretation of regional groundwater and sequential extraction of mineral phases, it was found that rock weathering, the geochemical environment and α-recoil of daughter products control 238U and 234U isotope distributions within the groundwater basins. The importance of each of these processes is determined by the sample’s location within the groundwater flow system, although due to the complexity of uranium isotope systematics it was not possible to conclusively determine at all locations the processes controlling the distribution of uranium isotopes. Nevertheless, interpretation of uranium isotope trends and distributions identified a number of recharge locations and helped to constrain groundwater flow processes, including a location of stagnant flow. However, in this groundwater basin uranium isotopes were not as successful in identifying inter-aquifer leakage as in other studies.

The final part of this study investigated diffuse inter-aquifer leakage using chloride and helium transport through the aquitard pore waters with one-dimensional analytical and numerical models. A core through the GAB and Arckaringa Basin was drilled in March 2015 and samples for noble gases, stable isotopes and major ion analysis of aquitard pore waters were collected. Models of chloride and helium transport showed that both diffusion and slow downward advection through the aquitard pores control solute transport. The concurrent use of chloride and helium constrained the models that would fit the solute profiles to the final model presented, and provided independent information about the hydrological conditions. Helium, being produced in-situ and influenced by groundwater residence time, provided evidence of solute transport rates and processes, and chloride also provided insight into palaeohydrological conditions. An increase in chloride concentration in the upper part of the profile is interpreted as being due to a reduction in recharge for at least 3000 years. Additionally, groundwater extraction from the lower aquifer since 2008, and resultant drawdown in the lower aquifer, has penetrated and moved through the aquitard, although the subsequent increase in chloride and helium concentration has only penetrated into the lower few meters of the aquitard. The aquitard properties and hydraulic head measurements gave an instantaneous picture of potential diffuse inter-aquifer leakage at the time of sampling while environmental tracer profiles provided a long-term perspective and insight into solute transport processes.

The three studies contained in this thesis provided field- and laboratory-focused research on both diffuse inter-aquifer leakage rates and localised enhanced inter-aquifer leakage between the GAB and underlying Arckaringa Basin, supported by modelling. This links inter-aquifer connectivity at the point-scale to whole-basin processes, which is rarely undertaken.

Keywords: Arid regions, aquitard, cross-formational flow, environmental tracers, hydrochemistry, inter-aquifer leakage, noble gases, solute transport

Subject: Environmental Science thesis

Thesis type: Doctor of Philosophy
Completed: 2018
School: College of Science and Engineering
Supervisor: Vincent Post