Numerical investigations of contaminant transport in permeable rocks: examining the effects of discrete flow features in density-dependent and density-independent systems

Author: Megan Sebben

Sebben, Megan, 2016 Numerical investigations of contaminant transport in permeable rocks: examining the effects of discrete flow features in density-dependent and density-independent systems, Flinders University, School of the Environment

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Discrete flow features (DFFs; e.g. fractures, faults, clay layers) are geological discontinuities of higher or lower permeability than the host rock that are of sufficient extent to impact groundwater flow. DFFs can provide preferential flow pathways (i.e. ‘preferential flow features’ (PFFs), wherein the permeability of the DFF is higher than the matrix), or flow barriers (i.e. ‘barrier flow features’ (BFFs), wherein the permeability of the DFF is lower than the matrix). The study of DFFs has thus far focussed on PFFs in low-permeability rock settings (e.g. granite, basalt), in which the majority of groundwater flow occurs within the PFF. There has been little research into the impact of both PFFs and BFFs on contaminant plume migration in otherwise permeable rocks (e.g. sandstone, limestone), despite that DFFs are widespread in high-yielding, permeable rock aquifers. The aim of this thesis is to examine within a modelling framework how DFFs influence the displacement and spreading of solute plumes, and the accompanying patterns of groundwater discharge, in idealised permeable rock aquifers.

Firstly, the influence of PFFs is investigated in a complex groundwater setting, to evaluate whether PFFs impact solute transport within commonly encountered situations involving seawater-freshwater mixing, such as those found in most coastal aquifers. Aquifers containing single fractures or regularly spaced discrete fracture networks (DFNs) are examined using modified forms of the Henry (1964) seawater intrusion (SWI) benchmark problem. The applicability of equivalent porous media (EPM) models for representing simple DFNs in SWI simulations is also tested. This study demonstrates that fracture effects on SWI are likely to be mixed, ranging from enhancement to reduction in seawater extent and the width of the mixing zone, depending on such factors as PFF location, orientation and density. EPM models are shown to be inadequate for inferring salinity distributions unless the density of orthogonal fractures is high and appropriate dispersivity values can be determined.

While the study of SWI showed macro-scale plume behaviour, the role of individual PFFs on solute transport was uninterpretable under the complex density-dependent conditions of the Henry problem. Therefore, numerical investigations of solute plumes passing through an individual PFF are performed under simpler conditions, to explore the local-scale PFF effects on plume migration. The numerical modelling results show that individual-PFF impacts on plume displacement and spreading can be considerable. The attenuation of plumes is likely governed by PFFs rather than flow through the matrix, given that a single PFF (representing a medium-sized fracture) produces the equivalent spreading effects of 0.22-7.88 m of plume movement through the matrix only.

Finally, the previous analysis is extended by accounting for DFFs as 2D flow features, and by including BFF situations. A simple analytical expression and numerical modelling are employed to quantify the displacement and spreading of a solute plume as it passes through a DFF. The results demonstrate that the attenuating influence of PFFs in permeable rocks is greater than for BFFs, and that PFFs are likely to have a more significant influence on plume distributions. DFF effects on plumes generally increase with increasing aperture.

Keywords: Preferential flow, fractures, flow barriers, seawater intrusion, variable-density flow, solute transport, permeable matrix, numerical model

Subject: Environmental Studies thesis, Environmental Science thesis

Thesis type: Doctor of Philosophy
Completed: 2016
School: School of the Environment
Supervisor: Professor Adrian Werner