Deterioration of Layered Coastal Aquifer Water Due to Density Dependent Flows

Abstract

Seawater intrusion in coastal aquifers threatens the development of coastal areas in many countries around the globe. In spite of the number of studies that have examined seawater intrusion in different heterogeneous coastal systems and hydrogeological settings, a situation where the coastal aquifer consists of two permeable layers separated by an aquitard with major abstraction from the upper aquifer (as is the case in the Al-Batinah area of Oman) has not been studied yet. This thesis focuses on such a situation and aims to extend existing knowledge about seawater intrusion dynamics and the consequent quality deterioration in such coastal aquifers. A good understanding of the seawater intrusion patterns is essential for improved management of coastal water resources. Shallow groundwater abstraction is expected to speed up seawater intrusion in the upper aquifer compared with that for the lower aquifer. As a consequence, an unstable condition with a complex flow and transport pattern develops. With time, a saline boundary layer (SBL) forms as salt is transported across the aquitard into the upper part of the lower aquifer, in time inducing instability and convective mixing. The system remains stable when the SBL is confined entirely within the aquitard. The extent and sensitivity of this unstable mixing and the consequential lower aquifer contamination are expected to depend on the hydrological and hydrogeological characteristics of the aquitard and of both upper and lower aquifers. Intensive two-dimensional (2D) numerical simulations have been performed for different aquitard properties and selected characteristics of upper and lower aquifers to gain an insight into the dynamics of the densitydriven flow. The impacts of different remedial strategies on the transport process are examined. The transport pattern in a three-dimensional (3D) system is also explored for selected cases. Both qualitative and quantitative measures have been used to analyse the results. These include selected salinity levels (standards for domestic and irrigation purposes), maximum depth of penetration (MDP), and total mass present in the lower aquifer (MtL). The literature presents several numerical codes for solving variable density groundwater flow and transport problems. In this thesis, a finite difference simulator SEAWAT-2000 is selected and sensitivity of its results to spatial and temporal discretizations and numerical solution techniques is first tested against the Elder-Voss and Souza benchmark problem (EVS). The results are shown to be sensitive to the level of spatial and temporal resolutions for the chosen numerical scheme. A high-resolution mesh is necessary to minimize incorrect seeding of fingers and consequential artificially induced salinization due to numerical errors. The numerical simulations show that the pattern of contaminant-spread from buoyancy induced mixing in the layered coastal aquifer has a complex structure, described as a moving zone of instability. The instability occurs first in the seaward part of the SBL and then moves landward. With time the seaward part of the system begins to stabilize as the vertical density gradient reduces, but instability develops in the landward direction. As a result, previously adopted dimensionless criteria such as Rayleigh Number (Ra), wavelength (@) and wave number (µ) are not readily suited to analyse system stability. Factors that affect the timing of instability onset include SBL thickness and its solute concentration (C), and the upward vertical (Vv) and horizontal flow velocities (VH) within the lower aquifer. Large upward Vv retards the formation of the SBL while large VH increases the dispersion term, which smooths out the initial perturbations and thus impedes the occurrence of the instability. Investigation of the influence of aquitard parameters shows that increases in some (e.g., aquitard dispersivity, and porosity) promote the occurrence of instability and intensify the convective mixing by promoting the growth of the SBL. On the other hand, decreases in some lower aquifer parameters (e.g., smaller lower aquifer hydraulic conductivity and dispersivity) enhance the growth of the initial perturbations of the SBL into fingers. The growth of the SBL alone does not imply occurrence of instability as large flow velocity or high dispersion within the lower aquifer can refresh the bottom part of the SBL, and hence impede the trigger of instability. Factors or conditions that speed up seawater intrusion within the lower aquifer decrease the degree of convective mixing. This is because the unstable vertical density contrast between the upper and lower aquifers does not then develop, and the system regains stability more rapidly compared with cases where the seawater intrusion in the lower aquifer is slow. A higher abstraction rate (Q) speeds up the seawater intrusion and promotes SBL development, and hence an early occurrence of instability. For a specific set of aquifer parameters, the time for the lower aquifer to become unsuitable for domestic or irrigation purposes shortens as the abstraction rate increases. Different management options like discontinuous abstraction, alternating abstraction from the two aquifers, and short-term abstraction regimes are examined for their effects on the convective mixing. For a management option to be effective in reducing the degree of convective mixing, the seawater intrusion in the upper aquifer should be retarded or reversed so that the salt supply to the SBL is reduced. Subjecting both aquifers to continuous abstraction of approximately equal rates results in relatively similar intrusion rates, thereby impeding the onset of instability. While the work in this thesis is not based on an actual case-study, the Al-Batinah region in Oman is used as a reference for model conceptualisation since seawater intrusion in layered coastal aquifers is of major concern there. Given that the rainfall rate is very small in such arid regions, natural recharge (direct from rainfall) is not effective in reversing seawater intrusion in a useful timeframe. However, artificial recharge (through injection wells) can improve the groundwater quality when abstraction is stopped. The dispersion zone in the upper aquifer decreases, thereby slowing the growth of fingers. If abstraction from the upper aquifer continues, obviously a higher recharge rate is needed to reduce the degree of the convective mixing as well as the seawater intrusion in the upper aquifer. When the lower aquifer is recharged, the fingers are smoothed of and the system can become stable even if abstraction continues. With higher recharge rates, the system becomes stable more quickly. Abstraction from the saline wedge is also investigated as a remediation strategy. With cessation of the inland abstraction, saline water abstraction retards the seawater intrusion and so diminishes the size of the developed fingers. Alternating inland abstraction and saline water abstraction does not assist in effective reduction of the convective mixing given that the isochlors recede toward the sea during saline water abstraction and intrude again during inland abstraction. For a management or remedial strategy to be effective in stabilizing, or reducing the intensity of, buoyancy-induced mixing in the lower aquifer, it must induce strong enough horizontal velocity, VH, in the lower aquifer (to eliminate the developed fingers and prevent perturbations at SBL bottom boundary), large upward velocity, Vv (to resist SBL development), or decreases in the seawater intrusion rate in the upper aquifer so that the unstable condition (dense fluid overlying less dense fluid) does not develop. Although the convective mixing patterns are different in 2D and 3D simulations, results show that there are similarities in the basic features of finger growth, coalescence, and their response to an increase in the abstraction rate. Based on the limited simulations, the 2D results are broadly similar to those of the 3D case and, with appropriate caution; 2D modelling can provide useful information for water resources management in the aquifer situations studied. The study provides an improved understanding of the fingering process in layered coastal aquifers and thereby contributes to the rational management of layered coastal aquifers