Influence of seaward boundary condition on contaminant transport in unconfined coastal aquifers

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Abstract

Contaminant transport in coastal aquifers is complicated partly due to the conditions at the seaward boundary including seawater intrusion and tidal variations of sea level. Their inclusion in modelling this system will be computationally expensive. Therefore, it will be instructive to investigate the consequence of simplifying the seaward boundary condition by neglecting the seawater density and tidal variations in numerical predictions of contaminant transport in this zone. This paper presents a comparison of numerical predictions for a simplified seaward boundary condition with experimental results for a corresponding realistic one including a saltwater interface and tidal variations. Different densities for contaminants are considered. The comparison suggests that the neglect of the seawater intrusion and tidal variations does not affect noticeably the overall migration rate of the plume before it reaches the saltwater interface. However, numerical prediction shows that a more dense contaminant travels further seaward and part of the solute mass exits under the sea if the seawater density is not included. This is not consistent with the experimental result, which shows that the contaminant travels upwards to the shoreline along the saltwater interface. Neglect of seawater density, therefore, will result in an underestimation of the exit rate of solute mass around the coastline and fictitious migration paths under the seabed. For a less dense contaminant, neglect of seawater density has little effect on numerical prediction of migration paths.

Introduction

The development of coastal resorts and coastal urban subdivisions has increased the incidence of groundwater contamination in the zone adjacent to the shoreline. The sources of contaminants can be landfills of solid wastes, tourist-generated wastes and usage of fertilizers and pesticides for agricultural purposes. These contaminants tend to enter the groundwater system and travel seaward in the ambient groundwater flow. They not only pollute the groundwater, but also endanger the environment of coastal beaches. The understanding and modelling of contaminant transport in coastal aquifers are vital to good management of the coastal environment.

Contaminant transport in coastal aquifers is inherently complex. At the seaward boundary, the actual sea condition includes saline seawater and tidal variations of sea level. The dense seawater tends to intrude into the fresh groundwater and consequently a saltwater interface results (Henry, 1959). The presence of the saltwater interface and tidal fluctuations affects groundwater flow in the area close to the shoreline, and hence will also affect the pattern of contaminant migration in that area. Little work has been done on contaminant transport in such a system. Schincariol and Schwartz (1990) and Oostrom et al. (1992b) presented experimental results on the behaviour of dense plumes in a horizontal and uniform flow field in porous media. Their results showed convection instabilities occurring in the dense plumes. Koch and Zhang (1992) investigated the effects of contaminant density on its movement also in a steady horizontal flow field by performing a series of numerical exercises. None of these studies has included the distinct features of coastal groundwater. Li et al. (1999) presented a theoretical model of water flow across the ocean–land interface. The model showed a significant contribution of local tide-driven water flow to the total groundwater outflow from the aquifer to the ocean. Further work is needed to investigate the influence of the seaward flow condition on solute transport and its discharge.

Numerical modelling is an efficient way to simulate contaminant migration. However, the modelling will be computationally expensive if both seawater density and tidal variations of sea level are included. Seawater is not likely to have the same density as that of the contaminant from surface sources, and extra computing time is needed for the numerical model to simulate two contaminants of different densities. Moreover, in order to properly model the transient groundwater response to tidal variations, a very small time step size should be used and this requires more computational effort (Volker et al., 1998). It is useful, therefore, to explore the consequence of simplifying the sea boundary condition on contaminant transport. If the sea condition can be simplified under certain situations, great computational savings will be made. The objective of this paper is to investigate the influence of the sea condition on the pattern of plume migration in unconfined coastal aquifers and to determine the conditions under which the sea boundary condition can be safely simplified. To do so, laboratory experiments were performed to obtain plumes with real sea conditions including seawater intrusion and tidal variations. These plumes are then simulated numerically by simplifying the sea condition. The impact of this simplification on plume migration is indicated through a comparison of numerical results with the experimental ones.

Section snippets

Sea boundary condition for coastal aquifers

In coastal aquifers, fresh groundwater normally discharges into the ocean if a seaward hydraulic gradient exists. In general, there exist three zones in the groundwater system: the freshwater zone, the mixing or diffusion zone and the seawater zone. The freshwater flows over the intruded saltwater and exits at the coastline. The diffusion zone is an area where hydrodynamic dispersion of salt occurs. The salt concentration in the diffusion zone varies gradually from that of the seawater to that

Experimental setup and techniques

The experimental setup is shown in Fig. 1. The flow tank is 1650 mm long, 600 mm high and 100 mm wide. The upstream freshwater head is maintained constant, while the mobile weir at the seawater end controls the seawater level. By connecting the weir to a driving mechanism, a periodically varying seawater level is generated. Glass beads were packed in the tank as the homogeneous porous medium, 535 mm high at the freshwater end and 365 mm high at the sea end. The vertical to horizontal ratio of

Computer code

As shown in Table 2, the density difference between the contaminant and the ambient freshwater is up to 3%. Therefore, this is a variable density contaminant transport problem. Density-dependent groundwater flow needs to be accounted for to simulate this system. In this study, the computer code, 2DFEMFAT Cheng et al., 1998, Yeh et al., 1994 is used to construct the numerical model. 2DFEMFAT employs the finite element method to solve variable density solute transport in water flow through porous

Comparison of numerical results with experimental ones

Comparisons of numerical predictions with the experimental results for contaminant plumes of different densities at different times are shown in Fig. 7, Fig. 8, Fig. 9, Fig. 10. The experimental results incorporate seawater density and tidal fluctuation, while the numerical results do not. These comparisons show the impact of simplifying the seaward boundary condition on contaminant transport prediction.

The above results show that for a density of 995.2 g l−1, the numerical prediction is in

Discussion

It is observed that in the comparisons of Fig. 7, Fig. 8, Fig. 9, Fig. 10, larger deviations of simulated results from experimental ones occur for the more dense plumes. This is due to the development of instabilities in the dense plumes. These instabilities complicate the behaviour of the plume and the solute mass distribution within the plume, and this was not simulated in the numerical model. The occurrence of instabilities may be related to some nondimensional parameters, such as the

Conclusions

Contaminant transport in coastal aquifers is complicated due to the presence of seawater intrusion and tidal variations. This paper presents experimental results of contaminant transport in an unconfined aquifer under realistic seaward boundary conditions. Numerical modelling was performed to investigate the influence of simplifying the seaward condition on the prediction of contaminant migration. It is found that before the contaminant reaches the saltwater interface, numerical prediction

Acknowledgements

Financial support was provided through the Cooperative Research Centre for Ecological Sustainability of the Great Barrier Reef.

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