Consistent unconfined contaminated disposal facilities dike tidal flow and transport model

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Abstract

A two-dimensional time dependent coupled free-boundary numerical model for tidally-induced flow and transport through the dike of a confined disposal facility (CDF) for contaminated sediments is presented in this paper. The model simulations demonstrate the major effect that tidal fluctuations have on the dynamics of the discharge flux. Sensitivity analyses demonstrate that tidal fluctuation hastens the rate of plume migration near the exit face because of the relatively high advective and dispersive fluxes induced by tides. With or without tides, the rate of contaminant migration increases with higher CDF area hydraulic gradient.

Introduction

Confined disposal is the placement of dredged material into a secure area where the sediment is physically contained. Confined disposal facilities (CDFs) are diked structures that have been built for the disposal of dredged material where in-water placement and beneficial use are not feasible or environmentally acceptable. The size, shape, design and level of complexity of these facilities vary widely depending on dredging quantities, methods of disposal, sediment contamination levels, state and local requirements and site characteristics. Confined disposal is the most commonly used management practice for contaminated sediments dredged for navigation and environmental remediation (IJC, 1997, USACE, 1997, USEPA/USACE, 1998a,b) and several computer models have been developed to support CDF design and operation (Dorkin et al., 1989, Myers, 1991, Myers and Brannon, 1991, Su et al., 2003, Su, 2004).

Many in-lake facilities have stone dikes constructed with layers of stone of increasing size. The center of the dike (the core) typically contains sand or gravel. The outer layers of the dike have stone with sizes increasing from several pounds to several tons to protect the facility from waves. Most existing, in-lake CDFs have no liners. The stone dikes are permeable upon construction. As dredged material is placed into the CDF, water is moved passively through the dike. The sand or gravel in the core of the dike functions as a filter and retains much of the suspended sediments. As the in-lake CDF becomes filled, portions of the dike become clogged as the sediments are pressed against it. The stone dike becomes progressively less permeable.

The primary objective of a CDF is retention of suspended solids, which are expected to contain the vast majority of the contaminants. The porous dike walls eliminate water retention but provide a pathway for untreated contaminated water to return to the fresh water body. Evaluation of water exchange and its implications for contaminant exchange is the main focus of our study.

The focus of the current work is development of a computer-modeling tool capable of evaluating the environmental significance of contaminant losses from in-lake CDFs. Data from a particular CDF near Chicago, IL and constructed in the manner described above were employed in the development of the model. Previous results of computer modeling had suggested that a CDF was efficient in retaining PCB's (Dorkin et al., 1989, Myers, 1991, Myers and Brannon, 1991). Here, we focus on the effects of tidal fluctuations on the temporal and spatial distributions of a contaminant plume within and through the dike wall. Lake waters infiltrating inward during a high-tide period and discharging outward during a subsequent low-tide period may significantly change the concentration levels of a plume as well as the rule of plume migration into the lake water. The model can also be used to examine the influence of low frequency lake water level fluctuation. Time-dependent variable water level changes in the lake produce time-variable dike-water flow into the CDF and contaminant transport outward through the dike.

This study presents a numerical solution of a two-dimensional porous flow model coupled with an advection–dispersion equation for contaminant transport in a flow domain with simultaneous CDF elevation control. This study also provides sensitivity analyses for the physical parameters that affect the transport processes of a contaminant plume within a tidally influenced zone of an aquifer.

Section snippets

Problem formulation

Fig. 1 shows an idealized version of the cross-section of a confined disposal facility dike. This idealization is made to render the problem for two-dimensions.

We will restrict our investigation to incompressible flow with constant density ρ and a homogeneous medium. In this case, we can define the hydraulic head h as a potential function for the flow:h=Pρg+y.The flow equation, assuming Darcy's Law, is of the form:Sht=x(Kxxhx)+y(Kyyhy),where h is hydraulic head (potentials), Kxx is

Hydraulic head and Darcian flow problem

As an example of the application of the described methodology, the self-consistent estimation of the flow region and flow characteristics governed by Eqs. (1), (4) is considered. First, we considered a flow boundary development without the tide; this produced no changes in the level of either reservoir. Next, a dynamic transformation and repositioning of the interior in-dike flow boundary due to the presence of a tide and corresponding changes of the free water levels in the surrounding

Conclusions

The simulation model predicts the tidal effects on a CDF, the dispersion of a dike, and the resultant rise in contaminant transport characteristics in the dike. A non-stationary tide dependent dynamic model of contaminant transport in a porous unconfined aquifer with moving dynamic boundaries was proposed for this problem.

A FEM numerical nonlinear solver algorithm was developed. This numerical algorithm describes nonlinear system dependence between flow geometry, other flow characteristics, and

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