Sorbing Vertical Barriers

ALAN J. RABIDEAU1, JOHN VAN BENSCHOTEN1, ASHUTOSH KHANDELWAL2, and CRAIG R. REPP3

Department of Civil, Structural, and Environmental Engineering, University at Buffalo, Buffalo, NY 14260

2S.S. Papadopulos & Associates, Bethesda, MD

3West Valley Nuclear Services Company LLC, West Valley, NY

Key words: barriers, sorption, slurry wall, zeolite, ion exchange

Abstract: This chapter presents an overview of vertical barrier technologies that use sorbing materials to remove contaminants from groundwater. Two classes of system are considered: 1) low-permeability earthen barriers, in which sorbing additives are used to reduce the diffusive flux of organic contaminants, and 2) high-permeability treatment walls designed to remove contaminants under advection-dominated natural groundwater conditions. The focus of the discussion is on the performance assessment of strongly sorbing barrier materials using laboratory tests. Emphasis is placed on the design and analysis of column studies to characterize the barrier sorption capacity and the appropriate formulation of mathematical models to extrapolate long-term barrier performance. Two case studies are considered: the amendment of soil-bentonite slurry walls with an organic-rich additives, and the use of natural zeolite to remove strontium-90 from groundwater.

Physicochemical Groundwater Remediation

Edited by Smith and Burns, Kluwer Academic/Plenum Publishers, 2001

1. INTRODUCTION

The severe technological obstacles to the restoration of contaminated aquifers have been well documented (e.g., NRC, 1994) and have led to a renewed interest in low-permeability containment barriers and the development of high-permeability treatment walls. Studies of both classes of barriers have included designs for which sorption is the primary mechanism for reducing contaminant flux, although few actual field installations of sorbing barriers have been reported. Reliance on in situ sorption as the primary remedial measure is complicated by the considerable cost and logistical difficulty of replacing a large barrier when the sorption capacity is exhausted. For this reason, the ability to predict the time-to-replacement is critical in evaluating whether the high capital cost of a barrier system is justified.

This chapter considers two particular types of sorbing barriers: 1) low-permeability slurry walls amended to promote the sorption of hydrophobic compounds (hydraulic conductivity cm/s), and 2) high-

conductivity zeolite treatment walls designed to remove inorganic compounds (K ~ 10"1 cm/s). These examples are selected, in part, because they represent the systems that have received the most attention from researchers and practitioners, but also because considering them together highlights the importance of conceptual issues common to both types of systems.

For any type of sorbing barrier, the development of a reliable predictive model necessitates accurate characterization of the sorption capacity of the barrier material. A basic premise of this work is that laboratory column tests are the most appealing tool for this task. However, obtaining meaningful column data for strongly sorbing materials can be quite difficult if the experiments are to be performed under groundwater conditions typical of actual contaminated sites. Over the past 5 years, our research group at the University of Buffalo, NY has conducted several studies related to the development of sorbing and reactive barriers. In performing these studies, we have considered several experimental and modeling issues common to low- and high-permeability barriers, and have noted similarities and differences between the two applications. This chapter presents some of these insights in the context of a general approach to barrier performance assessment.

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