Saskia Oosting, Eizabeth Greene, Patricia Haederle and John Enos

Abstract

In 1998, a vertical barrier wall (VBW) with reactive gates was installed to address non-aqueous phase liquid (NAPL) and dissolved-phase contaminant migration to the intertidal zone at a former manufactured gas plant (MGP) located on the coast of Massachusetts. The VBW extends into a low permeability clay layer at depth to minimize migration of separate-phase product, and the wall includes a NAPL collection system. The reactive gates were designed to address both low pH (less than 2 in some areas) and dissolved contaminant load in groundwater discharging to the abutting saltwater cove. Over the past eight years, we have monitored the performance of the system and made modifications to improve system performance such as minimizing surface flow into the structures and replacement of media in the reactive gates. The wall has performed well and has proven to be an integral part of the overall remedial solution for the property. Groundwater at the outlets of the reactive gates has generally met treatment goals, including 50 to 100% removal of dissolved organic loads. Generally, the time between changeouts of media has been longer than anticipated, and the methods of media replacement have been improved since it was first attempted. The aboveground portion of the NAPL collection system (e.g. belt skimmers, piping, temporary storage) was not installed with the original construction because the type of system to be installed would be largely dependent on the volume of NAPL collected by the system. To date, NAPL has only been detected once in the system, and the fact that the aboveground portion of the system was not installed represents a significant cost savings to the owners.

Key words:dense non-aqueous phase liquid, design support study, focused feasibility study, high-density polyethylene, manufactured gas plant, non-aqueous phase liquid, powdered activated carbon , vertical barrier wall, vertical recovery well

© 2007 EPP Publications Ltd

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Article code 754