Permeable Reactive Wall Remediation of Chlorinated Hydrocarbons in Groundwater

Abstract : A pilot-scale permeable reactive barrier was installed at Moffett Field in April 1996 and its performance was monitored over the following 16 months on a quarterly basis. The details of this study are described in a technology evaluation report (Battelle, 1998). This document provides a brief account of the technology evaluation. The objective was to capture and treat a small portion of the West Side Plume that contains chlorinated volatile organic compound (CVOC) contaminants, primarily trichloroethene (TCE), cis-1,2-dichloroethene (cis-1,2-DCE), and perchloroethene (PCE). The reactive cell in the funnel-and-gate type barrier is composed of granular zero-valent iron, a strong reducing agent. The flow-through thickness of the reactive cell is 6 ft and it is lined on either side by 2 ft of pea gravel. The reactive cell and pea gravel comprise the gate, which is 10 ft-long and is flanked on each side by 20-foot long funnel walls. The lowering of groundwater redox potential (Eh) and dissolved oxygen (DO), and the presence of nonchlorinated hydrocarbon products in the reactive cell, indicated conditions conducive to abiotic reductive dechlorination. Over the 16-month period after construction, the barrier consistently reduced groundwater concentrations of TCE, PCE, cis-1,2-DCE, and vinyl chloride to well below their respective maximum contaminant levels (MCLs) or to non-detect levels, which were the design criteria or performance goals for the barrier. The range of degradation half-lives of these compounds observed in the field system conformed well with the half-lives predicted during bench-scale column tests. The reactive cell did not contribute any significant levels of dissolved iron to the groundwater and the water exiting the cell contained below 0.3 mg/L of iron, the secondary drinking water standard.