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TECHNICAL REPORTS

Ground Water Quality

Hydrogeochemical and Biological Processes Affecting the Long-term Performance of an Iron-Based Permeable Reactive Barrier

^a DITAG–Land, Environment and Geo-engineering Dep., Politecnico di Torino, c.so Duca degli Abruzzi 24, 10129 Torino, Italy
^b DISMIC–Materials Science and Chemical Engineering Dep., Politecnico di Torino, c.so Duca degli Abruzzi 24, 10129 Torino, Italy

^* Corresponding author (rajandrea.sethi{at}polito.it).

Received for publication November 29, 2007. Despite the wide diffusion of zero-valent iron (Fe⁰) permeable reactive barriers (PRBs), there is still a great uncertainty about their longevity and long-term performance. The aim of this study is to investigate the biological and the hydrogeochemical processes that take place at a Fe⁰ installation located in Avigliana, Italy, and to derive some general considerations about long-term performance of PRBs.The examined PRB was installed in November 2004 to remediate a chlorinated solvents plume (mainly trichloroethene and 1,2-dichloroethene). The investigation was performed during the third year of operation and included: (1) groundwater sampling and analysis for chlorinated solvents, dissolved CH₄, dissolved H₂ and major inorganic constituents; (2) Fe⁰ core sampling and analysis by SEM-EDS, XRD, and FTIR spectroscopy for the organic fraction; (3) in situ permeability tests and flow field monitoring by water level measurements.The study revealed that iron passivation is negligible, as the PRB is still able to effectively treat the contaminants and to reduce their concentrations below target values. Precipitation of several inorganic compounds inside the PRB was evidenced by SEM-EDS and XRD analysis conducted on iron samples. Groundwater sampling evidenced heavy sulfate depletion and the highest reported CH₄ concentration (>5000 µg/L) at zero-valent iron PRB sites. These are due to the intense microbial activity of sulfate-reducers and methanogens, whose proliferation was most likely stimulated by the use of a biopolymer (i.e. guar gum) as shoring fluid during the excavation of the barrier. Slug tests within the barrier evidenced an apparent hydraulic conductivity two orders of magnitude lower than the predicted value. This occurrence can be ascribed to biofouling and/or accumulation of CH₄(g) inside the iron filings.This experience suggests that when biopolymer shoring is planned to be used, long-term column tests should be performed beforehand with initial bacterial inoculation and organic substrate dosing, in order to predict the effects of bacterial overgrowth and gas generation. During construction particular care should be taken in order to minimize the amount of used biopolymer so that complete breakdown can be achieved.

Abbreviations: BGS, below ground surface • DO, dissolved oxygen • EDS, energy dispersion X-ray spectroscopy • Eh, redox potential • FESEM, field emission scanning electron microscopy • Fe⁰, zerovalent iron • FTIR, Fourier transform infrared spectroscopy • PRB, permeable reactive barrier • VOC, volatile organic compound • XRD, X-ray diffraction