Hexavalent chromium (Cr(VI)) and trichloroethylene (TCE) are two of the most common groundwater contaminants. Researchers at the University of Waterloo have proposed that zero-valent iron filings can be a suitable treatment material for Cr (VI) and TCE contaminated groundwater. Laboratory batch and column studies conducted with simulated Cr- and TCE- contaminated groundwater (10 mg/L Cr(VI) and 2 mg/L TCE) and iron filings indicated succesful removal of both these contaminants to less than MCL values (0.05 mg/L Cr; 5 ug/L TCE) within a short time period. 

     Based on these laboratory results, an in situ permeable reactive barrier (PRB) composed of Peerless brand iron filings was installed for the treatment of Cr(VI) and TCE contaminated groundwater at the U.S. Coast Guard base, Elizabeth City, N.C. The installation project was initiated by U.S. EPA, in a collaborative research agreement with the University of Waterloo. The barrier was installed on June 22, 1996. The barrier is 46 m long, 7.3 m deep and 0.6 m wide and entirely comprised of Peerless brand iron filings. It was installed in approximately 6 hours using a continuous trenching technique which simultaneously excavated aquifer material and emplaced the iron filings media. The bottom of the barrier was chosen to coincide with a lower hydraulic conductivity silty clay layer, at 7 m depth, which is believed to bound the lower extent of aquifer contamination. The barrier contains 450 tons of granular iron and had a total installation cost of US $500,000. A cost comparison done by the USCG suggests that savings of nearly $4M in construction and long-term (20 year) maintenance costs will be realized by the use of this in-situ technology.
 

     Many reduced phases have the potential to reduce dissolved Cr(VI). Mineralogical study of the reacted solids obtained from experiments indicates that the principal mechanisms limiting Cr(VI) mobility is the reduction of Cr(VI) to Cr(III) and the formation of sparingly soluble Fe(III)-Cr(III) oxyhydroxide phases (Blowes and Ptacek, 1992; Blowes et al., 1995 and 1997). The mechanism of Cr(VI) removal by zero valent iron can be written

                     Fe^o + HCrO₄^- + 2H₂O + H⁺  <--> Fe(OH)₃ + Cr(OH)₃

     Zero valent iron is also used as a means of enhancing abiotic reductive dechlorination of organic compounds (Gillham and O'Hannesin, 1992, 1994). The results of laboratory batch and column tests indicates removal of halogenated hydrocarbons and Cr(VI) at rates that are suitable for groundater treatment using a reactive barrier. The University of Waterloo holds patents for these methods of in situ, passive remediation of metal and organic contaminated water.
 

     Contractors involved in the design and installation of the barrier include Envirometal Technologies Inc. (ETI; barrier design), Horizontal Technologies Inc. (installation) and Parsons Engineering Science (installation management).

     Results from sampling sessions conducted several times over approximately 2 years after the barrier installation indicate that Cr concentrations decline from upgradient values as high as 5.1 mg/L to less than 0.01 mg/L within the barrier. Concentrations of up to 5650 ug/L TCE, 290 ug/L cis-dichloroethylene (cDCE) and 66 ug/L vinyl chloride (VC), also decline significantly within the barrier. TCE concentrations decline to the MCL value (5 ug/L) within the barrier. cDCE concentrations decline to less than the MCL value (70 ug/L) within and downgradient of the barrier. VC concentrations drop sharply to values that approach the MCL value (2 ug/L), although concentrations of up to 5 ug/L are observed in some of the downgradient wells.

     Eh values as low as -580 mV SHE, and pH values as high as 10.9 are  observed in the vicinity of the reactive barrier. These low Eh, and high pH values indicate conditions suitable for the reduction-precipitation of sparingly soluble Cr(III) oxyhydroxides and reductive-dechlorination of TCE and its chlorinated daughter products.

   These results indicate that Cr(VI) and TCE contaminated groundwater that is intercepted by the reactive barrier is remediated to MCL values. In addition, concentrations of the chlorinated daughter products, cDCE and VC, produced by the reductive-dechlorination of TCE are themselves reduced significantly to values that approach their respective MCL values.

U.S. EPA site for Elizabeth City Permeable Reactive Barrier installation
Envirometal Technologies Inc., Waterloo, Ontario
Horizontal Technologies Inc. 
 
 

Treatment of Cr(VI) plume by the Permeable Reactive Barrier Copyright T. Bennett, 1997

Treatment of TCE plume by the Permeable Reactive Barrier Copyright T. Bennett, 1997

Simultaneous excavation and replacement of aquifer material with granular iron as horizontal trencher advances

Iron filings stored on site for use in the permeable reactive barrier

Bennett, T.A., 1997. An in situ reactive barrier for the treatment of hexavalent chromium and tricloroethylene in groundwater. M.Sc. Thesis, Department of Earth Sciences, University of Waterloo.

Blowes, D.W.  and Ptacek, C.J., 1992. Geochemical remediation of groundwater by permeable reactive walls: Removal of chromate by reduction with iron-bearing solids. In Proceedings of the subsurface Restoration Conference, Third International Conference on Groundwater Quality Research, June 21-24, Dalas Texas, pp. 214-216.

Blowes. D.W. and Ptacek, C.J., 1994. System for treating contaminated groundwater. U.S. Patent 5,362,394, filed March 3, 1992, issued Nov. 8, 1994, Canada patent 2,062,204, filed March 3, 1992, issued July 7, 1998, Europe Patent 92103559.8, filed March 2, 1992.

Blowes. D.W. and Ptacek, C.J., 1996. System for treating contaminated groundwater. Continuation in part. U.S. Patent 5,514,279, filed October 27, 1994, issued may 7, 1996.

Blowes, D.W., Ptacek, C.J., Hanton-Fong, C.J and Jambor, J.L., 1995. In-situ remediation of chromium contaminated groundwater using zero-valent iron. In Proceedings of the 209th American Chemical Society National Meeting, Environmental Chemistry Division, Anaheim, CA, April 2-7, Vol. 35, pp. 780-783.

Blowes, D.W., Ptacek, C.J., and Jambor, J.L., 1997. In-situ remediation of Cr(VI)-contaminated groundwater using permeable reactive walls: Laboratory studies. Environmental Science and Technology, 31 (12): 3348-3357.

Gillham, R.W., 1993. Cleaning halogenated contaminants from groundwater. U.S. Patent 5,266,213, filed Nov. 28, 1990, issued Nov. 30, 1993.

Gillham, R.W. and O'Hannesin, S.F., 1992. Metal-catalysed abiotic degradation of halogenated organic compounds. In Modern Trends in Hydrogeology, IAH, Hamilton, Ontario, May 10-13, pp. 94-103.