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   Weekly Abstract from AEHS Foundation West Coast Conference 2009  

In Situ Treatment of Hexavalent Chromium in Perched Groundwater

Gordon G. Alexander and Paula DeLong


Former chromium plating operations located above regional aquifers in the San Fernando Valley have generated concern over the potential impacts to regional groundwater supplies.  Since 2000, perched groundwater impacted with elevated concentrations of hexavalent chromium (Cr[VI]) exceeding 2,000,000 parts per billion has been extracted and treated at a former chromium plating site in Burbank, California.  Treatment of the chromium impacted groundwater has successfully mitigated the downward migration of hexavalent chromium (Cr(VI)) from the perched water-bearing zone to the regional aquifers.   

In situ treatment of Cr(VI) was pilot tested with the goals of: 1) shortening the remediation timeframe in the perched water-bearing zone; and 2) continuing to mitigate the migration of Cr(VI) from the perched zone to the regional aquifers.  This site was the first to implement remediation in the Los Angeles Region of the California Regional Water Quality Control Board under new General Waste Discharge Requirements (WDRs) for in situ remediation of Cr(VI).

For the pilot test, parallel test cells were installed in the source area at the site.  Two different methods for reducing Cr(VI) in perched groundwater were implemented.  Chemical reduction of Cr(VI) using calcium polysulfide (CaSx) injection was tested in one cell.  Biological reduction of Cr(VI) using molasses was tested in the second cell.  Each cell consisted of a single injection well and a pair of extraction wells pumped continuously during the pilot test to establish hydraulic control of the perched water-bearing zone.  Candidate solutions were amended with tracer compounds and injected over a nine-week period.

Physical and chemical parameters were monitored during the injection and subsequent six months.  The results have important implications regarding the use of these methods for remediation of soil, perched groundwater, and regional groundwater.  The successful results of the pilot test are being incorporated into the planned full-scale remediation of the site.

Gordon G. Alexander, PE

AMEC Geomatrix, Inc., 510 Superior Avenue, Suite 200, Newport Beach, CA 92663, USA; gordon.alexander@amec.com; Telephone: (949) 574-7501; Fax: (949) 642-4474.

Paula DeLong, PG

AMEC Geomatrix, Inc., 510 Superior Avenue, Suite 200, Newport Beach, CA 92663, USA; paula.delong@amec.com; Telephone: (949) 574-7626, Fax: (949) 642-4474.

Presenting author: Gordon G. Alexander

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