Iron Enhanced In Situ Reductive Dechlorination in High Sulfate Aquifers

Monday, December 3, 2018: 1:40 p.m.
N119/120 (Las Vegas Convention Center)
Daniel P. Leigh, PG, CHG , PeroxyChem, Walnut Creek, CA

The geochemistry of groundwater reflects the rocks through which it has migrated. Notably, the groundwater in the Las Vegas basin (LVB) migrates through sulfate containing carbonate rocks such as gypsum (CaSO4·2H2O) and anhydrite (CaSO4). As the groundwater comes into equilibrium with these rocks sulfate can accumulate to concentrations up to several thousand milligrams per liter (mg/L).

The application of in situ biologically mediated reductive dechlorination for treatment of chlorinated volatile organic compounds (CVOCs) establishes sulfate reducing conditions in the groundwater. During this process bioavailable ferric iron is reduced to ferrous, and sulfate is reduced to sulfide. Very high sulfide concentrations are toxic to bacteria and can inhibit biological activity. At many sites, sulfide is removed from groundwater by combining with naturally occurring ferrous derived from the aquifer matrix. However, carbonate rocks, such as those in the LVB, may not contain sufficient bioavailable iron to sequester very high concentrations of sulfide, resulting in inhibitory conditions and incomplete dechlorination of the CVOCs. In such cases, groundwater can be amended with iron to remove sulfide from the groundwater, thereby allowing the complete biological reduction of the CVOCs.

The sequestration of sulfide by ferrous also forms highly reactive ferrous sulfide species such as mackinawite (FeS) and pyrite (FeS2). Upon contact with these iron sulfide species, CVOCs undergo reductive dechlorination by the β elimination pathway, bypassing the production of toxic daughter products. The combination of biotic and abiotic processes can reduce cleanup time. This biogeochemical degradation process has been applied at numerous sites to enhance biological reductive dechlorination.

This presentation will provide a description of the technology, general biological and geochemical degradation processes for chlorinated organics and the methods and results of the bench and field studies and will suggest methods for applying this technology for treatment of CVOCs in high sulfate aquifers.

Daniel P. Leigh, PG, CHG, PeroxyChem, Walnut Creek, CA
Dan Leigh is a Technology Applications Manager for PeroxyChem Environmental Solutions. His primary focus is on application of biological, biogeochemical and chemical reduction technologies. He is a registered geologist and hydrogeologist and has over 30 years of professional experience in environmental assessment and remediation, primarily focused on biological and natural attenuation processes. He has over 27 years of experience in remediation of chlorinated organics. He has conducted site evaluation and remediation projects across the United States and Internationally. He has over 100 presentation and publications in technical areas including geology, hydrogeology, geochemistry, atoll island hydrogeology, natural attenuation, biogeochemical degradation, mechanochemical destruction of organic compounds, and in situ bioremediation of chlorinated and non-chlorinated organics.