CVOC Source Identification Through in situ Chemical Oxidation in Fractured Bedrock

Abstract

An in situ chemical oxidation (ISCO) pilot program, using Fenton’s Reagent (hydrogen peroxide and a ferrous sulfate catalyst), was performed to assess its effectiveness in destroying chlorinated volatile organic compounds (CVOCs) in a fractured-bedrock aquifer. This case study is unique because it was one of the first applications of ISCO in fractured bedrock. In addition, the targeted CVOC reduction from 1,500 to 100 micrograms per liter (μg/L) was relatively aggressive compared to most ISCO applications. This pilot program also provided the opportunity for an independent, third party evaluation of ISCO in a fractured-bedrock environment. The site geology consists of approximately 6 meters (m) of unconsolidated glacial deposits overlying fractured bedrock, with a groundwater depth of approximately 2 m. Initial characterization activities, including injection testing and multi-level packer sampling, identified a pre-ISCO CVOC plume extending approximately 90 m long by 45 m wide and spanning a vertical depth between 3 and 35 m. Packer sampling results indicated the pre-ISCO plume had an asymmetric configuration that was consistent with the injection-test results. The ISCO pilot program involved the injection of 14,237 liters of 50% hydrogen peroxide, combined with a ferrous sulfate and pH-buffering catalyst. Two injection events were performed, with overlapping performance sampling. Samples collected 30 to 45 days after each injection event showed CVOC concentrations below the treatment objective in many areas of the plume. However, samples collected 60 to 100 days after each event revealed significant rebound in most areas, at concentrations that approached initial pre-ISCO aquifer conditions. An assessment of the results suggests that the injected oxidants primarily influenced the more transmissive fractures in the treatment zone, whereas the less transmissive fractures were less influenced. Geochemical data and calculations indicate that the peroxide and catalyst may persist in the subsurface for prolonged periods (>200 days), thus complicating the assessment of rebound and the actual effectiveness of the technology. Although the success of treatment was limited, it proved to be successful in enhancing the conceptual site model of the subsurface, better defining the applications and limitations of ISCO treatment in fractured bedrock, and most importantly, clearly identifying the source of residual CVOCs at the site.