The site includes fractured, porous sandstone with double porosity. This condition presented the potential for back diffusion of the contaminants from the matrix into the fractures and had to be considered when designing the remedial strategy.
Targeted Environmental Media:
 Fractured Bedrock - Light Non-aqueous Phase Liquids (LNAPLs)
Not documented.
Major Contaminants and Maximum Concentrations:
- Total petroleum hydrocarbons (var)
- Trichloroethene (2000 to 8000)
- Pumping Tests
- Tracer (dye) Test
- Other (Compound specific isotope analyses)
Comments:
The groundwater beneath the site was contaminated. Numerous soil and groundwater investigations had been completed since 1988. Over 85 wells were installed on and off site and several wells were logged using geophysical methods. At least nine on-site chlorinated hydrocarbon (CHC) source zones were identified and differentiated using (among others) compound specific isotope analyses (CSIA). A pilot test was implemented for using in-situ chemical oxidation (ISCO). The pilot test included combined multiple well pumping and tracer tests to determine the CHC mass emitted from the source and the relevance of preferential flow paths. The test identified that the mass flux of the total CHC emission from the source zone is at a minimum 76 kg/a. During baseline sampling, CHC concentrations and ý13C values from CSIA were determined from 29 wells. In addition, compound specific (tetrachloroethylene [PCE], TCE, and cis-dichlorethylene [c-DCE]) enrichment factors were determined using a laboratory batch test with the goal of assessing the destruction of CHCs associated with the application of ISCO shown by changes in the isotopic signatures of the contaminants. The samples for isotope analyses were treated with sodium thiosulfate (to remove excess permanganate) and stabilized with sodium hydroxide.
- Chemical Oxidation (In Situ)
Comments:
Based on the feasibility study, the remedy of choice at this site is ISCO using potassium permanganate. Stakeholders decided to perform an ISCO pilot test in one suitable source area where TCE concentrations of around 2,000 to 8,000 micrograms per liter (ýg/L) were identified. The ISCO pilot test included injecting 40 tons of 2% potassium permanganate solution. Continuous sampling for CHCs was conducted during the first 9 months of the ISCO pilot test. Halfway through the pilot test (after injection of about 20 tons of potassium permanganate over 9 months), the effect of the ISCO application and the effect on the ý13C values were assessed and a second mass flux test was performed.
Not documented.
The pilot study is complete. The CSIA showed an enrichment of ý13C values for PCE and TCE in several wells where permanganate has been observed and downgradient observation wells where no visual evidence of permanganate was obvious. The CSIA also showed that oxidative destruction of CHC by permanganate caused the CHC concentration reduction. However, it was observed that where TCE concentrations were below 40 ýg/L no isotope fractionation occurred even if TCE was oxidized. The CSIA underestimates TCE and PCE destruction in more distant wells from the injection points as depletion of the ý13C values occurs.
The field results were able to demonstrate that the observation of the CHC concentrations in groundwater alone is not sufficient to demonstrate ISCO success. As an additional tool for evaluating remedial success, CHC mass flux should be considered.
Reference:
Meinke, R. 2012. "Assessing ISCO Success Using CSIA in Fractured Porous Sandstone". 8th International Conference Remediation of Chlorinated and Recalcitrant Compounds. May 21-24, 2012. Page A3.
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